ONYX Battery Design (Cells, Layout, and Limits)

This page is part of the ONYX Battery Guide

Intro

Battery discussions around the ONYX are typically reduced to voltage, capacity, and cell choice. In practice, these do not determine performance under load.

This post defines what 72V, 84V, and 96V batteries represent at the system level, how high-current packs are constructed using tabless cylindrical cells, and what determines whether a pack can realistically support 300A within the ONYX form factor.

This is not a general battery guide. It is a constrained analysis of a specific class of battery design.

At this level, performance is governed by system behavior under load, specifically internal resistance and current path design.

Summary

• Battery performance is defined by internal resistance, not nominal voltage
• Increasing voltage reduces current but shifts stress into other parts of the system
• Tabless cells shift the limitation from the cell to the pack architecture
• 300A capability is determined by parallel group design and interconnect strategy
• Nickel-only interconnects are not viable at this current level
• BMS, wiring, and connectors frequently become the system bottleneck
• Two batteries with identical voltage and capacity can behave differently under load due to resistance distribution
• Within a fixed enclosure, increasing voltage reduces parallel capacity and increases system stress
• Current defines component stress, while voltage determines how much power that current represents

Battery Selection Context

These recommendations are based on system constraints, not marketing specifications or nominal compatibility.

Design Constraints

• Must fit ONYX RCR 72V battery bay
• Must support 300A continuous current
• Must use tabless cylindrical cells only
• Must remain electrically stable under sustained load
• Must not rely on peak-only current ratings

These constraints are non-negotiable and define the design space.

These constraints eliminate most theoretical designs before they are ever built.

Every improvement in one part of the pack introduces a constraint in another.

Enclosure as the Primary Constraint

The battery is defined by the enclosure, not the cells.

The metal enclosure determines:

• maximum internal volume
• layout constraints
• structural integrity
• safety and containment
• thermal behavior

The enclosure is part of the system, not a container.

Structural Stability

• prevents movement under load
• maintains alignment
• reduces weld stress

Safety and Protection

• impact resistance
• containment of failure
• isolation from external damage

Thermal Behavior

• spreads heat
• may trap heat if poorly designed

Carrying and Integration

• mounting
• weight distribution
• structural support

System Constraint

All internal design must conform to the enclosure.

The enclosure defines not only what fits, but how current and heat are distributed throughout the system.

If a design does not fit within the enclosure, it does not exist as a viable system.

System Overview

This post is primarily a system-level analysis of high-current battery design within the ONYX RCR platform.

However, where necessary, it uses a concrete design example to illustrate real constraints and tradeoffs.

That reference design is:

• a custom 20s9p tabless 21700 pack
• targeting ~45Ah capacity
• designed around a slightly increased enclosure height

This example is not the only valid configuration.

It is used to make constraints visible:

• how enclosure volume affects parallel count
• how interconnect space competes with capacity
• how thermal and electrical limits emerge in real layouts

All principles in this post apply generally.

The 20s9p configuration is used to show how those principles play out in a real design.

These values describe the target configuration, not guaranteed performance.

Most packs that claim high current capability do not sustain it under real load conditions.

Power, Current, and Voltage Relationship

Power is defined as:

• Voltage x Current = Watts

At a fixed power level:

• higher voltage reduces current
• lower voltage increases current

Example:

• 72V system at 10kW = ~139A
• 84V system at 10kW = ~119A
• 96V system at 10kW = ~104A

Lower current reduces:

• resistive losses
• heat generation
• voltage sag

However:

• all current still flows through the same system components
• resistance remains the limiting factor

Increasing voltage improves efficiency, but does not eliminate system constraints.

Reducing current reduces losses across every component in the system.

This relationship defines how voltage choices reshape the entire pack architecture.

Current Does Not Scale Linearly With Voltage

A 300A load does not represent the same operating condition across 72V, 84V, and 96V systems.

At 300A:

• 72V corresponds to ~21.6kW
• 84V corresponds to ~25.2kW
• 96V corresponds to ~28.8kW

That is not the same stress condition.

At higher voltage, the same current corresponds to higher total power, higher thermal load, and greater system stress.

Conversely, for the same power output, higher voltage reduces required current and lowers resistive losses.

This distinction is critical:

• current limits define component stress
• voltage determines how much power that current represents

In practice, higher voltage systems should be designed to reduce current demand, not maintain it.

A 96V system does not need to chase the same amperage as a 72V system to produce high power.

300A is a stress ceiling, not a performance goal.

Most systems are not limited by how much current they can produce, but by how efficiently they can deliver it.

