Blog - Tests and diagnosis

FAQ: The discharge level of the 12V batteries with PCM

** Question: I keep discharging the 12V battery with PCM. But when the voltage gets to some 11V, the voltage suddenly drops to 0V. Is the battery defective?

No, the battery is not defective. The PCM board in side this type of battery will disconnect the output when the voltage level of the cells inside the battery is getting too low. That is why the voltage will change to 0V – the battery protection will disconnect the battery.

Note: we strongly recommend not using the PCM protection as a last resort to stop the equipment from draining the battery. Follow the suggestions at this article.

** Question: When I discharge the 12V PCM battery, the voltage will drop to 0V. After that the voltage stays at 0V. I need to disconnect the battery from my equipment to get the battery to work again. This is annoying. What should I do?

The LiFePO4 battery has the protective circuit – called the PCM. The PCM will physically disconnect the battery from the load to prevent the discharging. In order to release the PCM back to ordinary function, the load must be disconnected completely. (Note: in some cases the release may take some time, until the voltage of the cells increases to safe level again.)

If you keep draining the battery, the voltage will stay at 0V (disconnected) until the equipment is turned off or disconnected. This is a proper function of the PCM battery. If you have some equipment that keep draining the battery, we suggest: A) install a mechanical switch to disconnect the battery from this equipment B) start charging the battery BEFORE the battery is fully depleted.

** Question: I discharge the 12V battery with PCM. Some of the batteries will discharge till 10.5V some to 11.2V. The label on the battery says 10V. What is wrong?

As long as the battery has the nominal capacity, nothing is wrong. The battery consists of 4 cells inside the package. Each of the cells is monitored by the PCM. The nominal voltage of the low voltage disconnect is at 2.5V per cell. This means: in case all of the cells will be discharged identically, the disconnect voltage will be at 4x 2.5V = 10V. However the cells are seldom discharged at the same level. One of the cells will always be discharged earlier. In such a situation the voltage level of the total pack will be higher. For example: 2.5V + 2.8V + 2.8V+ 2.9V = 11V. However the difference may be even bigger: 2.5V + 3.0V + 3.0V+ 3.1V = 11.6V. This is not a fault of the battery; this is the normal function of the PCM board.

In addition to the various voltage levels of the cell, the tolerance of the PCM board is about 5%. This means the low voltage-disconnect level may be around 2.5V to 2.62 V per cell. This again can cause some difference of the results from different units of the battery with PCM.

Note: in earlier deliveries of the cells the low voltage was set at 2.0V per cell, making the disconnection level as low as 8V. Based on our long term tests the setting was changed to 2.5V per cell, making the level at 10V (with tolerance between 10V to 11.6V)