CTY2 & 72V: Battery Problems and Repair

Introduction

This post focuses on diagnosing real-world failure patterns in 72V and 60V ONYX battery systems, including 23Ah, 41Ah, and 45Ah configurations. It covers how to interpret voltage behavior, identify imbalance, understand BMS responses, and determine whether a battery can be corrected through balancing or requires physical repair.

Quick Diagnostics

Shared 23Ah and 41Ah Quick Diagnostics

23Ah Quick Diagnostics

41Ah Quick Diagnostics

System Overview

For the ONYX 45Ah and any battery with a QS8 connector, you do not need to disable the discharge port. For batteries with an SB50 lead, you need to disable the discharge port using the BMS app first.

Diagnostic Workflow

Step 1: Identify the Battery

Start by identifying the exact battery:

• RCR 72V 23Ah
• RCR 72V 41Ah
• RCR 72V 45Ah
• CTY2 60V 23Ah

This matters because the diagnostic path is not the same.

A 23Ah balancing problem is not handled like a 41Ah balancing problem. A 41Ah BMS wire fault is not the same as a 45Ah delta warning. A QS8 battery does not behave the same as an SB50 battery in the app.

Step 2: Check the Basic Failure Pattern

Watch for these ONYX-specific patterns:

• cuts out only under hard throttle
• cuts out on hills
• cuts out at relatively high pack voltage
• looks charged but feels weak
• top speed is suddenly lower
• one group drops faster than the rest
• one group reads impossibly high or impossibly low
• battery gets hotter than normal
• voltage sag is much worse than before

These patterns tell you whether the problem is:

• imbalance
• internal resistance
• BMS settings
• a bad balance lead or solder joint
• end-of-life cells

Step 3: Check Voltage, Delta, and Temperature

On batteries with BMS telemetry, check:

• pack voltage
• individual cell group voltages
• cell voltage delta
• battery temperature
• discharge behavior

Use voltage as your primary state-of-charge reference.

For ONYX 41Ah and 45Ah batteries using BMS apps, cell voltage delta is a key diagnostic metric:

• ~0.005V = optimal
• ≤0.030V = healthy

• 0.050V = imbalance present

• ≥0.300V = severe imbalance

Step 4: Test Under Load

A battery can look decent at rest and still fail on the road.

Test under throttle and watch for:

• sharp sag
• weak acceleration
• sudden cutoff
• one group collapsing faster than the rest
• instant voltage rebound after cutoff

If the battery turns back on immediately after you let off the throttle, that usually means the BMS protected the weakest group. It does not mean the whole pack is actually empty.

ONYX 23Ah Diagnostics and Repair

23Ah Aging Limitations

The ONYX 23Ah battery packs are now in an advanced stage of aging and no longer deliver original capacity or peak output.

Most riders will see:

• reduced range
• weaker acceleration
• higher voltage sag under load
• noticeably higher battery temperatures
• reduced tolerance for aggressive current draw

Using Kelly SICKO MODE or FarDriver JAWS MODE can cause permanent cell damage and irreversible capacity loss.

Safe 23Ah Current Limits

• DC Current Amps: 40A-50A
• AC Phase Current: 120A-150A

ONYX 41Ah Diagnostics and Repair

41Ah Aging and Load Limits

The ONYX 41Ah battery pack has reached an age where reduced performance is expected.

Riders will typically see:

• shorter range
• weaker acceleration
• heavier voltage sag under load
• higher battery temperatures during aggressive riding

Safe limits:

• DC Current Amps: 80A to 90A

• AC Phase Current: 180A to 240A

• never tune a Fardriver controller to pull more than 100A from the ONYX 41Ah battery

• pushing it to 200A overwhelms the battery

41Ah BMS App Diagnostics

The 41Ah battery uses:

• Overkill Solar BMS
• Xiaoxiang BMS

The apps provide:

• pack voltage
• cell voltage delta
• cell group behavior
• battery temperature
• cycle and capacity fields
• protection behavior

This is where most 41Ah diagnostics start.

41Ah BMS Configuration Checks

• Total Battery Capacity: 41000
• Total Cycle Capacity: 32800

For protection values:

• 80A BMS

• Charge Over Current: 90000
• Discharge Over Current: 90000

• 100A BMS:

• Charge Over Current: 110000
• Discharge Over Current: 110000

If these values are incorrect, the battery may show:

• inaccurate charge readings
• early cutoffs
• behavior that appears to be a cell issue but is actually a BMS configuration problem

41Ah False Reading Diagnostic

If one cell group shows something like 4.166V while the other groups are around 3.8V, that large discrepancy is very unlikely to be real.

A cracked solder joint or a detached BMS balance wire is likely causing the false reading.

That means:

• this is not a balancing issue
• this is a physical repair case

Repairing the battery involves reattaching the balance wire, which requires opening the battery, fixing the connection, and reassembling it securely.

41Ah Hard Failure Threshold

• if a cell group consistently reads below 2.5V, the battery is damaged

This is not a balancing issue.