What 72V, 84V, and 96V Actually Represent

A 72V battery is a 20-series configuration. An 84V battery is a 23-series configuration. A 96V battery is a 26-series configuration. Each series group increases voltage, not capacity.

Capacity is determined by the number of cells in parallel. A 45Ah pack is not a single entity. It is the sum of parallel cell groups.

Energy is defined as:

• Voltage x Capacity = Watt-hours

Two packs labeled with the same voltage and capacity can have identical energy but different performance under load due to:

• internal resistance
• interconnect design
• cell chemistry
• thermal behavior

Increasing series count within a fixed enclosure reduces available space for parallel groups and increases system voltage stress.

Increasing voltage within a fixed enclosure trades capacity and stability for electrical stress.

Voltage class changes pack architecture, not just pack labeling.

Tabless Cell Architecture

This design uses only the following tabless cylindrical cells:

• Tenpower 50XG
• EVE 50PL
• Reliance RS50
• Ampace JP50

Tabless cells differ from traditional cylindrical cells in that current is distributed across the full electrode surface rather than a single tab point.

This results in:

• lower internal resistance
• more uniform current distribution
• reduced localized heating
• improved high-current performance

At this level, the cell is no longer the limiting factor.

Tabless design improves cell-level performance, but does not eliminate limitations in interconnects, BMS, or overall pack design.

This shifts the limiting factor away from the cell and into the pack architecture.

Bottleneck Shift

In tabless-based designs:

• cells capable
• pack architecture limiting

The limiting factors become:

• busbars
• connection resistance
• BMS current handling
• output wiring
• connector interfaces

System performance is limited by the weakest component in the current path.

What looks like a cell problem is often an interconnect, BMS, or wiring problem.

Builder Archetypes

Battery packs built with similar cells and specifications can behave very differently in practice.

Differences in performance are often less about the components themselves and more about how the system is designed and executed.

Spec-Driven Builds

These packs are typically defined by:

• focus on nominal specifications (voltage, capacity, peak current)
• heavy reliance on nickel strip for interconnects
• limited attention to resistance distribution
• minimal consideration for sustained load behavior

They often perform adequately under light or short-duration use but show:

• significant voltage sag under load
• rapid heat buildup
• inconsistent performance over time

The limitations are not usually in the cells themselves, but in how current is managed across the pack.

Transitional Builds

These designs show a better understanding of system behavior:

• improved interconnect strategy (often hybrid nickel/copper)
• more attention to parallel group sizing and current distribution
• basic consideration of thermal behavior
• higher quality assembly and layout

These packs can perform well under moderate load and may approach high-current targets, but often still exhibit:

• localized hotspots
• uneven current distribution
• performance drop under sustained load

They are functionally capable, but not fully optimized.

System-Driven Builds

At this level, the pack is treated as a complete electrical system, not a collection of parts.

These builds are characterized by:

• interconnect design centered around low resistance and current distribution
• deliberate control of current paths and symmetry
• balanced positive and negative pathways
• thermal behavior considered as part of the electrical design
• enclosure used as a structural and thermal component

These packs do not rely on peak ratings. They are designed for:

• sustained load stability
• predictable behavior under stress
• minimal voltage sag relative to load
• controlled heat generation and dissipation

At this level, differences in performance are not subtle. They are immediately apparent under load.

System-Level Reality

Two packs can share:

• the same voltage
• the same capacity
• the same cells

…and still perform completely differently.

The difference is not the specification.

It is the design.

Parallel Group Design

Current capability is determined by the number of cells in parallel.

At a 300A target:

• current per cell must remain within safe discharge limits
• thermal load must be distributed across the group

Example considerations:

• theoretical current per cell is not usable current
• derating is required for sustained load
• uneven current distribution increases localized heating

Parallel group design directly determines:

• voltage sag
• thermal stability
• lifespan

Within a fixed enclosure, increasing voltage reduces parallel count, increasing stress on each cell and raising system resistance.

Increasing series count does not increase total energy unless capacity also increases. It reduces available parallel groups within a fixed volume.

For a fixed power target, higher voltage reduces required current, which changes what an optimal parallel count looks like.

Series Configuration

A 20s, 23s, or 26s configuration defines voltage behavior:

• 20s full charge: ~84V, nominal: ~72V, cutoff: ~60V
• 23s full charge: ~96.6V, nominal: ~83V, cutoff: ~69V
• 26s full charge: ~109.2V, nominal: ~94V, cutoff: ~78V

Under load:

• voltage drops due to internal resistance
• controller behavior changes based on real-time voltage
• performance decreases as sag increases

Voltage is not static. It is load-dependent.

Higher series configurations increase voltage stress on BMS components, wiring insulation, and connectors.

