# Measuring Discharge Rate of Li-ion cell

I am attempting to change the temperature that a 3.7 V rechargeable Li-ion cell is discharged in and am seeing how this affects the capacity and discharge rate of the cell. My setup currently uses an Ammeter and Voltmeter to log the data over time. However, from my research, I have found that I must use a constant-power load or a constant-current load for this setup. Since I do not have access to these, is there no way that I can find a discharge rate with what I have?

If not, what can I find out/compare at different temperatures with the data provided?

• Presumably you are using a resistive load. Doing so will mean that the discharge current varies as a function of voltage, but the variation over the useful discharge voltages is not so extreme as to create an invalid experiment. It simply means that you must base you calculations on the actual voltage and current at a given point in time, not a set point current. You will, however, need some sort of cut-off to stop the discharge before you damage the cell. Sep 24, 2020 at 15:27
• Maybe you have a buck (or boost) convertor spare. If you feed its output into a resistor, it will act as a constant power load for as long as the convertor can keep the output voltage constant. Don't discharge the LiPo cell too far. Maybe add a fan to cool the resistor and convertor. Sep 24, 2020 at 16:07

First, the Li-ion cell doesn't have a "discharge rate", so there is nothing to measure there. The load sets the current (at a given voltage), not the Li-ion cell.

Second, while it's true that laboratory test equipment tests at a constant current or constant power, nothing says that you have to. Even a constant-resistance load will work fine because the voltage of a Li-ion cell is pretty flat over State of Charge, so the current will be nearly constant. As long as you're logging the current throughout the discharge cycle, the integral of that current over the entire discharge cycle is the cell effective capacity at that load.

Third, make sure that you use a protector BMS with that cell.

Your tests will result in a table of effective capacity vs temperature and vs load current.

1. Get a few different power resistors to test at various levels of current. For example, for a 0.5 C current, 1 C current, and 2 C current.
2. Place the cell in the temperature chamber
3. Set the at temperature chamber the one of the test temperatures
4. Fully charge the cell, until the current drops below 0.1 C
5. Wait 1 hour for the cell temperature to settle
6. Connect one resistor to the cell
7. Log the current every 1 minute
8. Do so until the BMS shuts down the current
9. Integrate the current to get the capacity at that load and that temperature
10. Write it in the table of results
11. Change the resistor, and go back to #4
12. If no more resistors, go back to #3

What you need is a "constant current sink". It's a fairly straightforward circuit to build out of a transistor, an op-amp, and a couple of resistors:

https://circuitdigest.com/electronic-circuits/voltage-controlled-current-source-circuit-using-op-amp

https://circuitdigest.com/electronic-circuits/design-a-simple-constant-current-sink-circuit-using-op-amp

Good luck.