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I have an application for which I need a relatively high amount of current @5V. I was thinking of connecting four external USB battery packs like these in parallel, as they output a nice regulated 5V and are easy to charge. But I know lithium chemistries are sensitive souls and having "unbalanced" cells connected together is a no-no.

How can I avoid any individual pack with a higher charge accidentally trying to charge another? Is it as simple as putting a Schottky diode in series with the positive lead out of each battery? Do Schottky diodes have an intrinsic voltage drop (like LEDs) which will waste a significant amount of power and/or cause my circuit to see less than 5V? Is there a better approach?

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  • \$\begingroup\$ Yes, schottkys have a voltage drop. \$\endgroup\$ – Gunnish Sep 16 '13 at 15:39
  • \$\begingroup\$ Note that the external powerpacks aren't connected directly to the batteries, they're connected to the output of a boost converter. I think you'll actually be OK doing this, although I'd add a reservoir cap near the load and make sure to have "star" wiring to the packs. \$\endgroup\$ – pjc50 Sep 16 '13 at 15:42
  • \$\begingroup\$ That's an interesting thought @pjc50, but I get the feeling the circuitry in these battery packs is made down to a price and full of hacks. I'd really want to see a schematic I guess, but my chances are low :(. \$\endgroup\$ – Robert Atkins Sep 16 '13 at 18:15
  • \$\begingroup\$ @Gunnish How much is that voltage drop, typically, and what do I look for on a datasheet? \$\endgroup\$ – Robert Atkins Sep 17 '13 at 12:23
  • \$\begingroup\$ Oddly enough, Li-Ion cells will self-balance when placed in parallel given enough time (although you don't want them too imbalanced when you connect them). \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 17 '13 at 13:40
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Alternatively, you could use one larger battery with one regulator. It saves power, is even easier to charge, is still regulated, and you can scale the battery to your power requirements.

You could use a boost converter with a 4.5V source, or you could use a 6V battery with a buck converter, or you could use a linear regulator such as the LM7805 for a noise free source with more power loss. If you need more than the rated 1A, there are bigger, better regulators out there for sale.

Connecting regulated outputs is always asking for trouble in my experience.

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  • \$\begingroup\$ I dislike this solution because a buck regulator that can do 10A costs, well, big bucks. I take your counsel on connecting regulated outputs though. \$\endgroup\$ – Robert Atkins Sep 16 '13 at 20:29
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    \$\begingroup\$ Now having tried it and fried over $100 worth of batteries, I should have taken @Bob's advice here. Don't connect the outputs of two different battery packs' buck/boost regulators together. Don't even connect the outputs of the same battery pack's buck/boost regulators together. If you search hard enough you can find high current DC-DC regulators (OKR-T/10-W12, TDK-Lambda iAF) to step down the power from 3S remote control car Lipo packs, which can deliver plenty of current. \$\endgroup\$ – Robert Atkins Oct 21 '14 at 22:17
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There's no problem in putting lithium cells in parallel as long as they're similar, if you charge them in parallel they'll equalize, when you discharge them in parallel they'll discharge at approximately the same rate.

Usually what leads to issues is unregulated lithium cells being charged and discharged in a series.

The chemistry of lithium is sensitive but that shouldn't be a problem in your case, what might be a problem is that you don't know what are the electronics on those battery packs and so it might not work well, I feel like that is an unlikely scenario but it's possible.

I have a better idea for you (I think), use a computer power supply, it has a 5 volt rail that is smooth, cheap computer psu's sometimes put out like 40amps on the 5v rail, just shunt the green wire with the black and you're good to go.

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  • \$\begingroup\$ A computer PSU? That's a great idea! Related: have you got a two mile long extension cord handy? (It's for a portable application ;-) \$\endgroup\$ – Robert Atkins Oct 23 '13 at 5:57
  • \$\begingroup\$ 2 mile long? Not really, but I have a computer UPS ;), you can take it with you and directly connect the PSU to it, both the UPS and computer PSU are very useful devices that I've used often to solve all kinds of problems, like powering an improvised electric boat motor, powering appliances during power outages, testing lights and motors, etc... OR if you have a charged car battery or something like that, you can do what Bob suggested above, buy a suitable voltage regulator, hook it up to the battery and there you go. \$\endgroup\$ – wxiiir Oct 23 '13 at 13:23
  • \$\begingroup\$ Has that PSU got wheels then? Because when I say "portable", I mean "wearable". There's a reason the battery packs I originally specified are of the size that would fit into pockets. \$\endgroup\$ – Robert Atkins Oct 24 '13 at 15:54
  • \$\begingroup\$ Portable usually means that is something you can carry with you (because it isn't too big and heavy). Something that is wearable doesn't necessarily fit inside your pocket (like a jacket). You haven't specified the reason you've picked the battery packs (being able to wear them or carry them inside the pocket) in the question or in our dialogue and as you know I'm not supposed to guess it. Definitions and mistakes aside, the PSU and UPS fit with a lot of room to spare inside a small backpack and aren't very heavy, if this isn't portable enough for you, maybe you should get the battery packs. \$\endgroup\$ – wxiiir Oct 24 '13 at 23:16

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