Well, first 18650 laptop batteries are Lithium Cobolt Ion. Other 18650s, especially high current (10+amps) and Tesla 18650s are other chemistries. But I never seem to get my hands on those, only spare laptop batteries.
Normally, I try to build ebike battery packs with my 18650s. I need 100+ and I make a 13s10p pack (for those not in the know, when you see 13S10P or 5s2p it means series/parallel, ie. 13s10p is 13 in series (13×4.2 = 54.6volts) and 10 parallel (each cell is degraded to about 2000mah or 2ah so 10×2=20 amp hours aka 20ah).
Anyway, I test incoming 18650s by charging them up to 4.15 volts (not 4.2 because some 18650s aren’t designed to go that high) and then wait 2 weeks and measure the batteries. I then rank them by how much voltage they’ve self-discharged
4.13 – 4.15 Excellent
4.06- 4.10 Poor. Only for non-bike projects
<4.05 throw out, these obviously have internal damage and can be dangerous
These Poor quality is still good enough for non-critical projects like building packs for inverters or retooling NiCad tools to 18650s.
Converting Cordless with external pack
So I had 2 cheap drills who’s packs were dying and I converted them over to 18650s because I had some spares from a pack built (see Homemade Pack thread)
And I posted a thread about a dangerous voltage inversion on one of the 18650s that I pushed too hard, at which point it devolved into a tool conversion thread. Thus, here is a thread for DIY tool conversions.
For my 18v cheap drill, I opened the battery case, ripped out the guts, kept the connector and shoved 4s2p into the case and wired it up. No balance lead, I just want to see how it performs over time.
For the 12v drill, I did the same thing but only put in a 3s2p pack. You need to understand that typical laptop cells can only push 1C discharge continous and peak 2C . The average laptop cell these days is 2.4ah therefore 1C is 2.4 amps. However, they have degraded usually to 2ah so therefore 1C is 2amps and 2C is 4amps. If I have a 3s2p, that means I have 2 parallel for 4ah and capable of 4amps continuous and 8 peak. That is good enough for a cheap drill, but not a high quality drill. If you want to convert a high amp draw drill you may need to go to hobby king and buy a 10C 3-4ah pack. They will fit in the old drill battery pack (you must check reference sheet for dimensions!). In my case I had an idea for an external pack.
My 18v XRP Dewalt drill I’m more protective of. I have 3 batteries, 1 good, 1 bad, 1 dying at 15v. The bad one I ran with an idea someone mentioned of an external pack.
I opened the bad pack, ripped out all the cells but the one in the stem. The stem has a cell that fits perfect. It holds the connector fine, so I fixed it in place and soldered a bypass around it to the connector. Then ran these 18ga wire to an external pack and put them in a leather hip pouch that is thick enough to protect it in the field. I found the 18ga wire gets hot while using a sawsall. I’ll fix that with 14 or 10ga wire from my stocks.
The pack itself was 5s6p at about 12ah, with a balancing lead coming off it. I just used packing tape to tape that out of the way while in the pack.