Higher voltage increases electrical stress across all components in the system.

The choice of voltage class changes the load case for the entire pack.

Busbars and Interconnect Strategy

At 300A, interconnect design becomes a primary constraint.

Nickel-only interconnects are not viable at this current level.

Nickel strip alone introduces:

• excessive resistance
• heat buildup
• voltage drop across the pack

At this level, interconnect design is primarily a material problem.

• Copper provides low resistance and efficient current flow
• Nickel provides reliable weldability but significantly higher resistance

These are not interchangeable.

In high-performance designs, the current path is typically divided:

• copper used for primary conduction
• nickel or nickel-plated surfaces used only at the cell interface

Nickel-plated copper combines both:

• copper core for conductivity
• nickel surface for weldability

This allows low-resistance current flow while maintaining consistent weld quality.

Pure nickel interconnects, even when thickened, introduce significantly higher resistance and become a limiting factor under sustained high current.

Poor interconnect design results in:

• localized heating
• efficiency loss
• premature failure

Interconnect resistance is a primary contributor to voltage sag under load.

Poor busbar design increases:

• voltage drop
• heat generation
• instability under sustained current

Sag is not only a cell characteristic. It is strongly influenced by interconnect design.

As current increases, interconnect design scales from a secondary concern to a primary system constraint.

Interconnect design directly determines how efficiently current moves through the pack.

This is where many high-current claims stop being real under load.

At this level, interconnect design is often the difference between a pack that meets its specifications and one that actually performs under load.

BMS Constraints

A BMS rated for 420A does not guarantee stable operation at 300A.

All pack current passes through the BMS, making it a critical point of resistance and heat generation.

Limitations include:

• MOSFET thermal limits
• trace and internal path resistance
• voltage drop across the BMS
• thermal throttling under sustained load

Failure modes:

• sudden cutoff under load
• heat-induced shutdown
• progressive performance loss under sustained current

At high current, the BMS often becomes the primary system bottleneck.

The BMS is often the highest resistance component in the discharge path.

A 300A condition at 96V is a much higher total load state than 300A at 72V.

Wiring and Output Path

Current leaving the pack must pass through:

• internal wiring
• output leads
• connectors

Each introduces resistance.

Key considerations:

• wire gauge must match sustained current
• connector choice impacts voltage drop
• connection quality affects heat generation

At this level, small inefficiencies become system-level losses.

At this current level, wiring resistance can equal or exceed interconnect resistance if not properly sized.

Undersized wiring behaves like a resistor in series with the pack.

Poor wiring introduces voltage drop before power reaches the controller.

Thermal Behavior

Heat is generated by:

• internal cell resistance
• interconnect resistance
• BMS losses

Thermal behavior determines:

• sustained output capability
• safety margins
• lifespan

A pack that can deliver 300A briefly may not sustain it without overheating.

Thermal saturation occurs when:

• heat generation exceeds dissipation
• internal temperatures continue rising under constant load

As temperature increases, resistance increases, further accelerating heat generation.

Thermal behavior is also influenced by enclosure design.

A rigid metal enclosure:

• spreads heat more evenly
• reduces localized hotspots
• improves structural stability under thermal expansion

Poor enclosure design traps heat and accelerates thermal saturation.

Thermal behavior feeds back into electrical performance through resistance.

Heat is the physical manifestation of electrical inefficiency.

Assembly Method

Construction directly affects electrical performance, resistance, and long-term reliability.

Every weld and connection is part of the electrical path.

At this level, assembly is not just mechanical. It defines how current flows through the pack.

Key elements:

• consistent weld quality and penetration
• proper insulation (fish paper, kapton, terminal isolation)
• mechanical stability under vibration and load
• controlled compression where applicable
• rigid metal enclosure acting as the structural shell

Spot welding is required for reliable connections at this scale.

Poor welds introduce resistance at every connection point, which compounds across the pack under load.

Mechanical instability leads to:

• weld fatigue
• micro-movement between cells
• increasing resistance over time

The enclosure is part of the structure, not just a container.

A rigid metal enclosure:

• prevents pack flex under load
• maintains alignment of parallel groups
• protects against vibration and impact
• supports consistent compression across the pack

Improper enclosure design results in:

• shifting cell groups
• stressed interconnects
• uneven pressure distribution
• long-term reliability issues

Improper insulation results in:

• short risk
• localized heating
• long-term degradation

Assembly quality determines:

• total pack resistance
• thermal behavior under load
• consistency between parallel groups
• overall failure risk

Assembly errors are cumulative and scale across the entire pack under load.

Common Failure Archetypes

These failure modes are system-level issues, not isolated component failures.

These failure modes are all expressions of resistance, heat, and current imbalance.

