Portable 5V Power Supply

[Aaron]

Well, this sorta fits, cause its to run some of my computer equipment. Basically, I'm looking for a portable, rechargable 5V power source. I was thinking of getting 4 NiHM batteries, which would give me 4.8V, which should be sufficent.

The problem is, the AAs don't deliever much amperage, and putting them in parallel would be a pain, cause there'd be too many to recharge seperately.

So what can I do to get this power source? I'd be ok with something non-rechargable if it'd last forever (like running off a car battery with some sort of voltage conversion logic).

Anyway, hope I explained that ok.

Thanks

[Cool Matty]

Car batteries don't last that long, but they are rechargable http://forums.massassi.net/html/biggrin.gif

As for what you want to do, you might want to look into a higher voltage battery than you need, then maybe putting some resistors on it, to lower it to what you need.

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[Emon]

How about some sealed lead acid batteries? (http://www.allelectronics.com/cgi-bin/category.cgi?category=160400&type=store) That 6V 1.2AH one looks pretty good. About 4x1x2 inches, it's pretty small too. Put them in parallel for 6V and 2.4AH.

Or maybe some lithium ion cells (http://www.allelectronics.com/cgi-bin/category.cgi?category=160550&type=store)? If you ran two of that first one listed in series, then another set in parallel with the first, you would have 7.4V and 2.8AH. A cheaper option is using that second listed one, just getting two in parallel for 7.2V and 2.4AH.

Recharging the lead acids is easier since you only have two cells in parallel, you just hook it up to any 6V charger that can charge lead acid batteries. Check the same site for such a charger. They should also have quite a few lithium ion and NiMH chargers.

Speaking of NiMHs, this (http://www.allelectronics.com/cgi-bin/category.cgi?category=160200&item=NMH-2100&type=store) cell does 2.1AH, put four of those in series and then parallel that set and you have 4.2AH! All you need is a high voltage NiMH charger and you could charge the whole unit at once.

Infact, they should have just about any charger (http://www.allelectronics.com/cgi-bin/category.cgi?category=965&type=store) suitable for this application. With the high voltage NiMH and Li-Ion chargers you could easily charge your entire unit at once. Use some power resistors between the charger and the batteries if you feel it'll be putting too much juice in. Just measure what you'd need with ohm's law and test it with a multimeter to make sure it's getting the right voltage. All Electronics should also have plenty of power resistors. But putting a 5 or 6 volt source in a 5-9 volt charger should be fine.

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Bassoon, n. A brazen instrument into which a fool blows out his brains.

[This message has been edited by Emon (edited August 12, 2004).]

[Aaron]

Well, ok, not a car battery, but I have this latern battery that I've been using for years now.

Anyway, I like that lead cell you pointed out Emon. The power curve on lead-acid cells isn't great. Actually, that first lithium ion cell is pretty nice and portable too. Though if I can charge a lot of lead cells at once, I want to go that route.

So, my device runs at 5V and 2.5A. But, how can I apply Ohm's law here? Cause I have an input and output voltage, just not a constant one? Maybe it's something really obvious and I'm missing it. Here's what I can do:

V=IR
5=2.5 * R
R=0.5 Ohm

But that doesn't even consider the input voltage. Maybe I have the wrong idea, all I ever took was the really basic electronics course.

[Emon]

Wait, ohm's law? Er, I don't think it applies. Nevermind. Not sure what I meant.

You can charge a lot of lead cells at once so long as they're in parallel. For example, two 6V lead cells in parallel will work on a 6V lead cell charger, it should just take longer.

Lithium ions are probably better, they're much more efficient. I'd just take two in series, then two more and put them in parallel with the first two. Or if you're using the 7 something volt ones, throw two or three or even four in parallel and use a proper charger for 7 something volt li-ions.

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Bassoon, n. A brazen instrument into which a fool blows out his brains.