V50 Electrics : Towing electrics - charging capacity

[Simon J]

Having fitted the proper Volvo cable harness in my V50 for towing I'm wondering how the car controls the current through pin 9, the caravan/trailer battery charging circuit. If the trailer battery is heavily discharged it will try to draw quite a substantial current via pin 9. How is this limited to prevent overloading the wiring in the car?

[Clan]

Originally Posted by Simon J

Having fitted the proper Volvo cable harness in my V50 for towing I'm wondering how the car controls the current through pin 9, the caravan/trailer battery charging circuit. If the trailer battery is heavily discharged it will try to draw quite a substantial current via pin 9. How is this limited to prevent overloading the wiring in the car?

there is a fuse in the fuse box the line there is no control circuit just an ignition controlled live feed from the 12 volt power distribution system . What goes on in the trailer module is a grey area though !

[Simon J]

Indeed, perhaps it’s the trailer module that controls such things as one couldn’t just connect the alternator to the trailer battery without some control over the current.

[Clan]

Originally Posted by Simon J

Indeed, perhaps it’s the trailer module that controls such things as one couldn’t just connect the alternator to the trailer battery without some control over the current.

well you can as the control is in the alternator just as for the main battery … your trailer battery would effectively be connected in parallel with the main battery when running .. the system voltage will never go over the prescribed nominal 14v what ever the battery circumstances .. just the current will vary and reduce as the battery gets charged up . a flat battery will rarely take more than 40 Amps and even then that rapidly reduces as the charging gets under way ..

[Simon J]

My concern would be that the relatively light gauge wiring in the car that connects to the towing cable harness possibly couldn't handle 40 amps. But it makes sense that the charging current would be limited by the trailer module.

Does the trailer module also sense when the engine is running before activating the fridge circuit on pin 10?

[Whyman]

Originally Posted by Simon J

Does the trailer module also sense when the engine is running before activating the fridge circuit on pin 10?

Yes it does. Just to be sure I tested mine, with the engine running no power at pin 10 but as son as you connect, the caravan in my case, the pin is powered up and the fridge is working on 12v.

The Volvo system also checks the trailer lights and tells you if you have a failure on the trailer lights so no need to check them before you set off. No warning means no problem with the trailer lights.

[Clan]

Originally Posted by Simon J

My concern would be that the relatively light gauge wiring in the car that connects to the towing cable harness possibly couldn't handle 40 amps. But it makes sense that the charging current would be limited by the trailer module.

Does the trailer module also sense when the engine is running before activating the fridge circuit on pin 10?

The Trailer module is fed by a 40A fuse ...

[Simon J]

Originally Posted by Clan

The Trailer module is fed by a 40A fuse ...

Good point! Those wires must be tougher than they look!

With the trailer hooked up to the car I just checked the voltage at the pins and the battery/lights circuit (pin 9) is permanently live, i.e. even with ignition off it's live. But pin 10 (the 'fridge' circuit) is live with the ignition on, even if the engine isn't running. I had assumed that it would only be live once the engine was running in order to avoid power being drawn from the trailer battery if the car battery was weak but presumably it's not live when the engine is being cranked? As I didn't have anyone to help me I had no way of checking this but will do so later today.

[Simon J]

Re the 40 amp fuse, trailer and caravan wiring is rarely more than 2.5mm² (and often only 1.0mm² or 1.5mm² )which is rated at 29 amps. