Assembling my own battery pack out of 18650s

[microbug]

As you may well know I am making a custom bike light system. I ordered 10 Samsung ICR-18650-26F from Aliexpress (they are genuine) since they are cheap (£2 each) and have a higher capacity density (if that's a phrase) than RC li-poly batteries. All the batteries are from the same production batch and read around 3.8-3.9V: as far as I can work out they should all be fairly well matched. I have a power resistor and decent multimeter so I could charge the cells up, put them across the resistor and measure their voltages– but do I need to? I'm putting them in a 2S2P configuration, and I read that the internal electrodes of cells in parallel can become plated and therefore less effective if the cells aren't matched.

I'm hand assembling the battery pack so 2S2P is ideal, but I could do something else if I have to; I haven't designed the PCB yet so I can change the schematic.

[nctnico]

I'd use a readily made pack. Bare cells are very dangerous to handle because they can source a huge current.

[microbug]

Readily made packs are expensive and I'm well aware of the dangers. I have old cells to practice soldering and with the right technique it takes less than two seconds. The pack will be fully waterproofed and the connector is IP68 rated. While I don't plan to put protection inside the pack itself, there will be protection on the board it connects to. The connector is unlikely to be the same as any consumer products and the only end user is myself so I think I'll be fine. I agree that it would be much better to outsource the pack if it was for someone else, but I need 18650s for a different project anyway and they're cheap.

Besides, I've already got the cells so it's too late

[marshallh]

Matching is a good idea, if you can do it, do it. Differences will become magnified over the lifespan of the pack so it's important that they are minimized at the start. Of course, it's less of an issue if you can balance-charge the pack (each cell individually).

You won't blow up cells as long as you're careful. Do only use pre-tabbed cells. Unless you have a spot welder, absolutely do not try to solder directly to the terminals. For the tabs, higher wattage means less time heating and less time aggravating the cell.

You will need to buy a protection PCB for the pack. At minimum it should open the pack if there is a short. Better ones provide under and overcharging cutoff. I would strongly suggest that you use a protection PCB.

Lastly use a connector type that cannot be easily shorted.

I would be wary of any batteries purchased from Aliexpress. I'd only use them after characterizing them to make sure they match their rated specs.

[microbug]

EDITED FOR CLARITY

I have some single cell li-ion battery chargers which are based off modern charging ICs; they should get two batteries to the same point easily. For matching, my idea is to use a 2 ohm load connected to the battery. I would set my multimeter to average mode and compare the cells' average voltages, each after an hour of discharge. Will this work?

While I will use a balance charger to charge the two series elements separately, the parallel cells will be attached permanently to each other and therefore cannot be charged separately.

Unfortunately it's a bit late for pre-tabbed cells. I've been practicing practising with my soldering iron which is a Hakko FX888-D (65 watts); I could probably lay my hands on a high-end one. I found that if I use a Dremel to lightly scuff the contacts them it takes a second or so to apply solder to the cell. I can then tin and heat the wire/tab beforehand so it takes less than a second again to connect it to the solder. I could probably cut this time down with practice and a better iron, and the cell will have time to cool between the two heat pulses.

I did have a cursory look for protection PCBs that would cope with the max current (4-5A) but I couldn't find anything. If I really need one I could design my own and get the PCB manufactured; the size wouldn't be large so Oshpark would do. I need to put in a parts order anyway. I was planning not to put a protection PCB inside the battery pack as it will be sealed and hard to short... the pack will only be used with my own driver which takes care of over-discharge protection, so maybe a built fuse would do for over-current?

The battery connector is this one, if anyone's interested. I doubt it is actually IP68 but it will be fine for this purpose. I will use the female connector for the battery side since it is hard to short.

I'm pretty sure my batteries are genuine. They came in what appeared to be manufacturer-sealed 5-cell packs and their contacts are unmarked (they're not recovered cells). I bought them from this link. I will test the cells' capacity to make sure they're in the ballpark with my crummy Arduino battery capacity tester - it's not very accurate but it gives a good enough figure. Of course I can't be certain whether they're genuine (factory rejects?) but they definitely look the part. I have had a very good experience with Aliexpress in the past: I have only used them for a year or so so perhaps their mixed reputation is a lingering artefact from when they used to be worse (for they have at least got better). Either that or I'm just really naïve!

[mark03]

I know it's tempting to solder the cells directly, and the Internet is full of advice on how to do this with minimal damage, but my advice is DON'T.  Your battery system is one place you just don't want to cut corners.  Try looking for a local battery place with a spot welder who could weld tabs on for you.  Here in the US, Interstate Batteries did that for me for a very reasonable fee, the finished product is beautiful.

Second, are you sure you want 2S2P?  1SnP is easier to deal with and you'll probably want a switching power supply for the light anyway.  I happen to really like my own design: http://www.keteu.org/posts/bikelight.html, but your mileage may vary...

[microbug]

I just checked my Aliexpress order history, and thankfully I only ordered 10 batteries. I was going to order 20: I need 10 for a different project (using a holder, not soldering) but this means that I can buy some ones with tabs instead

I'll look into getting them spot welded locally (whether it's cheaper than buying them pre-tabbed), but I'm dubious: from my experience the US is a better place to be when it comes to services like that. It would probably be very expensive if any local company did offer to do it.

[richard.cs]

I've happily soldered 18650's before and wouldn't hesitate to do so again (carefully). If decided you want someone to so it with a spot welder I can reccomend Strikalite, http://www.strikalite.co.uk/ I've used them in the past for similar things at work and they're quite happy to do custom jobs. We sent them some parts, they welded some tabs and posted them back, price was very reasonable.

In terms of matching I would charge them all to the same state, discharge into a resistive load of a few ohms and compare the voltages after an hour. It might be unneccesary but (provided you've got 4 power resistors) it takes an hour or which 10 mins is actual work.

[microbug]

I've happily soldered 18650's before and wouldn't hesitate to do so again (carefully). If decided you want someone to so it with a spot welder I can reccomend Strikalite, http://www.strikalite.co.uk/ I've used them in the past for similar things at work and they're quite happy to do custom jobs. We sent them some parts, they welded some tabs and posted them back, price was very reasonable.

OK, I'll check them out. I'll send them an email asking for a quote... I suspect it will be cheaper to buy tabbed cells and solder them myself since shipping lithium batteries domestically in the UK is difficult and expensive due to safety restrictions.

In terms of matching I would charge them all to the same state, discharge into a resistive load of a few ohms and compare the voltages after an hour. It might be unneccesary but (provided you've got 4 power resistors) it takes an hour or which 10 mins is actual work.

Good, that was my plan. It shouldn't be hard; I can use the same circuit as for my capacity tester since it has an integrated MOSFET which would allow me to switch it off after exactly 1 hour.


FYI I'm planning to make two packs.