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So I'm working on a project where I'm wanting to take several 18V ryobi battery packs, put them in parallel to increase the capacity, and then use that to power my project. I think this should be fairly easily done, but my problem is when I want to recharge them, I don't want to disconnect them and charge them individually. Would it be alright to just take a single battery charger, and connect it to all of the battery packs in parallel? I wasn't sure if this would damage anything or not. I'm sure it would take longer to charge, but i'm okay with that.

Anyways, just thought I'd ask people that would know more about this than I would. Thanks!

  • It will cost more, but consider having one charger per battery. Then split the power supplying the chargers, instead of splitting the output of a single charger. If one battery needs more charging than the others, it will still get fully charged while the others will be able to shut off when they are ready. – alfreema Jun 30 '16 at 20:55
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Checking yonder google tells me that it's difficult and unwise for NiCd: http://www.powerstream.com/NiCd.htm, and unlikely to work for Lithium batteries unless you match the internal cells very carefully: https://electronics.stackexchange.com/questions/126673/charging-li-ion-batteries-in-parallel.

Either one could be overcome, but not for the minimal expense of just buying a cheap lawn tractor battery from Walmart, and putting a trickle charger on it.

  • Batteries in parallel can be problematic for charging the weak cell in 1 battery can cause over charging on a good battery. better to set them so you can charge them separately. + Joe – Ed Beal Jun 30 '16 at 19:05
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To expand a bit on the answer from Ed Beal, each 18v battery pack includes a BMS circuit which keeps the voltages of individual cells more or less the same. This helps prevent the identified issue with good cells getting over charged. When connecting multiple packs in parallel, there would be no such BMS to monitor and correct for differences between packs. In the end, you'd end up having to periodically charge individual packs no matter what. All that said, if you used all new packs with identical cells inside, they would probably stay pretty well matched for some time.

However, it still may not work with a standard Ryobi charger because we don't really know the charge algorithm. First of all, it detects what kind of battery (nicad vs lithium) is plugged into it. I don't know how it does that, but I can easily imagine there is a handshake between the charger and battery. That handshake almost certainly would not work with a bunch of packs connected in parallel. Moreover, chargers often have failsafe timers. If the pack is charging for a certain time, but is still not full, the charger stops out of an abundance of caution. So, it might not be a plug it in and forget it proposition because 4 packs in parallel will take 4 times longer to charge.

Last, I don't know what the application is or your price sensitivity, but you might consider an off-the-shelf ebike battery. These are generally 36V, but it's easy to step that down to 18. Ebike battery chargers are pretty cheap and indeed, are usually bundled with the battery. Likewise, these batteries include builtin BMS circuits to keep the cells balanced and prevent over/under charging. See ebikes.ca for some examples.

  • Thanks for your comments and suggestions! for my application i'm wanting to power a device (can be between 12-19 volts) for as long as I can, and i'm trying to fit all of my batteries in a pelican case (maybe 10"X6"X6"). my current solution is having three 6V batteries, but charging is a pain. I'd like as close to an off the shelf solution that i can find. I figured even if the ryobi batteries needed to be charged individually, that would still be easier/faster than my current solution. I'll give the E-bike battery a look. Let me know if you have any other suggestions! – gerrgheiser Jun 30 '16 at 22:20

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