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First, do not charge the cells to 3.7, especially at that low rate. Stop at 3.55 or 3.6 at most.
Second, the math is the math. As others have pointed out, if you parallel all the cells, 280ahx16=4.480ah at 3.2v nominal. If they were completely discharged, it would take 1,280 hours to recharge at 3.5a. Figure they were at 50%, it would take half that many hours, 640hrs.
When I was doing this, I took a 30a, 12v charger and brought 12v nominal, 4 series, 4 parallel to 3.45v (roughly) per cell for a total of 13.8v, then paralleled them all and charged to 3.55. I hooked up the 12v charger without a BMS and let it go but had a timer on my phone to check it every hour or so and did not let it run overnight. Don’t do that, it’s risky!
Even doing it that way was 280ahX4=720/30=24hrs, though I think it only took about 12hrs, so presumably the cells were at about 50%.
Second, the math is the math. As others have pointed out, if you parallel all the cells, 280ahx16=4.480ah at 3.2v nominal. If they were completely discharged, it would take 1,280 hours to recharge at 3.5a. Figure they were at 50%, it would take half that many hours, 640hrs.
When I was doing this, I took a 30a, 12v charger and brought 12v nominal, 4 series, 4 parallel to 3.45v (roughly) per cell for a total of 13.8v, then paralleled them all and charged to 3.55. I hooked up the 12v charger without a BMS and let it go but had a timer on my phone to check it every hour or so and did not let it run overnight. Don’t do that, it’s risky!
Even doing it that way was 280ahX4=720/30=24hrs, though I think it only took about 12hrs, so presumably the cells were at about 50%.
No need to let them sit paralleled of you’re going to charge them, that’s just a waste of time unless that’s your only balancing strategy.
I have also built some packs from used cells and just stuck them together at whatever state of charge they were at, but used a 4a active balancer and let it balance over time while using it.
I have also built some packs from used cells and just stuck them together at whatever state of charge they were at, but used a 4a active balancer and let it balance over time while using it.
anton
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