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Are there any backup battery packs with replaceable cells? (slrpnk.net)
submitted 6 months ago by thinkercharmercoderfarmer@slrpnk.net to c/permaculture@lemmy.world
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cross-posted from: https://slrpnk.net/post/33818672

I have just found that another one of my LiPo battery packs has started to develop a belly, and I'd like to replace it with one where I can just swap out the cells rather than junking the entire unit. I haven't had much luck finding something that meets my, I think modest, needs.

Needs:

  • USB 5V power (enough to charge one modern cell phone, slowly)
  • replaceable cells (18650s would be ideal but I'd settle for something that took AA or AAA batteries too)

Nice to haves:

  • USB-A and USB-C (though I can manage with either - I have a ton of cables for both)
  • Fast charging (the more watts the better but all I really need is ~10W at most)
  • Passthrough charging (can charge and keep the output hot at the same time)

Sky Pie:

  • Hot swapping cells (can selectively discharge one cell or bank of cells, then switch to the other while the dead cells are being replaced)

For that last one, I think if I can find a pack that just meets the "Needs" requirements I can rig up two of them and a raspberry pi to handle the charge monitoring and source switching. I just would like something that uses commodity hardware like the 18650 to store the power, can deliver enough voltage to run the pi, and is designed to have the cells be user replaceable. Is there something like that on the market?

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[-] SanctimoniousApe@lemmings.world 2 points 6 months ago

I've just had my first bank go puffy (RAV Power, I think?) not all that long ago, and it looked like there were 18650s used inside already. I just haven't gotten to trying to replace them yet, so I'm not sure. Have you looked inside yours? I don't see why you can't just exchange them if so. Might require a minor bit of de/soldering work.

I remember seeing charger bank shells on eBay.

Finally, I think a full blown RPi is serious overkill for your intended purpose. At least consider a Pico - much smaller, cheaper, and actually intended for simpler uses like this (although it's still kinda overkill).

I do find your hot-swap idea interesting, why would you need to do this if you had pass-through?

[-] thinkercharmercoderfarmer@slrpnk.net 1 points 6 months ago

I haven't taken this one apart, but it's too thin to accomodate 18650s. I think it's just a rectangular pack. I will probably take it apart just to see if I can salvage any of the components but I haven't gotten around to it yet.

My immediate concern is just charging my phone when I'm out and about, and for that I don't need passthrough or hotswapping or any of that stuff. If I have my Pi out in the field and I'm running a test, that's when I'd like things like passthrough or hotswapping so I can leave it out there for days at a time and not have to reboot it.

The hot swapping specifically is a long term ambition I have for some farm rovers. I want to do moisture sampling and maybe even some surveillance of weeds / insects with some cameras mounted to them, and then generate some reports based on the collected data. The idea is I'll have different units that do different things, and have different power requirements, but they all run on interchangeable cells that can be recycled through the same charger. It's extremely not necessary and it's definitely easier to just reboot the device once the new cells are installed, but I like the idea of keeping the on-unit compute active so I can see analytics from its perspective throughout the process. The Pi is overkill for what I'm doing, but those are the boards I have at the moment (those, and all my old android phones) so that's what I'm using to prototype. If I ever build something that I want to scale, that's when I'll start thinking about optimizing compute for the task I'm building for.

The pass through would be for something that doesn't offload the cells, but rather docks into a station kinda like a roomba does. I'd just hook the charger up to some contact plates and have it park on them (or set the rover in the dock myself), and the rover would be able to undock itself and get to work once it's charged. I'd still like those to use replaceable cells, just so I don't have to desolder anything when the cells start to fail. I'm planning to prototype both, which one ends up being easier depends a lot on what hardware I can buy vs. what I have to cobble together myself.

[-] SanctimoniousApe@lemmings.world 3 points 6 months ago

Interesting stuff - kinda jealous you have the time for such fun. Anyway, I think I answered your OP question with the eBay link. I've never built one myself - and I definitely can't point you to which are better - but at least you know they exist, and can hopefully find your way from there. Good luck!

[-] thinkercharmercoderfarmer@slrpnk.net 2 points 6 months ago

Thanks! It's very slow going, I've been at it more lately because I've been snowed in and had nothing better to do, but it is a fun diversion. I appreciate the link, those look very much like what I had in mind. If I ever make anything worth a writeup I plan to post a description here so stay tuned. At the current rate of progress I'll probably have a working prototype in 5 - 15 years.

this post was submitted on 06 Feb 2026
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