Got a couple of battery packs in the garage that have failed – one while I was hammering down a descent and one while charging. Each contains 4* 18650 cells. It’s occurred to me that I could possibly consider taking apart the existing packs, find and remove the duff cells, then use the remaining cells to make up a new battery. If only one cell has failed in each it might even be possible to make up a 6 cell battery giving me a longer life battery.

However, it’s far more than 20 years since I did GCSE science and while I’m ok with the basics of soldering I can’t quire recall the specific of series vs parallel when connecting the cells, and am a little wary of shorting anything out and killing more cells while disconnecting /connecting cells. Can anyone offer any useful, practical advice? Failing that, relevant gifs or amusing videos will also be appreciated

ta

Can’t help on fixing but I’ve got a 3.7v one for sale in the classifieds

You need to keep the voltage the same. To do that you need to stick with 4 cells in series (eg in a line). 6 cells in series would be 50% extra voltage which would likely blow circuits, etc.

If you had 8 cells, you could in theory run two 4 cell packs in parallel to make up a battery with twice the capacity but with LiIon you need to be careful…

Ta but the packs that failed are both 7.5V 4.4Ah Li-Ion Battery packs so I need to make up the same.

Failing that I’ll be in touch with Torchy The Battery Boy as friends have recommended his custom packs

ta, that’s what I couldn’t remember – cells in series add to the power, cells in parallel add to the runtime.

I was thinking that if the 4call batteries were made us as 2 parallel packs each of 2 cells in series, I could run 3 such packs in parallel? IS that not an option?

sticking with 4 cells is not a huge issue TBH as I’ve never run out of battery, except when it failed

You could in theory run three sets of two parallel cells but the voltage would be too low and also with liIon, you’d risk charge/discharge issues as the cells aren’t all matched.

The biggest problem you will face is that with 18650 cells the connection tabs between cells are normally welded on. The procedure of removing and re-fitting cells requires you to re-weld the link which is the tricky part of the whole thing as LiIon cells don’t really like the extreme temperatures involved.

I would strongly recommend popping Smudge at mtbbatteries an email to see if he could do anything with them. torch may be able to do something too but i don’t know if he make up the battery pack or not. LiIon cells are very nasty things when they go wrong so I would err on the side of caution if you don’t know what you are doing.

2nd using smudge.

There’s a lot of energy stored in a Lion battery and you don’t want to be letting all the smoke out at once as it could get messy.

You need to keep the voltage the same. To do that you need to stick with 4 cells in series (eg in a line)

Nah.

They’re 2S/2P if you wanted you could make up a 2S/3P or 2S/4P pack, the real limiting factor would of course be finding space for the cells and of course getting the buggers all charged…

Something like this could be a reasonable starting point…

And a separate charger… If you can put up with the faff…

Ah, I clearly have the voltage of a LiIon cell wrong 🙂 In which case, what he said 🙂

RIght, two bits to that

ok. got you on that. the packs are from the same supplier and *should* be matched, but better safe than sorry. Ditto other (later) comments about better safe than sorry

Why so? e.g. Assume that cells are 1.5v, 1000 mAh

1 cell = 1.5v, 1000 mAh

2 cells in series = 1.5v, 2000 mAh
2 cells in parallel = 3v, 1000 mAh

2 * 2 cell series pack in series = 1.5v. 4000mAh
2 * 2 cell series pack in parallel = 3v. 2000mAh

3 * 2 cell series pack in series = 1.5v. 6000mAh
3 * 2 cell series pack in parallel = 4.5v. 2000mAh

Is that not how it works? I only got a grade B for physics and that was *cough* years ago. (edit – got called away and there’s been an update since I typed this)

Sumdge’s customer service has been excellent on the failed batteries I’ve had to date, but there’s a (fairly obvious) reason I’m looking at using Torchy (or someone else) this time…

Oh, forgot the charger. The chargers I have will be setup for 2s/2p so would they work on 2s/3p but ju7sttake longer?

1 cell = 1.5v, 1000 mAh

2 cells in series = 1.5v, 2000 mAh
2 cells in parallel = 3v, 1000 mAh

2 * 2 cell series pack in series = 1.5v. 4000mAh
2 * 2 cell series pack in parallel = 3v. 2000mAh

Should be

1 cell = 1.5v, 1000 mAh

2 cells in parallel = 1.5v, 2000 mAh
2 cells in series = 3v, 1000 mAh

2 * 2 cell series pack in series = 6v. 1000mAh (basically 4 cells in series)
2 * 2 cell series pack in parallel = 3v. 2000mAh

3 * 2 cell series pack in series = 9v. 1000mAh (6 cells in series)
3 * 2 cell series pack in parallel = 3v. 3000mAh

The chargers I have will be setup for 2s/2p so would they work on 2s/3p but ju7sttake longer?

Yes

ok, so series affects power, parallel affects runtime. thanks

Basically yes (except the pedants will say voltage isn’t power though it’s directly linked Power = volts x current)

Remember that these cells have already been made up into a battery so I’m not sure what already exists in the way of tabs/welds. Will have to see if I can get some pics of one of the packs to show how they’re currently linked

(except the pedants will say voltage isn’t power

The real pedants will say potential difference rather than voltage 😀
My professor used to love saying there’s no such thing as voltage, but common usage and Wikipedia have finally won.

Have to say though the risk/reward of soldering Li cells even tagged just pushes me back toward EBay every time I think of it. I have some 18650 cell holders someone on here 3D printed or injection moulded which work nicely enoguh, my DIY stopped there.

I knew that someone would say that 😉

Ebay is of course, also awash with cheap 18650 cells as well as holders, they are probably about as Gash as the ones you’ll be reclaiming from a knackered Bastid battery pack but without the need to de-solder, so why not order yourself a few to experiment with and once you have a configuration that works go shopping for some decent cells from a reputable source.

Make your own packs from single batteries, don’t solder them, use this guys cases, dead simple. Use a Nitecore charger to charge them all.

http://www.customduo.co.uk/Pages/batteryholders18650.aspx

If you run 2 parallel pairs you’ll get what you need for your light. I get hours of use from a fully charged set with my Troutie light.

Sorry for the hijack but could you use the cells from a Dyson handheld to make a bike pack? I’ve no idea what to do so if someone fancies a challenge…