Voltage Sag - Metacycle/eBike performance

Voltage Sag - What is it and how does it effect ebike/meta bike performance.

Voltage sag is a term used for the result of loading a battery to its full discharge ability and the resulting drop in nominal voltage the battery can sustain. For example in ebikes, in general terms if running a 36 volt battery using standard 18650 cells will have a full charge of 42 volts.  As you ride the battery drains of electrical energy and the voltage drops to a cut off point for the cells usually around 33 volts to 29 volts. This produces an experience for a user of an average voltage above 36 volts. The unfortunate result of course, is that when you start your ride at 42 volts you will experience voltage going down until you stop your ride. This means the top speed and power at the beginning or the ride is much faster than at the end. When nearing this cutoff point the you will experience a power drop off, loosing climbing ability, speed at full load, and generally degrading performance. The performance degradation can be deduced from a voltage discharge curve often supplied by cell makers and battery pack makers.

As you can see the higher energy consumption results in faster discharge and reaching the voltage sag sooner (blue line at 3c discharge). The sustain rate of the voltage is most effected by the battery systems over all capacity (think volume) and the ability of the cells to handle high discharge rates. Worse yet, as the voltage drops the system will require more amps to put out similar torque at the motor, so you wind up gobbling battery capacity during a serious load on the motor toward the end of the discharge curve (towards the end of your ride).

For instance 40 volts at limit of 10 amps (a small 10 amp controller for instance with a cutoff at 12 amps) yields 400 watts(i) power to the motor which results in a torque value. But as the voltage drops to 37-34 volts under loading and discharge, to maintain the same torque the amp draw from the battery is increasing and is nearing the cut off point for the controller. Once that limit is reached the torque and the feeling of power and acceleration will drop off.


One most straight forward solutions is to carry a larger capacity battery which is more than what you will use on your average trip. What does that mean? It means finding out what your battery requirement is for your average riding trip. This can be hard to figure out on systems that don't offer tools to take the measurements. If you know you milage and what capacity your battery is then thats a start. If you think your battery should be around 500-600 watts, then try one thats 1000 watts just to give you better discharge over the range you project. 

Most batteries in the class for Metacycles and eBikes are rated for 1c discharge or better to give you power on demand and faster charge times. Yes there is the trade off, weight and price, you may feel like you are carrying more than you need and spent more than you want. But here are some of the benefits.

Better performance over the course or trip 
More effective hill climbing
Maintain top speed more efficiently
Better cycle life for the battery (up to 50% more cycles)
Safer regeneration capability, more resiliency for the battery
Less heat generation overall in the system

Next up: How does C rating work in performance for range.

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