Charging a spare additional battery

[Surf01]

At the rear of the Surf there is a 12v outlet and I was wondering if this can be utilised to charge a 85 or 110 amp deep cycle leisure battery.
Obviously it will only charge when the engine is running, but when you switch off the engine is there anything to prevent it draining back into the car system?
Is there a simple way to charge the leisure battery while driving around as it will be quicker and cheaper than buying and using a solar panel?

[spacast]

Have you got a Poundland nearby?...'cause they were doing these for £1
Caddy-Motion in car charger
http://www.motiontouch.com/case-stud...tery-recharger

An in-car caddy-cart battery re-charger. This is a cradle that can be securely and safely installed in the boot of your car for ease of use. The cradle has an advanced computerised charging technology which charges a Caddy Cart battery when the engine is running or in motion.

The cradle also incorporates two LED indicator lights which inform the driver of the state of the charge of the battery at any given time e.g. red indicator reflects that the battery is still charging, while green suggests that the battery is ready for use. There is also a safety coil that stops the charging when the car boot is opened.

As the battery lifespan is important to Caddy-Motion and to battery providers, there is a charging technology within the battery cradle that is designed to control the amount of charge that the battery receives, thus eliminating any over-charging or over-heating of the battery.

When the battery is not in use, both the caddy-cart battery and the car battery are disconnected from each other. This charging technology will eliminate any possibility of either system damaging or discharging the other.

As seen on Dragon's Den
They retailed at about £85, so not surprising they have gone bust

[BUSHWHACKER]

Install a split charge system......


The wiring in that socket isn't heavy enough to charge a battery, it'll melt!

[JD_975]

Caravan relay fella, like this one http://http://www.ebay.co.uk/itm/RING-Smartcom-Caravan-Split-Charge-Towing-Relay-RCT460-/110716269404?pt=UK_CarsParts_V ehicles_CarParts_SM&hash=item1 9c734335c

Plug in devices are only barely any good for maintaining a charge on an already good battery, trying to use one to recover a discharged battery will at best blow fuses, at worst..... well you have seen cars on fire before?

The caravan split charge relay wires up in a similar way to Bushwackers diagram the difference being it will only start to charge your spare battery when the charging goes above 13.8 (ish) volts, and will cut the spare battery off if the charging drops below around 13 volts so the spare battery cannot deplete the vehicle battery's in the event of an alternator failure.

[Maverick]

Caravan relay fella, like this one http://http://www.ebay.co.uk/itm/RING-Smartcom-Caravan-Split-Charge-Towing-Relay-RCT460-/110716269404?pt=UK_CarsParts_V ehicles_CarParts_SM&hash=item1 9c734335c

Plug in devices are only barely any good for maintaining a charge on an already good battery, trying to use one to recover a discharged battery will at best blow fuses, at worst..... well you have seen cars on fire before?

The caravan split charge relay wires up in a similar way to Bushwackers diagram the difference being it will only start to charge your spare battery when the charging goes above 13.8 (ish) volts, and will cut the spare battery off if the charging drops below around 13 volts so the spare battery cannot deplete the vehicle battery's in the event of an alternator failure.

Yep I used one to build a rig which I take to Scotland on holiday. I found out that a 110 battery would only last a day after three hours of in car charging though, so I've got a bigger solar panel for this year to trickle charge it.