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I understand that I can use 4AA batteries and a step down module to get 5V and power the Pi, however briefly. However, now I'm getting real outrageous and want to trying powering the Pi from just 2AA, even if just for 5 minutes.

The deal is, I have a Raspberry Pi working as my dash cam and powered by a USB cord, but I'm guessing the cord may be easily yanked out if I'm in an accident and my video would get cut off mid recording and corrupt the file (it saves in 10s video segments). However, I would like to have 2AA within my enclosure that take over power supply responsibility in the event main power is lost. I know 4AA is much easier, but takes more room. Instead, I want to use 2AA with this DC-DC step up module. As far as I can tell, I can use this module to change my 2AA batteries into 5V.

Am I correct in my understanding? Let's say the batteries are 1500mAh and in series provide 3v, so 3v/5v=0.6 * 0.85(inefficiency) * 1500mAh = 765mAh, which should power the Pi theoretically for nearly an hour, but I'd be happy for 5 minutes. Please tell me if this will work. Thanks!

Ghanima
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jake9115
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  • Yes, that will work. However how are you going to ensure this power supply takes over? – joan Jan 01 '16 at 10:02
  • @joan I'm not sure about how to make that battery take over. I see Adafruit has a rather expensive ($25) module that does this, I think it is called PowerBoost1000. Looking for a AliExpress equivalent! – jake9115 Jan 01 '16 at 23:08

2 Answers2

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Yes, should do

Just double checking the datasheet of the MT3608:

  • if using AA rechargables instead of batteries, mind the lower voltage (cell voltage 1.2 V), but for two cells in series still above the lower lockout voltage (2 V) of the MT3608
  • at very low input voltages the efficiency could be lower than 80 %, the datasheet (p. 5) lists 80 % at 3 V and a low output current - just to keep in mind if the theoretical runtime is lower than expected

As joan already hinted in the comment there is of course also the issue, how this circuit jumps in if "main" power supply is unplugged. Have a look around for UPS solutions to get an idea of how those ciruits work, e.g. How do I build a UPS-like - battery backup - system?

Community
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Ghanima
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  • Thank you for the info, you've done well to confirm my suspicion. I really find it frustrating that there are no low cost UPS modules readily available. It almost doesn't make sense to me... if you look at a cheap wholesale site like AliExpress, they have <$1 modules for a huge number of purposes, but not a single UPS module for less than $10. I'm very confused about this : ( – jake9115 Jan 01 '16 at 23:10
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I also have an outrageous setup, but with way bigger batteries and is in a server setup.

Your idea will work, as stated by joan. What I would do differently is I'd get a power bank instead. Attach said power bank to power so it's always charging. Then, attach the Pi to the power bank's output. Make sure the power bank supports charging and outputting at the same time. Some power banks disable output when charging.

With a big enough bank and a proper setup, your pi can happily run for hours days at a time.

Here are some numbers:

Power bank: 20000 mAh
Setup: Raspberry pi [MODEL] + 200mA camera

Pi 2        = 15 to 30 hours
B+          = 35 to 50 hours
A+ and Zero = 45 to 65 hours

Also, consider proper shielding from heat, since your Pi and batteries are in a car.

If you really want to use your own batteries, consider this. Oh, and this one definitely supports AA batteries.

Aloha
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  • Thank you for the idea. I do have a power bank already and considered this option, but really size is the factor. I think I'll use 2AAA NiCd with a multi-year shelf life as a backup for my Pi. The thought is, I'm using it as a dashcam, so if I'm in a crash, I just want to know it will keep power for a few seconds as to save my video file without corruption. A pair of AAA batteries up regulated to 5V should do this. Now to solve a cheapo UPS circuit! – jake9115 Jan 01 '16 at 23:12
  • @jake9115 or get a smaller powerbank ;) – Aloha Jan 02 '16 at 01:06
  • Another, perhaps more vital factor to consider than shielding from heat is that of vibration - auto-mobiles are subject (despite what the manufacturer's may say about the smoothness of the suspension systems!) to various forces in all three dimensions - so your unit must be built to take them - especially in the case of an accident for which you want it to remain functional even though there can be 10s of G applied... – SlySven Jan 03 '16 at 18:56
  • @SlySven The Pi (and the power source most probably) doesn't care about vibration at all. – Aloha Jan 04 '16 at 01:00
  • @SlySven In the case of an accident, the SD card is the most important thing (where the evidence is stored, OP didn't mention). It can easily survive 500G. A car crash at 200mph is about 100 to 200 G. – Aloha Jan 04 '16 at 01:15
  • But conventional batteries must be securely kept in their holder and not shake out or fly out in an accident - of course such things have a relatively high density so will experience larger forces than other parts of the assembly ( Force = mass x acceleration )! - and the OP may be in for a bit of a surprise if they really do think to "use 2AAA NiCd with a multi-year shelf life" - they may be better off with a pair of "Super-Caps". – SlySven Jan 04 '16 at 05:04