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If you don't have the split charge relay, then the caravan will be draining your car battery whenever it's hooked up. Not so much for the leisure battery (this will only charge when the alternator's running) but the fridge is a killer.
But with the engine running, you should still get a decent charge to the car battery. I put an ammeter on the feed to the split-charge relay on my last car but one, and it rarely pulled more than 10 amps, even with the fridge on. To put that in perspective, it's about the same amount of power as your headlights would use on dipped beam. The alternator should still be able to kick out plenty to keep the main battery topped up, and the fact that the alternator is connected to the main battery using a short, very thick cable run means that the main battery will effectively get preferential treatment when charging if both batteries are low.
I got the impression that this was diagnosed by the AA as the battery failing to retain a charge even with the caravan disconnected. If that's the case, then the caravan may be putting additional strain on the system and highlighting the fault, but the underlying problem is still with the car battery.
As for leaving your courtesy light on for a few minutes being enough to kill the battery, that's ridiculous! I've accidentally left mine on overnight before now, and had absolutely no problems starting the car.
Currently in Horatio's garage:
2010 Laguna III Initiale.
Last edited by Horatio; 28th June 2007 at 06:11 PM.