7s5p Lipo build

grythumn

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Apr 15, 2021
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I have 40 10Ah nominal LiPo cells from Battery hookup (most are in the 8-9Ah range) than I plan to rebuild into a 24V (7s5p) nominal pack for primarily DC UPS service. Current plan is to float them at 4.05 V/cell with a current-limited boost converter, and mount them in a metal ammo cam. Fused cigarette lighter sockets for primary output, 30A BMS, 30A DC breaker. Want to keep the option to use a small ~300W 24V inverter for other loads.

Haven't fully characterized the loads yet, but I believe the typical draw is 3-5A @ 24V total with a 30% duty cycle per day. On the list to verify. Other question mark is if the BMS will balance at less than 4.2V/cell (standard chinese HXYP BMS you see everywhere); may need a separate balancer if it doesn't.


I've desoldered and capacity tested the cells. They were soldered to a 4S BMS and glued together. Tab length is not very consistent... clearly done by hand.


Debating the termination. Soldering to buswires is simplest. Could lay out some simple boards as in the original configuration and solder to pads, or solder/spotweld to zinc strips. Don't currently own a spotwelder, though, or have zinc strips. Busbars seem excessive for these power levels.

Comments? Suggestions?
 

Roland W

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Oct 9, 2017
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106
Other question mark is if the BMS will balance at less than 4.2V/cell (standard chinese HXYP BMS you see everywhere); may need a separate balancer if it doesn't.
Sounds all good to me.

The BMS will not start balancing if the cells do not come close to your 4.2V max voltage, but if it really doesn't balance, that would be a good thing, as it would just mean that none of the string is ahead or behind. This would of course be very unlikely with used LiPo cells. So there will be for sure the one or other string out of sync and come closer to 4.2V and the balancer should do its work. The question is just if the balancing current is sufficient.
So you will see if there is a need of an extra active balancer. Just keep a good eye on the pack for a few days, until you really know that the BMS is having it under control.
 
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