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Old 11-09-2022, 03:14 PM   #4
Pat911
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Join Date: Dec 2020
Location: Melbourne, Australia
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Default Re: Lithium batter voltages different in parallel.

Hi Silenttrouble,

2A is an awfully low discharge current for 16 270Ah cells in parallel, no matter what the SOC is. You didn't answer my question but I'll assume you've connected the cells using wire and not bus bars. If that is the case, it would explain the differences in voltages when disconnected as the wire would have had significant resistance and balancing would be ineffective. You should also connect the discharger to the positive at one end on the chain and the negative to the cell at the other end of the chain to better equalise the current between cells. If connected to a single cell at one end then that cell will discharge first and the most furthest cell last.

In your scenario, I really think you should change your strategy to top balancing. Cells are never equal and balancing puts the cells into a state where they either get to 0% SOC at the same time (bottom balancing) or 100% SOC at the same time (Top balancing), it can never be both. Even when balanced, cells will drift and it's the job of the balance function in the BMS or external balancer that will keep it in check. Most will work on a top balance strategy.

What to do from here will depend on what you have available to you and how much time you have. The best way to do it would be to connect them in parallel using bus bars, or at least VERY thick cable, keeping it as short as possible. Get a 10A (or greater) laboratory style power supply, set it to 3.65v and start charging. Since your pack is quite depleted, It will take at least 18 days to charge at 10A but I would leave it like that for at least 30 days.

What BMS do you have as you could also get it to do the top balance for you but it may take even longer than that.
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