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This isn't really an electric vehicle question, and most people here don't know much about 18650s.

Some better communities:

To somewhat answer your question:

1 - Google the datasheets for your cells and look up their maximum currents.

2 - Laptop cells (especially old ones) are optimized for capacity, not power output. Tool packs are the opposite. Capacity-wise they might vary by 100%. Power-wise, they might vary by 6000%. If you use a laptop cell that can only supply a very low current without overheating, you'll overheat your battery (also, it won't work well). In other laptop cells you might be just fine.

3 - The more cells in parallel, not only the more capacity (linear with how many cells you add) but also power. So, if you have low-power (rate at which you can use the capacity) cells, having more of them allows them to work less hard each.

4 - You won't know what the capacity is of your old cells unless you test them. The datasheets will tell you what they were supposed to be. You want to roughly match groups of cells with similar capacities, else, chain is only strong as its weakest link. Worse, you might not notice that one cell in the chain is empty long before others, (the total voltage will still be in a reasonable limit), and so you'll reverse charge it and kill it, and maybe start a fire. If you have a capacity tester (I have a Zanflare C4 I think), then just go by the datasheet amounts. Individually charge them, then hook them up in series and put a load on it that you will expect to take, I dunno, 30 minutes to discharge them. Then check each voltage (each cell head and tail, no need to disassemble) every few minutes to see whether they're dropping in synch with each other. I.E. That they all start at 4.2v, and then 10 minute later they're all around 3.80v, then 10 minutes later they're all around 3.40v. If you find a cell that's particularly stronger or weaker than the others in the chain, yank it, replace it, start over. They'll all start at 4.2v, but you want them all hitting 3.2v (nearly empty) around the same time. A few hundreds of a volt difference is fine, a few tenths of a volt is not.

5 - You probably want a BMS. Get one off of ebay cheap, a few bucks. You'll probably want a 5-cell. It will help your battery stay synched up over time (they'll naturally drift apart even if perfectly matched).

6 - You can probably hijack the original charger. NiCAD chargers usually significantly overcharge their cells relative to their resting voltage. I.E. A NiCad cell is 1.2v, but its charger is often charging it to 1.5v, just to force it to accept current. So, if you had an 18v NiCad, and you feed it 21v lithium (up to 20% extra is probably fine I'd say), that lithium should be absolute max 21v, whereas the original NiCad charger is probably 24+v. So, just slap on a cheap buck converter to the original power supply, set the buck to 21v, and it should be fine converting the higher voltage to lower. Otherwise, a power supply isn't that hard to find.


I've done this lots before. It's not hard. It's great for tools you use occasionally. Most tools are agnostic towards battery input, voltage is voltage.

For example:
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