45Ah Battery Behavior

The 45Ah battery is ONYX’s modern battery architecture, with improvements in:

• connector design
• BMS capability
• current handling
• thermal stability
• long-term durability

It uses the SuperPower smart BMS and provides visibility into:

• pack voltage
• individual cell group voltages
• cell voltage delta
• current
• temperature

High imbalance on the 45Ah leads to:

• reduced capacity
• early cutoffs
• increased heat
• accelerated aging

Voltage, Sag, and State of Charge

41Ah Voltage Reference

23Ah Low Voltage Behavior

• keep voltage above 69V (20%) to reduce sag and prevent cutoffs
• a weak or unbalanced 23Ah may cut out around 74V, well before normal low-voltage behavior

Pack behavior matters more than the voltage number alone.

Sag Interpretation

• voltage sag reflects internal resistance and overall pack health
• excessive sag creates heat, wastes power, and damages cells

On an ONYX, sag diagnosis is practical:

• more sag than usual = battery aging
• severe sag under hard throttle = weak group or elevated resistance
• sag plus heat = stop pushing the pack
• sag that keeps getting worse = the battery is aging out or has a repair issue

Temperature and Safety Diagnostics

• 120°F (49°C) is the practical stress threshold to watch
• BMS units shut the battery down at 140°F (60°C)
• the battery typically needs to cool back to 135°F (57.2°C) before discharge resumes
• on hot days above 90°F (32°C), allow the battery to cool below 120°F (49°C) before continued use and into the recommended charging range before charging

Temperature behavior separates into discharge and charging conditions:

Discharge Temperature

Keep discharge temperature below 120°F (49°C) when possible.

Charge Temperature

• charge recommended: 40°F-105°F (4°C-40.5°C)
• charge limits: 32°F-113°F (0°C-45°C)
• do not fast charge a battery that is already hot internally

Temperature is diagnostic data. A battery that heats faster than it used to typically indicates rising internal resistance, excessive current demand, or both.

Hardware and False Positives

Not every battery issue is actually a cell issue.

Check:

• discharge connector condition
• charge lead condition
• balance lead condition
• BMS app configuration
• discharge enable behavior on SB50 systems
• controller current settings

A false high cell group reading can be caused by:

• cracked solder joint
• detached BMS balance wire

This is a repair issue, not proof that one cell group is actually higher than the rest.

Repair and Replacement Decisions

When It Is Still a Balancing Case

Treat it as balancing first when:

• 23Ah full voltage is low but the pack still behaves consistently
• 41Ah delta is elevated but still in the recoverable range
• battery response improves after balancing cycles
• no impossible cell readings are present

When It Has Become a Repair Case

Treat it as repair when:

• one group shows an obviously false voltage
• the same group keeps collapsing under load
• the battery cuts out even after proper balancing attempts
• a 41Ah group consistently reads below 2.5V
• the battery overheats too quickly
• BMS settings and balance attempts do not solve the problem

When It Has Become a Replacement Case

Treat it as replacement when:

• the pack has multiple failing groups
• balancing no longer holds
• capacity loss is severe
• current capability is no longer safe for the intended use
• the battery is at end of life and repair cost no longer makes sense

Many ONYX 23Ah and 41Ah batteries are aging out and will soon need to be replaced and disposed of properly.

Professional Battery Repair

ONYX Battery Repair

• Northeast Battery Systems

Northeast Battery Systems specializes in ONYX battery repair, diagnostics, and rebuilds for both stock and performance setups. They provide:

• battery diagnostics and testing
• cell group replacement
• balance lead repair
• full battery rebuild services
• BMS replacement and configuration
• custom battery builds

They work directly with riders to diagnose issues and recommend the correct repair path based on battery condition.

NEBS Contact Information

• Address: 200 Old County Circle, Unit 307, Windsor Locks, CT 06096
• Email: nebattsys@gmail.com
• Phone: (860) 951-1468

ONYX Batteries by NEBS

• Expert Consultations
• High Performance Battery Builds
• Battery Rebuild Services
• Durable Metal Battery Enclosures
• Cell Balancing Services
• Balance Lead Repairs
• Battery Health Diagnostics
• Performance Testing & Validation
• BMS (Battery Management System) Swaps
• 18 Month Warranty

Final Advice

Use the correct diagnostic path for the battery you actually have.

For the 23Ah:

• treat charger type and balancing behavior seriously
• remember that green charger light does not mean balancing is finished
• use outlet wattage to confirm real balancing behavior
• watch for the 81.6V-82.8V fake-full pattern
• watch for premature cutoff around 74V

For the 41Ah:

• use the BMS app
• check delta first
• correct BMS calibration if percentage or Ah readings are inaccurate
• follow the full unbalanced fix procedure exactly
• revert settings after correction
• know the difference between an imbalance problem and a balance-wire or solder-joint repair problem

For the 45Ah:

• use the SuperPower BMS app regularly
• monitor delta and temperature
• diagnose from real data before assuming the battery is bad

And when the problem has moved beyond balancing, move to a proper repair path.

Detailed Guides

• 72V: 23Ah Battery Balancing

• 72V: 41Ah BMS Calibration

• 72V: 41Ah Battery Balancing and Recovery