How To Find Out The Capacity Of A LiFePO4 Battery

By measuring the actual capacity of a new LiFePO4 battery, you can find out if it matches the one declared by the manufacturer. If the battery has already been in operation, then such measurements will help to assess the degree of wear and tear and suitability for further use for its intended purpose.

Safety Precautions When Checking The Capacity Of A LiFePO4 Battery

The process of measuring the capacity of lithium-iron-phosphate batteries is an order of magnitude safer than similar manipulations with the more common Li-ion in everyday life.

First of all, this concerns the control of the temperature regime during charging and discharging with high currents. Some LiFePO4 batteries remain completely safe even when heated to +70°C. However, this applies only to high-quality branded drives. For ordinary low-cost batteries, the average heating threshold is at around + 50 ° C.

However, when measuring the capacitance of LiFePO4 cells, the following safety rules must be observed:

1. The battery charge voltage must not exceed 3.65 V.
2. Do not allow the battery to discharge below 2.5 V.
3. You can charge the cells only at a positive ambient temperature.
4. The charge and discharge current should not exceed the battery under test specified by the manufacturer.
5. In the process of charging and test discharge, it is necessary to control the temperature of the cell.
6. In case of overheating, stop the process immediately.
7. It is strictly forbidden to close the battery contacts without load.

In addition, when checking the capacity of a lithium-iron-phosphate battery with manual devices (including in a self-assembled circuit), the process must be constantly monitored.

Simple Ways To Measure The Capacity Of A LiFePO4 Battery

The easiest way to measure the actual capacitance of a cell is to use a special device designed for this purpose. Such devices “know how” to properly charge and discharge the battery, control the temperature, voltage, time, avoiding critical operating modes.

The capacitance can be measured without such devices, using a multimeter and a load that consumes a current of 1-2 A. The sequence of actions, in this case, will be as follows:

1. The battery must be fully charged to 3.6V.
2. The multimeter enters the voltage measurement mode and is connected in parallel with the battery.
3. The load (2 A) is connected in series, after which the time is recorded.
4. The discharge is performed until the voltage at the battery terminals drops to 2.5 V.
5. The resulting time in hours is multiplied by the load current, and the result will be the approximate actual capacity of the tested LiFePO4 battery.

Example:  A battery under a load that draws 2A was discharged from 3.6V to 2.5V in 2 hours and 30 minutes (2.5 hours). Its actual capacity is 5 Ah, or as manufacturers often indicate 5000 mAh.