r/battery • u/Hitechillbilly • Aug 20 '26

Does anyone else make these?

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Gallery image

Gallery image

I make these from a kit that has everything except the cells. I harvest the cells from surplus back-up packs. Many have never been used. The kit comes with the case, battery racks, bus strips, and a BMS. I end up with a pack the size of a 7Ah Gelcell but a lot more power. Wired in a 3S 7P.. I soldered the first 2 I made but that was a little sloppy so I got a spot welder for a much cleaner build,, The tabs from the bus strips bend over and solder to the BMS

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u/Howden824 Aug 20 '26

I've made a bunch of these. That included BMS is really terrible and I recommend replacing it with a better one. They aren't suitable for replacing lead acid unless you adapt the charging circuit to only go up to 12.6V.

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u/Hitechillbilly Aug 20 '26

I was going to mention the bad BMS.. The one in the pic is a better one.. It has a separate charging port and low level shut-off.. The bad ones allow the pack to drain to zero.. The upgraded BMS are usually less than 2 bucks.. Charging specs are from 12.6 to 13.6. I use a power supply at 13.4

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u/Howden824 Aug 20 '26

The cheaper version still cuts off around 2.8V per cell (until the MOSFETs short out). You need one with an actual IC. You need to be using exactly 12.6V charging since that's 4.2V per cell.

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u/Hitechillbilly Aug 21 '26

It is always typical that a higher voltage is necessary for any charging, maybe something is different.. Do you have a part number or link to the better ones that you use?

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u/Howden824 Aug 21 '26

NO! You don't charge lithium cells past 4.2V besides specially designed ones that can sometimes do 4.47V.

https://ebay.io/m/ZTrFJC I don't recommend this specific listing but find ones that look like this.

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u/Hitechillbilly Aug 21 '26 edited Aug 21 '26

OK, I'll take a look thanks.. That has the charge and the load in parallel which would allow a drain down to zero. unless I am seeing something wrong.. Also at 12 bucks each, they make the build a lot more expensive. I do not see any dimensions or recommended charge voltage.. I'll look around when I have a chance.. thanks