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Cells for high capacity, high C-rate? 48V, 250A continuous

aybabtme

New Member
Joined
Apr 10, 2020
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Hi folks!
I've built a 48V pack made of 16x CATL 280Ah cells, with a REC Q BMS. The cells were purchased with Jenny Wu from DocanTech in July. The purpose of the pack is to power 2x6kW outboards to propel my boat. However in low-power tests (3kW) I'm noticing that the cells are showing an impedance of at minimum 0.4mOhm. I suspect that many factors are at play here, one of which is the very minuscule surface area available on the CATL 280Ah cells for the contact, and the soft threads on the bolts that limit the torque that can be applied to the busbars. Since I expect to put 250A of load on this pack, I'm worried that these cells are just not going to work, and I'm starting to wonder if I shouldn't redesign the pack with different cells with larger contacts, or put multiple cells in parallel, or idk.

What other cell (or configuration) would you recommend for a space/weight efficient, high power (12kW continuous) and high density pack (weight sensitive)?

The pack otherwise works well as a house bank at low C rate. I haven't actually had a chance to test it at higher C rates, right now my fear is entirely speculative.

Here's the impedance as seen by the BMS (note: due to space and weigth distribution constraints, the pack is split in two at cell 8 and 9, joined by a 1 meter 2/0 cable, hence the really high impedance).

1666041814346.png

In addition, one of the cell appears to misbehave despite the pack being top-balanced. The cell reaches its 3.6V level much before the rest of the pack. At least so far, after having manually top-balanced to 3.6V before assembly, and this week again after letting the BMS top-balance for a while.

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In your application 250A is almost a 1C continuous discharge rate. If you have the budget, another 16 cells in parallel would make a much better arrangement but that would double the weight and space needed.
The other choice would be LTO cells which can discharge at 10C and higher but they are only 2.4V and about half the energy density of LFP. 75Wh/Kg vs. 150Wh/Kg.
 
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