anyone done this to a 300tdi????

i have a split charge relay and a spare battery and was hoping to use it to power the lamps on a roof bar and then posibly a small winch (but not sure yet)

is it worth it or will the standard alternator and battery cope with normal electrics plus 6 more 100+watt lamps? (2 on the bull bars anbd 4 on the roof bar)

Split charge systems work perticulaly well if you are not running the engine all day but want to have the power on for fridge or inverter etc.

It may be just simpler to get a larger battery that can cope with a deep cycle if you are not using the power for too long without the engine running.

It's a good idea if you can fit it in. At least you won't affect the cars normal system if you melt the battery down. Don't expect split chargers to give too much current to the spare battery, usually enough to trickle charge and run a fridge also most standard car batteries will not cope with several hundred watts being drawn. So be warned, they may overheat and explode. Years ago I did the same with a Ford Transit fitted with 4 sets of triple air horns and 1200 watts of spots. It worked well when we wanted to make a path down the motorway but the smell of battery fumes and wiring got a bit worrying. The combined rated current was around 150 amps. Without the proper wiring, relays and battery you'll get poor results. As James said, you need special batteries and they must be fitted somewhere safe because the discharge could cause them to vent explosive fumes. It wouldn't hurt to check with a battery specialist first.

Watts = amps x volts
tot up the total watts you want to run and divide it by the 12v of your car. This will tell you how many amps you will need. Check the amp rating of your battery and alternator output.

Second systems connected to winches and things are quite good as you can drain a batterry while winching if you are by yourself, then when free from the difficulty you have no power to start the vehicle. Other great thing about a second battery system is that if you leave your headlights on at work all day, you can come out and jump start yourself from the 2nd system. The split chargers are only one way usually so charges both but any current draw is not coming from both.

the unit i've got has a little box full of dancing elves that charge both but disconnect the 2nd one when the engine isn't running to stop such a thing happening.

check out the marine sites for examples of split charge systems. esp ones like the narrowboat sites.
ie,

http://www.tb-training.co.uk/SPL-BAT.html

this one is better.
http://www.tb-training.co.uk/MarineE09.html

kinell dormouse i read that and was trying to work out why the royal marines would want a split charger

nah, thats the household cavalry.