twin isolated but charging battery project

[wickermonkey]

Right, ive got an idea but im not sure if its worth doing. Ive just bought a new battery for the minor as the one that was in there was a bit weedy and was struggling to start the car in the sub zero arctic conditions. anyway i have got a new slightly higher rated battery and the original one thats is still ok to use.
What i wanted to do was to have a second battery in the boot that was charged while driving but was seperate from the rest of the car, this would be so i could hook up a stereo plus as i do like my sound quality (not booming bass) a couple of amps will be used. Plus i go camping quite often so i could use this battery for various things without draining the main battery.
problem is in not sure wiring wise how/where to hook it up the the alternator (running a 1275 marina engine) so that its isolated from the rest of the electrics

Any ideas/soloutions?

[GeordieDave]

Do a quick search on ebay for split charge unit or circuit. They range in complexity and price but something simple should do the job for you. Make sure you use suitably rated wiring and that the second battery is securely fastened. You'll find lots of information on ICE websites as what you're planning is quite common. I bought my split charger from nutsaboutvw on ebay. Mine charges my GPS tracker backup battery.

[beero]

I believe you can get similar from a caravan spares retailer as this is a common setup for caravan batteries.

[bmcecosse]

As above - or you can just charge it manually with a switch that you put on when you want the second battery to take a charge. You would do this after you thought your main battery had recovered from starting the engine - perhaps after a few minutes, and would switch off in traffic that sort of thing. You can also use a largish diode - connected through your ignition circuit to the second battery. It will always hold that battery 0.7 volts below the main battery - but since it will rise to 14.5 volts under charge - that still pulls the spare battery up to 13.8 which will give it a nice charge.

[kennatt]

if you want a simple system,go to a camping/caravan supplier and buy an electronic charging relay,Run a line from your battery via the relay,they are voltage sensitive so dont switch untill the charge is above 12 volts,to the battery in the boot,earthed of course. Then as soon as the relay gets enough voltage it will switch and also charge the spare battery.But I would think you would need an alternator to do this, a dynamo would struggle.I have this system on my van,saves running wires to the ignition to feed a mechanical relay .

[wickermonkey]

Thanks for all the replies, gone for the NUTSABOUTVW's off ebay at the moment. Id looked on google lastnight but could only find people (4x4 people) who want more power for winches n stuff.