My RasPi running off AAA batteries

[MattPurland]

Just thought I'd share my breadboard-based battery pack powering my Pi





Seems to be working a treat, no idea how long they'll last! They're 1800mah Ni-MH rechargeables running through a 5v 1a switching regulator. I might post exactly what I did if I get more time, though it's pretty basic

[blc]

Neat!

Out of interest, what regulator are you using? 8 AAAs seems a bit hefty for a device that needs 5v - are these wired in parallel to the regulator? Only ever used fixed linear regulators in the past...

[MattPurland]

I kinda cheated on the regulator, I got it off ebay:

http://www.ebay.co.uk/itm/390327745637? ... 1497.l2649

Unfortunately it's quite expensive, but I only bought it to save time in building the circuit myself. It's probably cheaper to buy the components and build that onto the breadboard.

The regulator drops out at 7v so I needed something above that, so 1.2 * 8 = 9.6. Could've used 6 but that would only just provided enough (7.2) and that would drop out pretty quick once the batteries started to drain.

[blc]

At least you'll get better power efficiency with a switching regulator over a linear; linear regulators are not exactly known for being the most efficient devices...

Now the challenge is to utilise the GPIO pins in order to monitor the battery level and report that back to the OS...

[MattPurland]

Exactly, I wanted it to be efficient as possible.

That was also on my mind, not sure how to do it exactly but I'll give it a go at some point!

[alexeames]

These regs are pretty good (can't speak for longevity, but they work) and can be had for less from Hong Kong...

http://www.ebay.co.uk/itm/230698024555? ... 1423.l2649

I've seen others on here recommend them too.

[MattPurland]

Gah, how did I not find that before!

[blc]

These regs are pretty good (can't speak for longevity, but they work) and can be had for less from Hong Kong...

http://www.ebay.co.uk/itm/230698024555? ... 1423.l2649

I've seen others on here recommend them too.

Nice find - they may come in handy for other projects of mine!

[holmez]

Apologies if I'm wrong here but...

The voltage on rechargeable batteries doesn't drop until the batteries are as good as drained, I think you're thinking of non-rechargeable batteries.

So I think you could have used 6 batteries quite happily.

[MattPurland]

Ah that's news to me... thanks!

[nadnerb]

Nice work Pedro

[MattPurland]

Hah, cheers mate

[Pencoys]

Nice one Matt. A current project. I could'nt resist it....
Now what about the monitor? Tell All
73 Mike

[Jim Manley]

Apologies if I'm wrong here but...

The voltage on rechargeable batteries doesn't drop until the batteries are as good as drained, I think you're thinking of non-rechargeable batteries.

So I think you could have used 6 batteries quite happily.

The discharge curve depends on what kind of rechargeable battery technology is used, and it always is a curve. Lithium polymer keep their voltage pretty close to maximum for about 90% of the discharge, and then begin falling off pretty rapidly in the last few percent of capacity. Nickel metal hydride starts dropping off noticeably at around 80% of full discharge, and nickel cadmium start drooping at around 70% of discharge. They all slowly lose voltage over their earlier parts of their discharge curves, with the slope being most shallow for Li-poly, somewhat steeper for Ni-MH, and a bit steeper for Ni-Cd. The switching regulator will keep the output voltage constant no matter what the input voltage is for the portion of the curve above the regulator minimum input voltage. So, the only real difference in the battery technologies will be how much discharge memory effect builds up (most for Ni-Cd, Ni-MH has less, but, it does build up over a longer period of time than Ni-Cd, and almost none for Li-Poly).

[MattPurland]

The screen isn't running off the batteries as it needs 12v, I'm gonna rip it open and see if it can be powered from 5v (it'll have a regulator in there somewhere which I can hopefully bypass).

I might look into using higher capacity batteries, I get the feeling the AAAs won't last too long

[MattPurland]

FYI, here's one for £1.39!

This also takes a minimum voltage of 4.5v and an output of 2-3 amps, which should be a lot easier to work with!

http://www.ebay.co.uk/itm/LM2596-DC-DC- ... 248wt_1397

[cowfodder]

The screen isn't running off the batteries as it needs 12v, I'm gonna rip it open and see if it can be powered from 5v (it'll have a regulator in there somewhere which I can hopefully bypass).

I might look into using higher capacity batteries, I get the feeling the AAAs won't last too long

You should look at some of the higher voltage lipo batteries, like a 14500.

http://www.lighthound.com/AW-14500-Prot ... _p_96.html

3.7v, 750mah and the size of an AA battery. One of my ecigs uses them running through a 5v regulator, and it lasts me about 2 days between charges. I'm sure that the Pi would put considerably more drain on them though.

Could also consider an 18650, but they're a bit bigger I think.

http://www.lighthound.com/RediLast-1865 ... _3998.html

3.7v, 3100mah

[alexeames]

FYI, here's one for £1.39!

This also takes a minimum voltage of 4.5v and an output of 2-3 amps, which should be a lot easier to work with!

http://www.ebay.co.uk/itm/LM2596-DC-DC- ... 248wt_1397

That looks like what I've got Matt. By the way, I've done some measurements and my Pi uses ~2 watts at idle* and ~2.5 watts when watching 1080p video. Using a 3 cell lipo (~12v) and one of these LM2596 regs I get ~85% efficiency.

