How long can a car battery power delivery 500 Watts for[?]

[cloneman]

I don't know how to calculate this. I'm wondering how long I can run a 500 Watt load (430Watts + 12V inverter inefficiencies) on a decent car battery.

First of all, I guess I need the amp-hours rating of the car battery (can't be derived from cold cranking amps I'm assuming)

any suggestions (other then running an experiment, heh)

[andymcca]

You probably want a deep cycle battery (marine type, like you'd have on a boat). These are built more for charge-discharge scenarios. Normal car batteries are not really designed to be drained fully.

As for the A-h rating, I can't think of any off-hand, but you can actually buy several different sizes.

[Erelyes]

Andy is completely right. Draining a normal car battery more than a few times will kill it. You want a deep cycle.

I did some researching and some Exide deep cycle commercial batteries have a 200 minute reserve capacity @ 20A. (about 240w) (http://www.exide.com/portal/server.pt/c ... commercial)

So call it an hour and a half. But those are very heavy duty, they have double the CCA of your typical car battery.

[Greg F.]

Erelyes, that sounds about right. I use 100aH Gel batteries and they won't last all that long with that kind of draw without being recharged. I know people who have had batteries such as mine for ten years and they tell me to get that kind of life out of them you need to never let them go below 50%.
So you are looking at a "usable" reserve. First thing you need to do is see if you can lower that 430 watt load.

[jhhoffma]

A lot of car audiophiles use Optima Spiral Cell batteries when doing dB competitions. The deliver a lot of power, can be deep cycled and keep the voltage up as the battery drains. Most batteries will drop the voltage as the charge is used up, which most electronics don't like so much...