When there is a high demand for power, all batteries experience a phenomenon called voltage sag.

What is Voltage Sag?


Voltage sag in a lithium ebike battery is a temporary drop in the battery's voltage during periods of high electrical demand, such as when accelerating or climbing steep hills. This phenomenon is common in lithium-ion batteries and is primarily caused by the internal resistance of the battery cells.


Here's a more detailed explanation of voltage sag in a lithium ebike battery:


  • Internal Resistance: Every battery has internal resistance, which is a measure of how much the battery's voltage drops when a current flows through it. In lithium-ion, the internal resistance is relatively low, but it still exists. When you draw a large amount of current from the battery, such as when accelerating quickly or going uphill, the internal resistance causes a voltage drop.
  • Ohm's Law: According to Ohm's Law (V = I * R), where V is voltage, I is current, and R is resistance, a higher current (I) results in a higher voltage drop (V) across the internal resistance (R). This voltage drop is what we refer to as voltage sag.
  • Impact on Performance: Voltage sag can affect the performance of your e-bike. When the battery's voltage sags significantly, the power output of the electric motor may decrease temporarily. This can result in reduced acceleration, lower top speed, and decreased overall performance of the ebike.
  • Recovery: The good news is that voltage sag is usually temporary. Once the high current demand subsides (e.g., when you stop accelerating or reach a level road), the battery's voltage recovers to its nominal level.
  • Battery Management Systems (BMS): Many ebike batteries are equipped with Battery Management Systems (BMS) that help monitor and manage the battery's performance. The BMS can limit the discharge current to prevent excessive voltage sag, which can prolong the battery's lifespan and ensure safer operation.
  • Battery Capacity: Voltage sag can be more pronounced in batteries with lower capacity or those that are heavily used and have aged. A well-maintained and properly charged battery will typically exhibit less voltage sag than one that is heavily depleted or damaged.


To mitigate voltage sag and maintain optimal ebike performance, it's essential to maintain your battery properly, charge it regularly, and avoid drawing excessive current for extended periods. Understanding how voltage sag works can help you plan your rides and manage your e-bike's battery more effectively.



Signs of Voltage Sag


The extent of the voltage drop, or sag, depends on the amount of power drawn from the battery and sent to the motor. When the motor draws power from a battery, the voltage displayed on the battery readout will be lower than the actual voltage. Even after the power draw stops, the voltage may remain low for a while. If you come to a stop at a red light, the voltage may rise slightly within a minute or so. 


When there is a high demand for power from a battery, it becomes difficult to read the remaining voltage accurately. However, once the high-power draw ceases, the voltage will adjust slightly and increase. Even if you continue using the battery but switch from high power usage to low power usage, such as from PAS 5 (600-800W) to PAS 1 (250W), the voltage will gradually increase/recover over time. If you let the bike sit for a few minutes after coming to a stop, the voltage will be more accurate.


If the voltage of your ebike's battery drops too low, the controller will stop sending power to the motor until the voltage returns to normal. This is more common with smaller batteries, especially if you frequently use the full range of your battery and use high PAS levels. However, with larger batteries, voltage sags are less likely to occur unless the battery is at a low state of charge.


If this happens while you are riding and your battery is low, it is recommended to switch to PAS 1 and continue riding. In case your bike powers down even in PAS 1, it is best not to turn it on again. Instead, return home and charge the battery immediately.


Here is a 3rd party take on voltage sag from the Electric Bike Review (EBR) forums: 

(EBR) Voltage sag explained