Voltage sag is the drop in battery voltage when a load is applied. Some sag is expected. The useful question is how much it changes with load, temperature, charge level, and battery condition.
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:
| System | Full Charge | Low Sag | Moderate Sag | Severe Sag |
|---|---|---|---|---|
| 72V (20s) | 84V | ~80-81V | ~76-79V | <75V |
| 84V (23s) | 96.6V | ~92-94V | ~88-91V | <87V |
| 96V (26s) | 109.2V | ~104-106V | ~100-103V | <99V |
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.