Spec Sheet Pack:

• strong nominal specs
• poor real-world performance due to resistance

Nickel Bottleneck Pack:

• limited by interconnects, not cells

BMS-Limited Pack:

• cutoff under load despite capable cells

Thermally Saturated Pack:

• performance drops after short high-load periods

Diagnostics

Common symptoms and causes:

Voltage sag under load:

• high internal resistance
• insufficient parallel groups

Sudden cutoff:

• BMS limit reached
• voltage drop below threshold

Heat buildup:

• interconnect resistance
• poor current distribution

Performance inconsistency:

• uneven cell loading
• connection variability

These symptoms translate directly to riding behavior:

• voltage sag → reduced acceleration and top speed
• sudden cutoff → loss of power under load
• heat buildup → performance drop over time
• inconsistency → unpredictable throttle response

Electrical issues present as performance issues.

Diagnosis should focus on identifying where resistance is introduced in the system.

All diagnostic symptoms are manifestations of resistance and heat.

Diagnosis is the process of locating resistance within the system.

Buy vs Build

Buying:

• consistent
• validated design
• limited visibility into construction

Building:

• customizable
• potentially higher performance
• increased risk if improperly designed

Building is only justified when:

• performance requirements exceed available packs
• design constraints are fully understood
• construction quality can be maintained

Most performance issues in custom packs come from execution, not design intent.

Voltage Sag Under Load

Voltage sag is one of the clearest indicators of real-world battery performance and total system resistance.

Voltage sag is the drop in pack voltage when current is drawn.

It is caused by resistance across the entire system:

• cell internal resistance
• busbar and interconnect resistance
• BMS resistance
• wiring and connector losses

Sag is not a defect. It is a measurable behavior of the system under load.

What Sag Looks Like in Practice

Using fully charged packs under a high load (e.g. ~300A), sag can be roughly interpreted as:

These are not absolute values, but they provide a practical reference for evaluating pack behavior.

• Low sag → strong pack, low resistance, stable output
• Moderate sag → acceptable performance, but not optimized
• High sag → excessive resistance, heat buildup, and reduced usable power

At higher voltage, the same percentage sag represents a larger absolute voltage drop, but a lower percentage loss relative to total system voltage.

What Causes Sag

Every component in the current path resists flow.

At 300A:

• even small resistance values create measurable voltage drop
• losses compound across the system

Higher resistance results in:

• larger voltage drop
• increased heat
• reduced usable power

Why Sag Matters

Performance is based on real-time voltage, not nominal voltage.

Under load:

• controller sees reduced voltage
• power output decreases
• acceleration and top speed are affected

Two identical packs on paper can behave differently due to sag.

Parallel Groups and Sag

Increasing parallel count reduces sag:

• more cells share the load
• lower current per cell
• reduced voltage drop

Insufficient parallel groups result in:

• higher per-cell stress
• greater voltage collapse under load

Voltage vs Sag

Higher voltage systems reduce current for the same power.

This results in:

• less total voltage drop across the system
• improved efficiency under load

However:

• poor interconnect design will still cause sag
• resistance remains the limiting factor

Increasing voltage does not eliminate sag. It reduces its impact.

Thermal Link

Sag and heat are directly related:

• voltage drop = energy lost as heat
• higher sag = higher temperature

Thermal saturation increases resistance further, creating a feedback loop.

Real-World Behavior

Sag is most visible:

• during hard acceleration
• at low state of charge
• in poorly designed packs

A well-built pack:

• maintains stable voltage under load
• recovers quickly after load is removed

A poorly built pack:

• drops voltage aggressively
• feels inconsistent
• limits usable performance

System Constraint

Sag is not determined by one component.

It is the result of:

• cell selection
• parallel design
• interconnect quality
• BMS capability
• wiring and connectors

The pack performs as well as its total resistance.

What a Well-Designed Pack Feels Like Under Load

A well-designed pack does not just look better on paper. It behaves differently in use.

Under repeated acceleration and sustained load, a strong pack feels:

• consistent rather than variable
• stable rather than abrupt
• predictable across state of charge
• resistant to sudden power collapse

A key characteristic of a strong pack is how it behaves as state of charge decreases.

At high state of charge (100% → ~75%):

• peak performance is available
• voltage remains high under load
• acceleration is strong and immediate

At mid state of charge (~75% → ~50%):

• performance remains consistent
• slight reduction in peak output
• minimal change in overall feel if the pack is well designed

At lower state of charge (~50% → ~35%):

• voltage sag becomes more noticeable
• peak power is reduced
• the system becomes more sensitive to load

Below ~35%:

• available power drops more rapidly
• voltage collapses more easily under load
• performance becomes increasingly limited

A well-designed pack maintains usable, predictable performance through most of its discharge range and degrades gradually.

