Charging low voltage cells is risky. Internal damage may not be apparent. Take appropriate precautions such as charging in a well ventilated area away from anything flammable. If you do try this procedure, you do so at your own risk.
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I have taken stored cells from as low as 0.4V and fully recovered them, as long as they weren't puffy and there was no visible damage or deformation compared to good cells. If you charge them normally, they will be destroyed. If the cell is 0V, it is hopeless.
I recently experimented with several cells with starting voltages like 0.3, 0.4, 0.5, and 1.7V. I started with a 40 Ah cell that was at 1.7V and a 5A charge was way too much for it and made it go puffy. So on a whim, I tried 1/ C rate. It's probably possible to charge a little faster than my 1/ C rate recommendation, but I ran out of low voltage cells to play with.
The trick is to start out at a super low charge current. I suggest 1/ C rate. For example, if your cell is 100Ah, charge at 50mA until it's over 2.5V. Then from 2.5V to 3.2V, charge at 1/200 C rate until 3.2V. That's 0.5A in this 100Ah cell example. From 3.2V until 3.4V, charge at 1/20 C rate (5A in this example for 100Ah cells).
I've seen other posts here with much higher current recommendations. While still lower than normal, they weren't low enough for extreme cases like this. So I thought I'd add my two cents.
In one case, I recovered an 8-cell (8S1P) 100Ah battery where the total pack voltage was 3.7V measured directly on the battery (yes, bypassing the BMS). Each cell was around 0.5V. I actually started by charging the entire pack at 40 mA. After the above recovery process, I ran a 40A constant current discharge/charge cycle on the tester and it did the full 100Ah on both discharge and charge. That was a full recovery! If my friends from work read this, they'll know who I am on this forum. Don't tell the boss!
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