Mountain Bike Reviews Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
2,152 Posts
Discussion Starter · #1 ·
So, I've got about 18 18650 cells out of two dead battery packs from my laptop. I recently read a post on some website that said normally just one cell will fail, and that's enough to make the whole pack quit working. Then it went on about how to identify the bad cell and replacing it, but I'd much rather use the remaining good cells to make a Li-Ion pack for my diy halogen lights.

So, my question has to do with the PCB. I've been doing a bunch of looking, trying to understand what I can about what I need to do it right, and I've found this PCB which look like a winner, but it's says it's only for four cells. Now, I'd like to build an 8 cell, 14.8v pack. How would that work? would I need two pcb's? Or when it says it's for four cells, does that just mean multiples of four in series.

I have successfully built a few NiMH 14.6v packs that I soldered together. I home Li-Ion's aren't any less tolerant of the momentary heat of a soldering iron. I know spot welding is the hot ticket (pun intended) but the costs would outweigh the benefits of doing this myself. I'd just buy pre-made packs if that were the case, unless I can find one I could rent or borrow.

Anyway- I did a search and was overwhelmed with the number of not quite right results so I thought I'd ask. I'm more than happy to hear any personal experiences or tips and tricks form some of you who have already done something like this.

Thanks,
BM
 

·
Registered
Joined
·
596 Posts
I would try and find a battery shop with a spot welder. If you can cut the tabs and solder to them things are better. If you're brave; clean, liquid flux, lottsa heat quickly, and lastly a hit with the damp sponge YMMV. Make up packs of 2 cells in parallel, then 4 of these in series along with the the PCB.
 

·
Registered
Joined
·
46 Posts
I got most of my initial experience with li-ion batteries from laptop batteries too, but I found out that once you separate them from the pack you cant wire them back up. They are too dangerous to solder, and can be explosive under many many circumstances. I used mine in a wired battery holder, and pop them into separate charger. i now don't use them, and only use cells with a protection circuit build in to each cell.

I work with single cell packs and don't have any experience with multi-cell circuits, to my understanding your looking for a 4 series x 2 parallel pack. I think that a doable setup. Depending on the current you intend to use you could just make 2 4-cell packs, and might not need all 8 cells for one outing. Ive got a single XML and running full blast, it works fine for over an hour on 2 cells.
 

·
Registered
Joined
·
124 Posts
I've built a 4s4p 18650 pack using laptop cells before. Does it work? Yes. Would I do it again? Hell No!

I prefer to use only single cells in my lights now. Most laptop packs have "pairs" of cells attached together. I would try to keep those pairs together if possible.
 

·
Registered
Joined
·
2,374 Posts
the 4 cell PCB is for 4 cells in series, so if you need a 14.8V pack, that means 2 lots of 4 cells (in series) wired parallel. I prefer to have each parallel pair wired to each other as opposed to having to separate 4S packs wired in parallel at their outputs. The PCB should work fine, just be aware of the postage costs.

As for soldering li-ion cells direct, it's easy. Rough up the terminals carefully with a file or a dremel. Tin the terminals and the wire with good quality lead solder. Melt the two together for no more than a couple of seconds then put a pair of pliers (usually what I'm holding the wire with) onto the join to sink the heat away. Keep the heat exposure to no more than 5s and you'll be fine. I've done it for quite a few packs with no problems.

For old laptop 18650 cells sometimes you get lucky and it's just one bad cell out of 4 or 6, Most of the time they're all duff. I ended up giving all mine away as spares for lights I made as presents as the average capacity (1200-1500mAh) was just too low. To test them, test the voltage of each cell and throw away any that are under 2.5V. Then charge each cell individually to 4.2V, ideally in a metal container inside another one filled with sand. Test each one every day after that for a week, throw away any that drop >0.2V over a week. If you have a friend with an RC charger, bribe him with beer to do discharge/charge capacity measurements. If they're less than 1Ah, just recycle them, they're not worth using.

Fingers crossed, after all that you'll have enough for what you want :)
 

·
Registered
Joined
·
2,152 Posts
Discussion Starter · #6 ·
Thanks for all the replies...

As to using single cells for lights, am I to assume that's for led lights? I've got a pond scum halogen setup that I've been powering with 24cell AA NiMH battery packs. I haven't done Li-Ion because of the cost involved and I'm hoping this could be an inexpensive way to drop some battery weight.

But still, if I have to buy a single cell charger to individually charge each cell, and still need a suitable smart charger for the completed packs, and pcb's...I can see that running up the costs...

As far as wiring to the pcb- if I'm understanding correctly, wire pairs up in parallel and treat them as an individual cell when attaching to the pcb?

thanks again-
BM
 
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top