Fully discharged lipo battery

Hello guys!
Two months ago I bought two (3s, 11.1 v) lipo batteries, and didn’t use them for flights yet, I just used them for testing motors rotating direction. and I have made sure that the voltage is nominal (3.6-3.8v/cell). however, today I found that one of the batteries is fully discharged! even though I didn’t use it since last charge check, but I left the battery connected to PRM module with open end. I don’t think that this may cause the problem, but this is the only difference between the conditions I left the two batteries in! Also, I think that the battery is swollen a bit. can any one help me with the probable causes? I’m still newbie.

Update: I tried to charge then measure the voltage of the battery, I found out that two cells are charged (3.7), and one is damaged.

It sounds like the battery has been discharged and damaged. I’m not sure why that happened. Storing them at 3.8v/cell should have been fine. I don’t know what a PRM module is and whether that had an influence. If it’s open loop it shouldn’t, as you wrote.

If it’s swollen, it’s likely damaged and should be properly disposed of. Charging at that point can be dangerous, so I wouldn’t recommend it.

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If you have two batteries exactly the same and one looks swallow.
Can you take same images of both side by side.
Also measure the voltage on both batteries
A BMS within the battery will not allow the battery to be charged to prevent damage or potential thermal runaway.
How if the battery is over discharged the following procedure may help.
Using a fully charged battery like for like same voltage
First measure the over discharged battery voltage
Connect fully charged battery to the over discharged battery for at least one minute
If the voltage, that’s rested voltage or static voltage has increased
Try using the charger again
If it starts to charge
System pass
If it does not charge
Check your warranty

The images show the puffy battery beside the normal one.
can I still follow your procedure?

The swollen battery looks precarious therefore I would not proceed for your safety.

On a positive note, you have probably helped a lot of people by showing the difference between a swollen battery and a normal battery as a comparator.

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