A poorly designed pack may feel strong at full charge but becomes inconsistent or unstable much earlier, often due to resistance and poor current distribution.

That difference is not subtle under real use. It is the result of resistance, current path quality, and thermal behavior across the entire system.

Energy, Runtime, and Range

Watt-hours define how much energy a battery stores, not how far the bike will travel.

Energy is calculated as:

• Voltage x Capacity = Watt-hours

A ~72V 45Ah pack:

• ~3240Wh

An ~84V 45Ah pack:

• ~3780Wh

A ~96V 45Ah pack:

• ~4320Wh

This is total stored energy.

However, stored energy is not the same as usable energy under load.

A portion of that energy is always lost as heat due to resistance throughout the system:

• cell internal resistance
• interconnect losses
• BMS losses
• wiring and connector losses

Higher current increases these losses.

Real-world runtime is always lower than theoretical capacity, and the gap increases as load increases.

Advertised capacity describes stored energy. It does not describe how efficiently that energy is delivered.

Energy vs Power

Energy determines how long the system can run.

Power determines how quickly that energy is used.

At higher power levels:

• energy is consumed faster
• losses increase due to higher current
• usable energy decreases due to heat

This is why high-power riding reduces both runtime and efficiency.

A battery does not just “drain faster” at higher power. It also becomes less efficient while doing so.

Runtime

Runtime is determined by average power draw, not peak power.

Example (72V ~3240Wh pack):

• 3000W average → ~1 hour
• 1500W average → ~2 hours
• 6000W average → ~30 minutes

These are idealized values.

In practice:

• higher current increases losses
• voltage sag reduces usable output
• thermal behavior further reduces efficiency

As a result, actual runtime is always lower than calculated values, especially under sustained high load.

Real Power Usage

Actual riding power is highly variable.

Typical ranges:

• light cruising: 1500W to 3000W
• normal riding: 3000W to 6000W
• aggressive riding: 6000W to 10000W+

Short bursts may exceed these values, but sustained power is what determines runtime and thermal behavior.

Higher sustained power:

• reduces runtime
• increases heat
• amplifies system inefficiencies

Range Is Not a Battery Spec

Range is not determined by voltage or capacity alone.

It depends on:

• average power draw
• speed
• terrain
• rider weight
• aerodynamic drag
• throttle behavior

Two riders using the same battery can see significantly different range because they are using energy at different rates.

Range is a usage outcome, not a battery specification.

Voltage, Current, and Efficiency

Higher voltage systems:

• reduce current for the same power
• reduce resistive losses across the system
• improve efficiency under load

However:

• total stored energy still defines total runtime
• higher voltage does not increase runtime unless total watt-hours increase

Efficiency gains from higher voltage come from reduced current, not increased energy.

Poor system design can eliminate these gains entirely.

Load, Losses, and Usable Energy

As load increases:

• current increases
• resistive losses increase
• more energy is converted to heat instead of motion

This means:

• high-power riding consumes more energy
• and wastes more energy at the same time

Usable energy decreases as load increases.

This is why two identical batteries can produce different real-world range depending on how they are used.

Practical Interpretation

A battery does not provide a fixed distance.

It provides a fixed amount of energy, delivered with varying efficiency depending on load.

Higher power:

• increases energy consumption
• increases system losses
• reduces total usable output

Higher voltage:

• reduces current for the same power
• improves efficiency
• reduces loss per unit of power

But:

• energy is still finite
• and system design determines how efficiently that energy is delivered

The difference between a good pack and a poor one is not just how much energy it stores.

It is how much of that energy actually reaches the controller under load.

Thermal Imaging and Hotspot Detection

Thermal imaging is one of the most effective ways to evaluate a battery under real load conditions.

A thermal imaging camera (e.g. FLIR) allows you to visualize how heat is distributed across the pack, revealing resistance, imbalance, and inefficiencies that are not visible through voltage or current measurements alone.

However, thermal imaging is only useful when applied correctly.

When to Use Thermal Imaging

Thermal behavior changes over time. A single snapshot is not enough.

To get meaningful data, observe the pack at multiple stages:

• Cold state (before use)

  • establishes baseline temperature
  • reveals ambient conditions

• Early load (first few minutes of riding or testing)

  • shows initial current distribution
  • identifies immediate imbalance

• Sustained load (extended riding, hill climbs, or racing conditions)

  • reveals true system limitations
  • exposes weak interconnects, BMS heating, and wiring losses

• Post-load (immediately after stopping)

  • highlights retained heat
  • makes hotspots easier to identify

Thermal issues often do not appear instantly. They develop as the system approaches thermal saturation.

