First post, by adalbert

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Let's say we have a 14,4V Ni-MH laptop battery pack with 12 x 4/3A sized 1,2V Ni-MH cells. They are wired in series and use a dumb charger. There is no balancing circuitry at all. There usually is only one temperature sensor. Voltage may be uneven on some cells, which causes generation of heat until charging is completed. The cells can withstand that, but it decreases their life expectancy. Long-term storage is a problem, because voltage goes out of balance pretty quickly, and when you plug the charger after a period of no use, you are degrading the cells even more, because they are overheating while re-balancing.

Could Ni-MH cells be replaced with 4 x 18650 Li-Ion 3,6V cells? That should provide the same voltage and similar capacity. Obviously they would be connected through BMS charging/balancing circuit.
We should have 2/3 space of the battery pack empty now, so there would be plenty of space for the balancing and charging circuitry. There would be also less cells, so there should be less possible points of failure.
There should be even enough space for a thermal fuse touching the case of each cell. For maximized safety I would also like to use Panasonic 18650 cells with internal "current interruption device".

I don't want to just plug li-ion cells in series and call it a day, I know that this is dumb and a fire hazard. But can it be done the way I described in this post?

//edit: updated the schematic a bit


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Reply 1 of 4, by Mister Xiado

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I think a resettable fuse and a new AC adapter would be required, to prevent the majority of headaches. My over-engineering senses also cry out for heat sinks and a forced air cooling system, provided the battery casing can hold a fan, and has decent airflow potential inside of it.
Interesting idea, and it definitely seems workable. Lamentably, such planning often translates directly into grossly inflated costs. I've seen people casually mention having performed such a project, but they didn't stick around to provide details. I have a few old devices with MASSIVE batteries that could become much more useful if I could cobble together a lithium ion battery pack for them, but the idea of carrying a potential IED made from Chinese batteries on my person disincentives the idea pretty effectively.
I recommend testing the idea with a test bench, with test loads rather than a computer, and if it proves to be reliable and non-explosive/incendiary, then encase it all. Smells like a hundred dollar battery pack, though.

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Reply 2 of 4, by luckybob

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Just buy cells with an internal protection. You are making a mountain out of a mole-hill.

example: https://www.batteryjunction.com/samsung-26j-b … jxoCL6oQAvD_BwE

It is a mistake to think you can solve any major problems just with potatoes.

Reply 3 of 4, by debs3759

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Mister Xiado wrote on 2020-12-29, 21:13:

Smells like a hundred dollar battery pack, though.

Shouldn't cost more than $30. Maybe $20 with access to batteries at discount prices. I've been looking at similar on a larger scale for storing enough power to go off grid.

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