* depends what USB devices you have plugged in - logitech usb kbd/mouse dongle takes ~ 0.1 watt

[MattPurland]

I looked into other batteries including lithium ion and polymer, but I decided to go with batteries that could easily be replaced if they died. It's hard to say without doing proper measurements but I think the AAAs I'm using last about an hour with average use so I definitely need to look at something with more amperage... 1800mah just isn't enough!

EDIT: I think the 18650 lithium ions are the best choice:

http://www.ebay.co.uk/itm/4x-18650-3-7V ... 500wt_1416

You can get cheap chargers and battery clips for these, so the above running in pairs in series would be a good choice giving me 10000mah at 7.4v - 8.4v

[alexeames]

When you say pairs in series, do you mean just two cells at a time or two parallel sets of two?
Just asking because you would only get 10 Ah (@ 7.4-8.2) if you had 4 cells.

Still, let's do some sums for fun
If you had just two of these in series with the same reg I use LM2596S @ 85% efficiency, you'd have...

~5 Ah * 8V * 0.85 = 34 Wh

RasPi uses about 2.5 Watts (we've already stripped out the regulator losses, which were based on 3s lipo measurements).

So 34 Wh / 2.5 Watts = 13.6 hours - not bad. Are those cells for real though? (I mean their capacity).

[MattPurland]

That's correct, the auction is selling 4 batteries, so 2 parallel sets in series. I might try it with just the 2 in series to see how long it'll last. No doubt they're not actually 5000mah, but they should be close

The AAAs I'm using seem to die quite quick, but I think that's down to the USB hub I'm running off it too

[alexeames]

That's correct, the auction is selling 4 batteries, so 2 parallel sets in series. I might try it with just the 2 in series to see how long it'll last. No doubt they're not actually 5000mah, but they should be close

They are suspiciously cheap - that's my concern. But the flip side of that is that it's a low risk outlay. The cheapos are usually a lot less capacity than they state. Most 18650s are about half what these claim.

[MattPurland]

Indeed, but it's worth a pop!

[error404]

The very best 18650 cells available from a legit manufacturer right now are about 3000mAh, 5000mAh is a joke (as are your 1800mAh AAAs). I'd expect maybe 1800mAh out of them if you're lucky, probably less. Also those cells don't seem to have protection, and for a DIY project I wouldn't use unprotected cells unless you are already confident using Li-Ion technology (in which case, add an external protection circuit), it's just too easy to connect something up wrong or jostle something and end up in a bad situation and this chemistry is legitimately dangerous if mishandled.

You can get some decent (~2000mAh measured capacity) protected cells from DealExtreme for not much more money; they also sell chargers and holders. I recommend these cells and here's a holder (similar ones for 3, 4 cells available). I don't really trust their chargers, but they do sell them. You could buy the high quality Sanyo or Sony cells used in laptop batteries, but they're more than double the price for 25-30% extra capacity and unprotected.

A pair will get you somewhere over 6Wh, probably similar to your AAA pack, or maybe somewhat more (good NiMH AAAs are about 1000mAh, yours probably aren't 'good' if they say 1800mAh on them).

I use these regulator modules for...all kinds of stuff. They output 3.3V by default, but a 10k resistor from ADJ to GND gets you 5.05V. They can be had for about $0.50 a piece if you buy 5 or more (they're handy), are pretty efficient (should get around to measuring one) and come in a plastic case. Search eBay for MP2307 or KIS-3R33S.

[blc]

You can get some decent (~2000mAh measured capacity) protected cells from DealExtreme for not much more money; they also sell chargers and holders. I recommend these cells and here's a holder (similar ones for 3, 4 cells available). I don't really trust their chargers, but they do sell them. You could buy the high quality Sanyo or Sony cells used in laptop batteries, but they're more than double the price for 25-30% extra capacity and unprotected.

A pair will get you somewhere over 6Wh, probably similar to your AAA pack, or maybe somewhat more (good NiMH AAAs are about 1000mAh, yours probably aren't 'good' if they say 1800mAh on them).

Sorry to dig up an old thread, but the subject of powering a raspberry pi from batteries has interested me recently...

Cheers for those links; those cells look ideal...

How do you get to the figure of 6Wh for two cells though?

In theory, if you had two pairs of these cells running in parallel, you'd get approx. 4Ah @ 8.4; once you account for voltage regulator efficiency, you'd get approx 27Wh - with the RasPi alone, that's roughly 10 hours of battery life. Unless of course I'm getting my sums wrong: 4(Ah) * 8(v) * .85 (regulator efficiency) = 28.56Wh. Even with only two cells in series, I still get ~13Wh - still 5 hours of battery life.

I know this assumes that everything is running at optimal voltages/efficiencies; I haven't accounted for voltage curves, so there is some margin for error there - but with such a high capacity, would that really make much of a difference...?

My application may require more than 5v though, possibly even 12v - I can't really be sure until I get my "new" toy through! I could end up needing 4 cells in series, and we start to get some hefty battery weights (and costs) if you double that up in parallel...