Powering stuff from a car battery

[Toby]

How much current can I safely draw from a car battery before I need to worry about actually starting the car?

Basically am going to be away from a mains power source for five days and will need to use an invertor to charge things like cameras, phones & laptops. Whilst I can run the engine, it's far more convenient to just leave such things on charge in the car overnight. I can't imagine such items would draw much current so I'm hoping I'd be safe leaving things like this charging without the battery running without fear of flattening it?

I have a power meter so I could measure the actual draw with various items plugged in but how do I find out how much is safe to pull before I need to start the engine?

[LeperousDust]

I can't help really but take a spare (or some jump leads) ?

[iCraig]

I think it all depends on the age and performance of the battery, and how much is needed to fire up your car. Remember that a battery can look perfectly charged by having your lights and radio on etc, but it takes just that little bit more to start the actual car.

In regards to your actual question, my radio has a timer on it that won't go more than 2 hours on battery power before switching to stand-by. Maybe that's a rough idea than any more than two hours worth of draw will risk the upper power of the battery needed to restart the engine?

[Toby]

Spose I could borrow a battery charger and do a test run one weekend - if I end up with a battery too flat to start the engine then at least I can get it going again with the charger

[Darrin]

Coleman Jumpstart pack. Something like a 25 quid one from Tesco's. They generally have two power sockets on the front and can either be recharged with a cord to your lighter socket or direct connect to your battery (reverse jump start).

Mine is over 7 years old and will still run my 100W inverter powering my son's Nintendo DS for 12 hours.

[Toby]

Hmm interesting, not considered that.

So you charge it up in the car whilst the engine is running then use it to power other stuff?

EDIT: Hmm, can't seem to find any links to a Coleman Jumpstart, possibly an older discontinued product?
Halfords seem to have this type of thing but they're £70+

[Dr. Z]

You can get them for cheaper than that, have a look about ebay and the likes.

The problem you have is one of "how long is a piece of string". If you know the power draw of the device and the efficiency of the inverter you can calculate how many amps it will draw from the battery and from that you can work out roughtly how long it will last until it does totally. Then you can guess how long you will actually get before you need to stop using it in order to start the car.

[Toby]

Well I can easily measure the draw of the various devices I need to power but how many Ah is a typical car battery? (Spose I could actually look at it and see if there's a rating on the top )

[Dr. Z]

Mine is 110Ah but then its a huuuge 6cyl BMW-style battery. Yours will probably be around 80 I would have thought, with puny engined things being about 55 or so.

Not like its difficult to check anyway!

[Darrin]

Check with Asda. If they truly are like Walmart, there'll be a section back in automotive where they sell batteries and chargers. In that same area will be the jumpstart battery packs. If all you're really doing is charging a few small items off of it, you won't need all that big of a battery. I don't remember mine exactly, but I don't think it's much over 10 or 20Ah. And it will power a 3W flouresent light for well over 14 hours. Hell, mine operated an 800W inverter for a couple hours!!