Preparing for Inspection

For accurate inspection, the battery should be removed from the bike after testing.

• this allows full access to all surfaces
• eliminates heat influence from surrounding components
• makes hotspot patterns easier to isolate

Inspection should be done immediately after load while the pack is still warm.

Delays reduce temperature contrast and can hide problem areas.

How to Scan the Pack

For accurate results:

• scan under real load conditions (not idle testing alone)
• observe temperature changes over time, not just peak values
• compare similar regions (parallel groups, busbars, connections)
• look for asymmetry, not just absolute temperature

Key areas to monitor:

• busbar paths
• weld points
• parallel group transitions
• BMS region
• wiring exits and connectors

Heat should be distributed relatively evenly across equivalent sections.

Interpreting Hotspots

Heat is a direct indicator of resistance.

Localized hotspots typically indicate:

• increased resistance
• poor connections or welds
• uneven current distribution
• undersized conductors or bottlenecks

A well-designed pack:

• heats gradually and evenly
• stabilizes over time
• shows minimal localized hotspots

A poorly designed pack:

• develops concentrated hot areas
• shows uneven temperature distribution
• continues increasing in temperature under sustained load

Temperature difference between similar areas is more important than absolute temperature.

Metal Enclosure Considerations

A metal enclosure changes how heat appears in thermal imaging.

• the enclosure spreads heat across its surface
• hotspots may appear diffused rather than sharply defined
• external temperature may lag behind internal heating

However:

• heat still originates from internal resistance points
• localized warming of the enclosure often corresponds to internal hotspots
• repeated patterns or asymmetry still indicate underlying issues

Even with a sealed metal case, thermal imaging can reveal:

• where heat is concentrating
• whether one side of the pack is working harder than the other
• whether heat is building unevenly over time

Surface temperature is useful, but it does not always represent the hottest internal point.

System-Level Insight

Thermal imaging evaluates the entire system:

• cells
• interconnects
• BMS
• wiring
• connectors

It shows how current actually flows through the pack under load.

Voltage and current measurements tell you what is happening.

Thermal imaging shows you where it is happening.

Practical Use

Thermal imaging is most useful for:

• validating new builds
• comparing different pack designs
• diagnosing performance issues
• identifying early failure points

It provides a direct view of where energy is being lost and where the system is constrained.

Heat is not just a byproduct.

It is a map of resistance across the system.

All of these constraints converge inside the enclosure.

Enclosure, Internal Volume, and Material Stack

The battery is defined by the enclosure, not the cells.

The enclosure does not just determine what fits. It determines how current flows, how heat accumulates, and where the system will fail.

This design uses an expanded internal volume of approximately:

• 14.0 x 7.4 x 6.2 inches

The increased height is intentional.

It allows a 20s9p configuration (~45Ah using 21700 5Ah cells) without forcing compromises in:

• interconnect design
• thermal behavior
• wiring and BMS placement
• structural stability

This volume is not allocated to cells alone. It is shared between:

• cells (capacity)
• conductors (current capability)
• insulation (safety)
• wiring and BMS (control and output)
• structure and compression
• thermal mass and heat distribution

Every one of these consumes space and contributes resistance.

The enclosure defines the performance ceiling of the system.

Packaging efficiency is not just mechanical. It is electrical.

If capacity is increased within a fixed volume, something else must be reduced:

• conductor cross-section
• spacing and thermal margin
• insulation thickness
• or structural stability

Those reductions increase resistance and heat.

In this design, enclosure volume is increased to avoid those compromises rather than compressing them into a fixed space.

You cannot maximize:

• capacity
• current capability
• thermal stability
• compactness

at the same time.

The enclosure determines which of these are prioritized.

Cell Packing

Using tabless 21700 cells such as the Tenpower 50XG:

• Diameter: ~21.5 mm
• Length: ~70.5 mm

A 20s9p configuration requires:

• 180 total cells
• tightly controlled packing
• deliberate allocation of space for current paths and insulation

Increasing parallel count improves:

• current distribution
• voltage stability under load
• per-cell stress

But increasing density also increases:

• thermal coupling
• resistance sensitivity
• routing complexity

The additional enclosure height allows higher parallel count without collapsing spacing for conductors and insulation.

The highest capacity configuration that fits is not the highest performing configuration under load.

Performance depends on how current moves through the pack, not how tightly it is packed.

Busbar Material and Interconnect Strategy

At 300A class current, interconnect design defines system performance.

Material choice is the primary driver of resistance.

