Aux Battery

Hi guys, first post here, so hello everyone.

Here's what I am trying to achieve:

I've got a 17aH Sealed Lead Acid Battery in the boot (Trunk, to you guys) of my car. I will be using it to keep an in-car-PC (for mp3s/GPS) on standby in my car over night and stuff (to avoid long boot-up times when I get in and drive off). I'm just wondering how to wire it to my main battery.

I want the computer to only be able to drain the Aux battery when the ignition is off. Was thinking about connecting the two batteries together through a relay which is switched by the car's ignition live.

I don't want the car to be able to take power from the Aux battery (eg during cranking). Was thinking diodes could be used here, but have no idea which ones would be best suited as the PC would be pulling up to 15A from the alternator/main battery (plus current to charge AUX battery) so would need a bloody big diode (or smaller ones in parallel?)

The other thing I am not sure is an issue or not, is this: If I didn't use the car for a few days, and the Aux battery was drained down, is it safe to effectively just connect it to the car's battery/alternator to re-chage it, or would the current draw be too great and result in damaging the Aux battery. Do i need to put a resistor in-line temporarily or something until it is part charged?

Can anyone recommend a circuit I could make? I have a few car relays knocking about that I could use, and I'm sure I could get other stuff too if need be. If not, are there any (cheepish) all in one solutions?

Sorry for such a long post!

Welcome.

Any time a battery is installed in the trunk you want to make sure it's got adequate ventilation (most cars already have a vent to the outside somewhere in the trunk).

For what you want to do, I recommend using a battery isolator rather than a 30 amp relay.  A battery isolator is a relay, but it's designed for higher current applications.  They come in different varieties (flavors)- from 80 amps to 200 amps and up.  While you could get away with a simple relay for what you're doing (since you're not using the aux battery for starting or other high current uses), using an isolator will allow you to use bigger wire (like 4 gauge perhaps, as opposed to 12 or 14 gauge) to connect the batteries than it would if you didn't.  This will keep you from having as much a voltage drop in the long run to the back.  Alternatively, if you have an amplifier installed in the back of your car, you could connect a simple relay to it's power terminals since you've already got a sufficient power feed, and all you'd have to run is a trigger wire.

You can ground both batteries to the frame, and use the isolator to connect the positive's together in parallel.  I recommend using an accessory wire to trigger the isolator.  An accessory circuit is nearly identical to an ignition circuit, but the accessory circuit loses power during cranking. 

The car PC would still pull current from the auxiliary battery, but it wouldn't matter because it's isolated from the primary battery under the hood (bonnet), so it wouldn't be taking any current away from the starter.  No need for diodes or anything.

If you will not be using the car for extended periods of time, and fear the battery may get drained, use a deep cycle battery.  Deep cycle batteries are designed to stand up to abuse, and are used to run stereo systems at length while the vehicle is not running.  This does not damage the battery, and even if it's drained below 10.5 volts, it can still be charged back up to full capacity.  That's where deep cycle batteries shine.  A typical car battery will lose around half it's capacity any time it's drained this low.  Another thing you could do is invest in a small trickle charger to keep the battery charged while not in use.

Thanks for the reply geepherder.

Good point about the ventilation. Will have to have a look into that. Didn't even think about that one!

The accessory circuit, I think is what I was going on about (from the back of the headunit)

I presume the battery I bought (Sealed Lead Acid type) is a deep cycle one as it was intended for electric golf carts.

I will look into these battery isolators. Hadn't heard of them before, so thanks.