Why does the battery voltage drop when you accelerate: due to internal resistance
When you accelerate, the battery voltage drops, then rises again as soon as you release the throttle. This phenomenon, known as voltage sag, is caused by the internal resistance of the cells and the connections. The higher the current, the greater the voltage drop during acceleration, and the more the battery heats up. This is normal to a certain extent. It becomes a problem when the voltage drop causes a loss of power, triggers a warning light, or causes the battery to shut down. Here's where it comes from, what makes it worse, and how to reduce it.
The essentials
- Voltage drop = current × internal resistance: doubling the current doubles the voltage drop.
- The heat generated increases with the square of the current: doubling the current multiplies it by four.
- The internal resistance increases with cold temperatures, low battery charge, and aging.
- A fall that is too forceful can cause a loss of power, followed by the BMS or controller shutting down.
- More cells in parallel, along with power cells and good connections, will reduce the voltage drop.
The mechanism: internal resistance
A battery is not a perfect voltage source. The cells, nickel strips, welds, cables, and connectors all offer a small amount of resistance to the flow of current. This resistance "consumes" a portion of the voltage as soon as current flows, according to Ohm's law:
`Voltage under load = No-load voltage - Current x Internal resistance`
Here's an example calculation, using a chosen value for illustrative purposes: If the total resistance of a battery were 0.1 ohms, a current of 100 amps would cause its voltage to drop by 10 volts during acceleration. And this resistance would dissipate 100 × 100 × 0.1 = 1000 watts of heat inside the battery during that time.
In a battery, the resistance of a single cell is multiplied by the number of cells in series and then divided by the number in parallel. This is why a battery with many cells in parallel maintains its voltage much more effectively.
What increases the voltage drop
| Factor | Why? |
|---|---|
| High current | The drop in performance is proportional to the current: controller reset, dual motor configuration, starting on an incline |
| Cold | The chemical processes within the cells are slowing down, and their internal resistance is increasing significantly |
| Low battery | The no-load voltage is lower, and the resistance increases towards the end of the discharge cycle |
| Aging | The internal resistance of the cells increases with the number of charge cycles and over time |
| "Energy" cells | Designed for capacity, they have a higher resistance than power cells |
| Worn connections | A corroded or loose connector, or a cracked solder joint: these all create additional resistance, which causes localized heating |
The impact on the scooter
Melting power
The available power is the product of voltage and current. If the voltage drops during acceleration, the controller can no longer deliver the set power, especially when going uphill or with a low battery. The scooter will feel "weak."
A gauge that's spiking
Many displays estimate the remaining battery charge based on voltage. With each acceleration, the gauge drops, then rises when you release the throttle. It's not the battery that's suddenly draining; it's the voltage that drops under the strain.
Power cuts
When the voltage of a group of cells drops below the BMS's low voltage threshold, the BMS cuts off the battery to protect it: everything shuts down. The controller can also reduce power or cut off if it detects a voltage that is too low. This scenario is detailed in our guide Battery and BMS that cut off when accelerating. In all cases, it is the weakest group of cells that determines the outcome: a single group of cells that is weakened is enough to cause the entire battery to shut down.
Heat
The heat generated increases with the square of the current. A battery that experiences a significant voltage drop gets very hot, and this heat accelerates the aging of the cells, which further increases their resistance. This creates a vicious cycle.
How to reduce voltage drop
- `Matching the controller to the battery: Limit the battery current to what the cells can comfortably handle.`
- Riding with a tempered battery: In winter, a battery stored in a warm environment maintains its voltage much better.
- Check the connections: A blackened or loose connector can cause a voltage drop and overheating.
- Adding a battery in parallel: An external battery shares the current with the internal battery. Our Performance (S) 72V 15Ah external battery, with its high-discharge cells, is designed for this purpose.
- **Upgrading to a power battery:** High discharge cells, more cells in parallel, generous wiring. Our Performance 80V 40.5Ah battery for Nami Burn-E, using Molicel P45B cells, delivers up to 450A peak compared to the original battery's 98A. The difference is explained in Performance Battery vs. Range.
Measure the sag
A connected BMS (Battery Management System) or a programmable controller allows you to record voltage and current during acceleration. Comparing the no-load voltage to the voltage at the peak of the effort gives you an idea of the battery's internal resistance: divide the voltage drop by the current. Repeat the measurement under the same conditions (charge, temperature) from one season to the next: if the voltage drop increases significantly at the same current, the battery is aging, or a connection is degrading. The interpretation of individual cell readings is detailed in read the technical datasheet for a lithium cell.
Workshop tip
A battery that discharges more quickly than before under the same usage usually indicates a group of cells that are nearing the end of their lifespan or a connection that is overheating. Before replacing the battery, have the voltage of each cell group checked and inspect the condition of the connectors. And if you really want more power, consider our electric scooter batteries Performance line before modifying the controller.
Frequently Asked Questions
Is it normal for the voltage to drop with each acceleration?
Yes, all batteries experience voltage sag under load. What matters is the extent of the sag: if it causes the battery to cut out or significantly lose power, then the battery is undersized, too cold, has a low state of charge, or is nearing the end of its lifespan.
Why does the gauge reading increase when I release the throttle?
Because the gauge reads the voltage, and the voltage increases as soon as the current decreases. The actual remaining charge is read when the system is at rest, after a few moments without accelerating.
Why is the drop-off in performance more pronounced when the battery is low?
At the end of the discharge cycle, the no-load voltage is lower, and the internal resistance of the cells increases. Consequently, the same acceleration will cause the voltage to drop further, often reaching the cutoff threshold.
Does a more powerful controller solve the problem?
No, it actually makes it worse: it demands more current, which increases the voltage drop and heat. The solution lies with the battery.