• Copper provides the lowest resistance and best current handling
• Nickel provides reliable weldability but significantly higher resistance

These are not interchangeable.

Increasing nickel thickness improves current handling, but does not approach the conductivity of copper. At this level, resistance is a material limitation before it is a geometric one.

For high-performance packs, the current path is typically split:

• copper used for primary conduction
• nickel or nickel-plated surfaces used only at the cell interface

This approach allows:

• low-resistance current flow through copper
• consistent weld quality at the cell

Nickel-plated copper combines both:

• copper core for conductivity
• nickel surface for weldability

This is often the most effective solution where direct copper welding is not practical.

Direct copper-to-cell welding is possible, but requires more advanced equipment and tighter process control. It is not the baseline assumption for most builds.

Thickness and Geometry

Conductor thickness and width determine:

• voltage drop
• heat generation
• current distribution

Increasing cross-sectional area reduces resistance, but consumes internal volume.

That volume competes with:

• additional cells (capacity)
• insulation and spacing (thermal stability)

In constrained enclosures, interconnects are often reduced to fit available space. This increases resistance and heat under load.

In this design, increased enclosure height allows:

• proper conductor sizing
• improved current distribution
• reduced voltage drop

without sacrificing parallel count or insulation.

System Impact

Interconnect design directly affects:

• voltage sag under load
• thermal behavior
• current balance across parallel groups

Poor interconnect design results in:

• localized heating
• uneven current distribution
• reduced usable power

At this level, the limitation is no longer the cell.

It is how effectively current can move between cells and out of the pack.

Positive and Negative Paths

Current does not distribute evenly by default.

It follows the lowest resistance path.

Both positive and negative paths must be treated as identical systems:

• equal cross-section
• similar length
• consistent material

Imbalance creates:

• uneven current distribution
• localized heating
• asymmetric voltage drop

These effects compound under sustained load.

Path design determines whether parallel groups actually share load evenly.

Insulation Stack

Insulation occupies fixed and limited volume within the enclosure.

• fish paper
• insulating rings
• kapton
• structural wrapping

This volume is not optional and cannot be reduced without consequence.

Insulation competes directly with:

• conductor cross-section
• wiring paths
• thermal spacing

Reducing insulation to gain space increases:

• short risk under vibration or compression
• localized heating due to reduced separation
• long-term degradation from material breakdown

Increasing insulation improves safety margins, but reduces available space for:

• current paths
• parallel group layout
• heat dissipation

Insulation also affects how heat is retained and distributed within the pack. Tighter insulation and reduced spacing increase thermal coupling between cells and interconnects.

At high current, this accelerates temperature rise and amplifies resistance under load.

Insulation must also maintain integrity under:

• compression
• vibration
• thermal cycling

Failure is often not immediate. It develops over time as materials shift, wear, or degrade.

Insulation is not just a safety layer.

It is a structural, thermal, and spatial constraint that directly affects reliability and performance under load.

Wiring and Output

At 300A class current, wiring is part of the primary resistance path, not a secondary detail.

• 4 AWG preferred
• 6 AWG minimum for short runs

All current leaving the pack must pass through this path.

Resistance is determined by:

• length
• routing geometry
• termination quality
• connection interfaces

Even small increases in resistance at this stage result in:

• measurable voltage drop
• heat generation at exit points
• reduced usable power under load

Wiring is often one of the most constrained parts of the system due to limited space within the enclosure.

Routing decisions directly affect:

• path length
• bend radius
• mechanical stress
• thermal concentration

Poor routing introduces:

• unnecessary resistance through longer paths
• localized heating at bends and terminations
• fatigue at connection points under vibration

Terminations are critical failure points.

Poor crimps or connections increase resistance and create localized hotspots that worsen under sustained load.

Wiring must be designed as part of the current path from the beginning.

If wiring is treated as an afterthought, it becomes a bottleneck regardless of cell or interconnect quality.

Connectors

The connector is the final restriction in the discharge path.

All current leaving the pack must pass through this interface.

At this level:

• QS9 and QS10 class connectors are appropriate for high-current applications

Connector selection directly affects:

• voltage drop
• heat generation at the interface
• sustained current capability

Connector limitations include:

• contact resistance
• contact surface area
• internal conductor size
• thermal behavior under sustained load

Even with proper internal design, a connector with insufficient capacity will:

• introduce additional resistance
• generate heat at the output
• limit usable power under load

Dual Output Strategy

Single connectors concentrate all current through one interface.

