Testing the battery
is it necessary?
The lithium-ion battery requires performance checking right after construction and from time to time during operation.
Why should you test
The characteristics of li-ion cell operation and their ease of damage require adequate battery protection, e.g. by using a BMS (Battery Management System). This device, like any other, can break down or have a factory defect that can damage the battery or cause a fire! Therefore, it is necessary to check the operation of the basic security functions.
What to check?
Lithium-ion cells don’t like:
overcharge – above 4.25V
excessive discharge – below 2.5V (3V is used because very little capacity remains between 3V and 2.5V, and it is safer)
excessive temperature – above 65 degrees Celsius
In addition, it’s worth knowing what capacity our battery has. As you know, over time and subsequent cycles of charging and discharging our battery will have a smaller capacity, and at some point it may turn out to be insufficient.
How to do it the easiest way?
On this blog I deal with the issue of electric bicycles, which is why I will be referring to them. The battery can be tested directly in the bike with proper precautions and frequent voltage control with a meter. During charging, we check the voltage of each section on the BMS plug and make sure that none of them overcharges above 4.25V. Finally, BMS must disconnect the charger when one of the sections reaches a dangerously high level of charge
Next, while driving, every few kilometers we stop and check the voltage on each section and when we reach the level of approx. 3.3V / section, we compact the measurement frequency to check if BMS cuts off the power in the area of 3V / section.
You can also try to remove one of the wires from the BMS plug. This simulates a sudden drop in voltage on the battery section and at this time the voltage should not go to the motor controller.
Most BMSs have thermal protection, which should be attached directly to the battery cell. If this is not the case, we can add our own thermostatic fuse and plug it into, for example, the controller’s e-lock or brake circuit. After exceeding the maximum temperature, the fuse will cut off the controller power supply, and thus the current consumption from the cells, and thus prevent further heating by discharging.
How do I test the battery?
Unfortunately, testing while cycling takes a long time, requires our constant commitment and is unpleasant. You can test one battery like that, but when we make more bikes / batteries, we must go a step further. I decided to build my own discharg station. Thanks to this, I can test the battery without leaving my home. In addition, the discharger is useful when you want to discharge the battery for some purpose, and do not want to go cycling (e.g. to store the bike during the winter or when we perform maintenance charging to level the battery section).
The discharger controller saves on the SD card the measurement results from which a discharge graph can be generated. It is possible to monitor the temperature of four sensors, and for each one you can set a different discharge disconnection threshold. The testing process is also interrupted when the packet reaches a voltage below the number of sections * 2.5V (BMS should cut off the battery earlier).
I built my discharger based on:
- Arduino Nano micro-controller with display and SD card modules
- 30A ACS712 current sensor
- power resistors 50W 4.7Ohm (25 pcs 5s5p) total 1250W
- switch on MOSFET (currently I have used IRF 3205, but its max voltage is 55V, therefore it is suitable for max 13S package. I plan to change the transistor to be able to test packages above 13S. I will update this description when I find the right transistor and test it)
The construction cost was about PLN 200 (~$50).
I have a schematic diagram, PCB design and arduino code, but it is all in a very early beta version, so for now I will not put them here.
Below is a video in which I show the process of testing the battery, using a discharger.
Stay tuned 😊 :