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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!

user391339
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  • 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. – David Jun 24 '15 at 22:06
  • 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. – user391339 Jun 24 '15 at 22:18
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    If they held enough charge you wouldn't need multiple batteries... – David Jun 24 '15 at 22:19
  • 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. – user391339 Jun 24 '15 at 22:23
  • The USB batteries tend to be pouch or 18650 lithium cells. If you want a stack of them you can buy them individually and just have one charge / stepdown circuit. Beware: Ah values printed on packaging may not be real. – pjc50 Jun 24 '15 at 22:28
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    (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) – pjc50 Jun 24 '15 at 22:29
  • @pjc50 Very good point well made. I have bought cheap packs in the (far) past just for their circuitry, to then replace them with a large array of high-grade cells. Of course, these days just buying the circuit has become an easy and viable option :-) – Asmyldof Jun 24 '15 at 23:56
  • Why is solar out? Note that a **very** small wind turbine can easily average > 1 Watt in many cases. | If you have LiIon in parallel (which needs to be done with care and understanding and all cells should be of same capacity type and condition), you can charge all to say 4.1V to 4.2V and then stop charging. This gives say 70% - 80% of capacity and all batteries are "happy enough". This has issues but may not be terrible :-). Charging one of N is easy with relays (gasp). Use a dpdt per cell to switch it off parallel group onto charger (or SPDT with eg grounds still connected). OR ... – Russell McMahon Jun 25 '15 at 01:57
  • DPDT or SPDTs to connect one battery at a time onto load. Care need with changeover. Small series R's can help balancing. eg a VERY SMALL resistor per battery will allow very brief joining of two cells with acceptable intercell currents. eg one cell at say 4V, one at 3V. if C = 1000 mAh then 1A flows when R = V/I = (4-3)/1A = 1 Ohm so each battery has 0.5 Ohm in series. In normal operation at say C/10 this drops V=IR = 0.1A x 0.5 Ohm = 0.05V. This allows brief paralleling during changeover with eg relay. MOSFETS can be used as SPST's to connect cells as desired to commin buses. – Russell McMahon Jun 25 '15 at 02:01

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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.

Spehro Pefhany
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    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. – user391339 Jun 24 '15 at 23:25
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    Yes, that would be a valid approach for sure! – 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.

gzix
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