At higher current levels, this increases:

• contact resistance load
• thermal concentration
• risk of connector heating and degradation

Dual output configurations distribute current across two paths:

• reduced current per connector
• lower interface resistance per path
• improved thermal behavior

This can be implemented as:

• dual connectors
• split output leads

However, this introduces additional complexity:

• balanced current distribution between paths
• increased wiring and routing requirements
• more connection points (additional failure surfaces)

Placement and System Impact

Connector placement must be considered as part of the enclosure layout.

It competes for space with:

• wiring paths
• busbars
• structural elements

Poor placement increases:

• wiring length
• routing complexity
• resistance and heat

At this level, the connector is not just an interface.

It is part of the current path and can become the system bottleneck if not properly sized and integrated.

BMS Volume and Placement

The BMS is part of the current path, not just a control system.

All discharge current flows through it.

This makes the BMS one of the most critical sources of:

• resistance
• voltage drop
• heat generation

A 400A-class BMS does not guarantee stable operation at 300A.

Real performance is limited by:

• internal trace and bus resistance
• MOSFET characteristics
• thermal handling under sustained load

The BMS introduces a concentrated point of resistance within the system.

Volume and Integration

A high-current BMS requires space for:

• current-carrying paths
• balance leads
• main leads
• insulation clearance

This volume competes directly with:

• cell packing
• busbar routing
• wiring paths

If insufficient space is allocated, compromises occur in:

• wire routing
• conductor size
• thermal exposure

The BMS must be designed into the layout from the beginning.

Placement and Routing Impact

BMS placement directly affects:

• wiring length
• path resistance
• thermal behavior

Poor placement results in:

• longer current paths
• increased resistance
• additional heat under load

Placing the BMS in high-heat zones compounds the problem:

• elevated temperatures increase resistance
• thermal throttling may occur
• failure risk increases over time

System Limitation

Even with strong cells and low-resistance interconnects, the BMS can become the primary bottleneck.

Common failure behaviors include:

• voltage drop across the BMS under load
• heat-induced performance loss
• sudden cutoff at sustained current

At this level, the question is not the BMS rating.

It is whether the BMS can sustain current without becoming the highest-resistance component in the system.

Mechanical Stability

Mechanical stability directly affects electrical performance.

Movement under load causes:

• weld fatigue
• increasing resistance
• long-term degradation

Controlled compression improves:

• contact reliability
• structural integrity

The enclosure acts as a structural component, not just a shell.

A mechanically unstable pack becomes electrically unstable over time.

Thermal Behavior

Heat is generated wherever resistance exists:

• cells
• interconnects
• BMS
• wiring
• connectors

Higher density increases:

• heat retention
• thermal coupling
• risk of localized saturation

Thermal behavior is a function of:

• resistance distribution
• material selection
• packing density

A pack that cannot dissipate heat cannot sustain high current.

Temperature rise increases resistance, which increases heat.

This feedback loop defines real performance limits.

System Constraint

Within the enclosure, nothing is free.

• every material consumes space
• every connection adds resistance
• every path generates heat

These are not independent factors. They compound under load.

The pack does not perform based on its specifications.

It performs based on:

• total system resistance
• how evenly current is distributed
• how effectively heat is managed within that volume

As current increases, small inefficiencies scale into:

• measurable voltage drop
• localized heating
• reduced usable power

The enclosure defines the boundary within which all of these effects interact.

You cannot optimize one dimension without impacting the others.

Increasing capacity affects:

• available space for conductors
• thermal behavior
• current distribution

Improving current paths affects:

• available space for cells
• packing density

Managing heat affects:

• spacing
• material choices
• structural design

Every decision inside the enclosure is a tradeoff between:

• resistance
• heat
• space

The pack performs as well as the balance between these constraints, not the maximum of any single one.

Final Advice

At this level, battery design is not defined by voltage, capacity, or cell selection.

Those are starting points.

Performance is defined by what happens under load:

• how current flows
• where resistance is introduced
• how heat accumulates
• how the system behaves over time

Tabless cells increase potential, but they do not remove constraints.

They shift the limitation away from the cell and into the system.

Two packs with identical specifications can behave completely differently because:

• resistance is distributed differently
• current paths are not equivalent
• thermal behavior diverges under load

At high current, performance is not defined by peak capability.

It is defined by what the system can sustain.

Small inefficiencies scale into system-level limitations:

• resistance becomes voltage drop
• voltage drop becomes heat
• heat reduces usable performance

The system does not fail instantly.

It degrades under load.

And that degradation defines real performance.

The pack behaves as a single system, limited by its weakest path.

Design is the process of deciding where those limits occur.

You cannot eliminate constraints.

You can only control:

• where resistance exists
• where heat accumulates
• how the system reaches its limits

Performance is not a specification.

It is the result of those decisions.

A battery is not defined by what it can do once. It is defined by what it can continue to do under load.