Im not great with car electrics tbh.

I was in Go Outdoors in Coventry yesterday and they sell big leisure batteries cheap and inverters. Id rather a BIG battery that can do more power for longer. No sure about sourcing wiring yet but i guess anything will do.

I dont think Maplins sell big relays. At least i cant find them on their site. I have found that there is a shop called Jacksons nearby that may be helpful tho.

I ultimately want to try to avoid repetadly discharging the starting battery as they arnt designed to take such beating whilst the leisure ones are to a better extent.

What exactly would a reply be doing is it was between the two batteries? My understanding of relays it that they use small voltage to control a larger voltage. Your going to have to explain in kiddie terms

I think i might start a work-log/website for all this ... if anything to motivate me

I get massive Vauxhall diesel batterys for about Ã,£22.

Quote from: soopahflyI get massive Vauxhall diesel batterys for about Ã,£22.

The problem with car batts is that they die if you discharge them often, which is what would happen if i was watching a DVD on my laptop whilst running say few lights and a fridge.
Leisure batts can take the repeated charging for this kind of stuff.

Ok bumpage for the Electrics Help. Going to add a basic wiring system next weekend if i got time.

My plan it to heat it with small a mobile gas heater and get some plugs in the back. Any suggestions/links?

ok, the relay...

its just like a big switch, that you switch on and off with a smaller switch

a normal car relay will take about 30amp, which should be plenty for you tbh... you can get bigger ones that are 110amp tho (biggest ive seen)
you can get a solinoide that will do hundreds of amps... but you cant have them "turned on" for too long or they burn out

(you can always connect up 2 x 30amp relays to give you 60 amp if you like)

ok, so you want to charge your leasure battery from your alternator, then use/drain it wihtout draining your main battery
so you need to connect the 2 batterise together... but only when the alternator is putting our power

your relay will need 4 wires
a negitive connection (from anywhere)
and thick positive wire from your main battery
a thich positive to your leasure batter
then a positive wire that "engauges the relay joining the 2 thick wire connections together

the only clever bit is the positive to power the relay....
on your alternator youll probably have 2 wires, maybe 3... you need the thin one... when the alternator is NBOT putting out power this will be a negative, but when the alternator starts putting out power it will be a positive.
so join the positive for the relay to this.

that way, when you you start the engine and the alternator starts putting out power the 2 batteries will be joined together (and to the alternator) so both will be charged up !

this IS the BESt way to do it.... some people will say use a switch and do it manually... but yuou can forget to switch it on/off and end up with 1 or 2 flat batteries
some people will say power it from the egnition, but then if you leave the egnition on you can flatern your main battery by using it when the engine iusnt running.... and more importantly, when you go to start the engine, it will use both batteries, and try to take 1/2 the starting power from each batterie.... which will mealt your wires to the 2nd one !

for the size of wire to use.... look at the thicker wires on your alternator and use the same size as that, you should never need more power than that  ..... and if you do your doing something wrong !

another colple of things...
wheres the leasure battery going ? when its charging/de charging itll probably vent explosive gas so its better under the bonnet where theres some ventilation
and your invertor will need venterlation too, so HAS to be under the bonnet.

plus.... most leasuree batteries are the same as normaly car batteries so will die with repeated total discharging... there jsut big fookers so its harder to do !
if you want to run it flat repeatidly you need a gel batterie... but there about 3 times the price iirc....
you should be fine wiht the big leasure batterie tho

run everything you can right from the 12v.... the lights etc... its more efficent, and your invertor will have a max output you dont want to exceed.... what that max is depends on how much you want to spend...

what do you want to run from it ? if its jsut the laptop and lights youd be better off getting a car charger for the laptop and not using the invertor for it...
you can use as much 12v power as you like... but youll be limited by the invertor for the 240v supply and the power your using adds up quick !

where abouts are you again ?
I could wire the lot uo abnd show you how to do it at the same time really easily, plus, Ive got all the stuff needed lying around in the garage

sorry about they typing/spelling/babbaling.... i;ve had a few drinks

just realised, i said power the lappy from a 12v car charger if you can/// but yuoull need to make sure one of those can charge it as fast as youll use it....

my brother is a lorry driver, and i know his lappy car charger couldent keep up with him using it so he was planning on getting an invertor and using the mains charger instead !

p.p.s. loads or brand new cheap invertors on ebay !