Nice find - they may come in handy for other projects of mine!alexeames wrote: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.
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).holmez wrote: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.
You should look at some of the higher voltage lipo batteries, like a 14500.MattPurland wrote: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
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.MattPurland wrote: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
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 wrote: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
Sorry to dig up an old thread, but the subject of powering a raspberry pi from batteries has interested me recently...error404 wrote: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).