Thread: highout, small capacity battery?

Originally posted by jshrieve:
<STRONG>

I'd go DC-DC if there were inexpensive ATX ones, but there aren't afaik.</STRONG>

You could use a capacitor and a diode.

Or you could buy some C sized Nickle metal hydride cells and make your own 12v battery, or you could use 10 nicads from a hobby shop that are used for RC cars.

What would you consider an inexpensive ATX DC-DC power supply costing? 20 bucks? 60 bucks? 100 bucks?

Jeff_

You'd need 1.5 or 2 farads to cover 3 seconds without dropping too much voltage.
That's probably $150-$250.

C sized nimh, you need a lot, as you'd not only have to put them in series to get 12V, but you'd have to put more in parallel to get the necessary minimum current. The max discharge rate of most small nimh batteries is too low.

I would pay probablu up to $100 for an ATX DC-DC that supplied at least 55W and all the proper voltages.

I was looking at http://www.ibsa.com/estore/view_prod...number=SLA0846
Two of those in parallel might due. The maximum discharge rate isn't listed, but it's likely to be able to put out at least 6A since it's an lead acid battery.

Or maybe this: http://www.ibsa.com/estore/view_prod...number=SLA1000

Originally posted by jshrieve:
<STRONG>
C sized nimh, you need a lot, as you'd not only have to put them in series to get 12V, but you'd have to put more in parallel to get the necessary minimum current. The max discharge rate of most small nimh batteries is too low.

</STRONG>

You could use C sized Battery packs for RC cars. The ones that run with modified motors use 7.2 volt packs with 3000 mAH batteries. They drive RC cars for 6 minutes at 15 amps or so. You could pick up the lower quality type for 15-20 bucks.

I have a panasonic LC-RB126R5P Sealed lead acid battery sitting right here. It is 6.5 amp hours @ 12 volts and is 6" by 2.5" by 4" big. It would do the trick. You can buy them at the local hobby shop for RC Plane field kits, they run 20-30 bucks, 6.5 amps for one hour. How pressed for space are you? I'm pretty sure you could even use AA sized nicad batteries, your only talking about a few seconds here, the max discharge rate is just a ballpark figure given over some unit of time so you don't kill the life of your battery. If you know anyone into Electric RC cars, find out how many amp hours their batteries are and see how long they run their car for. I think you'll find that some of them pull 15+ amps out of C sized cells.

That battery you found it the link would probably do just fine, but you could probably rig something up for less than 40 bucks. How are you going to keep it charged?

Jeff_

Here ya go..

This one

or

Here's one

or

Maybe this one, Probably not at 6 amps for very long.. :-(

or

If you only need 3 seconds...

Jeff_

Originally posted by jshrieve:
<STRONG>You'd need 1.5 or 2 farads to cover 3 seconds without dropping too much voltage.
That's probably $150-$250.
</STRONG>

Actually I just did some calculations, and if you want 3 seconds, and your cap is charged up to 13.8 volts, and your inverter cuts out at 11 volts, you need 6 farads to cover 6 amps.... :-)

Chances are that your battery voltage isn't lower than 11 for more than a few hundred milliseconds at a time during cranking however...

Jeff_

Thanks for the links and tips Jeff, very very handy.

Looks like maybe I'll grab that first 4AH one. That'd do the trick and let me run my system under a typical load (3A includes inverter overhead) for over an hour before touching the main battery. It'd fit pretty easy too.

As for keeping it charged, that's a good question. I'll have to think a bit more about how I want it to work.