For those of us that have played with electric RC hobbies, I think it helps to think of the big LiFePO4 cells as oversized hobby batteries rather that using lead acid batteries as a baseline reference point for charging, storage etc.
I am working on top balancing 16 of these cells at the moment. I’m building them as 2, 24v packs which will initially be wired in parallel driving a 24v, 2000w inverter. At some point in the future, if I feel the need to go to a 3000w inverter, I’ll wire them in series for 48v. When I started, I decided to parallel one set of cells and charge them in parallel. The other set, I wired in series to compare the process both ways.
My resources for charging the batteries include a the cheap Tuffion 30v, 10a power supply that Will semi-recommends as at least being durable
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, an ISDT T8 DC smart charger, and another generic 36V, 27A fixed power supply that I can use to drive the T8 to near full capacity when charging in series. The T8 supports a max current of 30a and up to 8s batteries, with 2.2a balancing current. It also does a nice job of stopping the charge automatically once the set voltage is reached. I would describe it as providing adjustable constant current power, up until just before the target voltage is reached, at which point, it drops the charge rate to avoid overshooting the target voltage.
I’ve been charging the paralleled and series packs in blocks of a few hours at a time since I’m not entirely comfortable leaving the setup unattended when charging. I’ve been charging the packs at 27a to stay 10% below the max rated capability of the T8.
As can be expected, the series pack has made much faster progress since it pushes about 750W into the pack when charging. When I was using the T8 to charge the series pack, I’d use the Tuffion to keep feeding the parellel pack about 9.3 amps.
Today, I got the series pack to the point that the highest SOC cells started to climb quickly, so I stopped charging them as a pack, and used the T8, set to 14a to peak each cell at 3.60v. I then re-seriesed the pack and allowed the T8 to repeak the whole pack to 3.60v, giving it time to top balance the pack using its balancing capability. It took about 40 mins for it to reach a state that it considered balanced and shut off the charging. I then told it to repeak at 3.65, which only took it a few minutes to do. I’m not sure how well balanced I think it really is, but I’m going to capacity test, cycle it a few times and see what it looks like. If the parallel top balanced cells behave more uniformly as a series pack, then I’ll break this one down, wire it up in parallel and rebalance it that way. Right now I am using the series pack as a power supply for the T8 to keep 27a pushing into the paralleled pack, so I get some capacity testing and finish charging the other pack at the same time
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