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I need suggestions or help thinking this problem through. I want to battery power a Raspberry Pi with multiple USB battery packs (for a remote sensing application) with no need to do anything manually. Solar re-charging of one battery is not really an option. I am talking about off-the shelf USB batteries sold by companies such as Anker.

Most USB batteries do not allow simultaneous charging while using a battery, so daisy-chaining multiple USB batteries (with one plugged into the RPi's USB) is tempting, but probably out.

I am wondering if I need to design a charge controller circuit that detects when a given battery is low, then switches instantaneously. Does this sound like my best bet? Is there an off-the shelf circuit or reference design that I should look into for this?

Another idea I had is maybe to connect all the batteries in series. This would of course produce a higher voltage than the original 5V. I could use a step-down buck converter to supply the 5V to the raspberry Pi.

Any ideas or strategies are welcome! Thanks for your help!

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  • \$\begingroup\$ Does it absolutely have to be a USB battery? Small sealed lead acid (SLA) batteries have large capacities and could be regulated to 5V easily. \$\endgroup\$ – David Jun 24 '15 at 22:06
  • \$\begingroup\$ In my experience dollar for dollar the consumer market USB batteries hold a lot of charge compared to lead acid. I will follow up on this and make sure to account for the higher voltage of the lead-acid battery. \$\endgroup\$ – user391339 Jun 24 '15 at 22:18
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    \$\begingroup\$ If they held enough charge you wouldn't need multiple batteries... \$\endgroup\$ – David Jun 24 '15 at 22:19
  • \$\begingroup\$ I will look into lead acid more closely. I think one of the other reasons for my wanting to get USB (lithium) batteries is that they will be more versatile for other projects/applications down the road. \$\endgroup\$ – user391339 Jun 24 '15 at 22:23
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    \$\begingroup\$ (Also, the Pi's power consumption compared to microcontrollers is terrible. Depending on what you're sensing you could do it on 2 AA batteries for months with a PIC/AVR/MSP etc) \$\endgroup\$ – pjc50 Jun 24 '15 at 22:29
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A Raspberry Pi model B draws about 1.21W with nothing plugged into the USB ports.

So (say) 3 months is about 2200 hours, so you're talking about 2.6kWh if you want to power it continuously.

A full-size laptop Li-ion battery pack might hold 48 to 80Wh, so you'd need at least 33 to 54 of them.

A 76 lb deep cycle battery might hold 1.2 or 1.3kWh so you might only need a couple of them.

A 5.2Ah USB battery pack might supply 25Wh so you'd need almost 100 of them.

Doesn't sound very practical at all, I'm afraid.

A properly designed low-power microcontroller device can run for several years on a small button cell, waking up briefly and doing things once in a while.

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    \$\begingroup\$ Thanks. I am going to look into these deep cycle batteries but I will likely go with a micro controller based on this info. If I need to take a hybrid approach, I could have the micro controller trigger boot of the RPi for a short period of time. \$\endgroup\$ – user391339 Jun 24 '15 at 23:25
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    \$\begingroup\$ Yes, that would be a valid approach for sure! \$\endgroup\$ – Spehro Pefhany Jun 24 '15 at 23:26
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Instead of adding in series why not add them in parallel so that voltage will stay constant.

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  • \$\begingroup\$ Is that basically hooking up all + and - battery leads to the + and - on the raspberry PI? \$\endgroup\$ – user391339 Jun 25 '15 at 0:33
  • \$\begingroup\$ Yeah exactly..All + connected and in other end all - connected \$\endgroup\$ – gzix Jun 25 '15 at 0:35
  • \$\begingroup\$ Still won't get months worth of capacity. \$\endgroup\$ – R Drast Jun 25 '15 at 11:31

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