A matchbox sized device providing 12V, 3A for a minute? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: October 15, 2017, 12:04:58 am
content: I have a little computer that i keep plugging back and forth between different places, and i thought it would be nice to not have to power cycle it every time for this.That can be achieved by putting a small UPS inside of it.The thing runs from 12V DC (10.8V-20V, so direct power from a 4S li-po is possible), and can pull up to 3A (about 0.8A idling).However, there isn't much space inside and i'd rather not have a brick dangling from it, so i'm looking for something suitably power dense (2x2x2cm block, or 8x3x1cm block) to fit in.Supercaps can do the power, but they are quite bulky.Li-ion cells can be small enough, but i haven't seen ones with high enough power density. Essentially we are talking about a 50 mAh 60C battery, which does not appear to exist.So, the question is - are there batteries and/or power sources dense enough to do this?What should i be looking for?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2017, 12:16:20 am
content: 12V * 3A * 60s = 2.16 kJ8 * 3 * 1 (cm^3) = 24 cc = 0.024 L2.16 kJ / 0.024 L = 90 kJ/L = 0.09 MJ/LNot counting control circuitry (if applicable), that rules out supercaps:https://en.wikipedia.org/wiki/Energy_densityYou also need power density 1.5 kW/L. If density is about 2, then you need 3000 W/kg, which rules out most batteries. Maybe LiFePO4 will do. You get a lot more energy density than you need, so you can afford excess capacity to make up for the weak power capacity.I say shop for model airplane batteries, see what will fit your volume, and see if they will supply your demand.Probably you'll find one that works, and, that offers a huge margin like 10 minutes or more run time!Tim

author: mikerj
author_rank: Super Contributor
timestamp: October 16, 2017, 11:56:27 am
content: I would look for a higher capacity 3S Lipo rather than a lower capacity 4S. This will increase the available discharge current, and provided you keep the total discharge within bounds there should be enough voltage.e.g. ThisE-Flite 450mAh 3S 30Cpack has a significantly smaller total volume than your 8x3x1cm block specification (though it is 15mm thick, which might be a problem?). You would only be using a small fraction of it's capacity so the voltage should stay well above the 10.8v threshold.

author: ebclr
author_rank: Super Contributor
timestamp: October 16, 2017, 12:14:59 pm
content: I hope this is what you are looking forhttp://www.ebay.com/itm/SOS-Mini-Jump-Starter-Car-Emergency-Charger-Battery-Booster-Power-Bank-20000mAh-/122155271313?var=&hash=item1c71058491:m:moOTTRYdQltWIDKTdJIbazw

author: Artlav
author_rank: Frequent Contributor
timestamp: October 16, 2017, 08:01:07 pm
content: After some looking around and finding nothing really that well fitting, i considered the problem again, and realized that i can achieve the same seamless relocation if the PC is put into a suspended mode and then woken up again.In suspended mode it only consumes 70mA, which is trivial in comparison.Thanks all for suggestions!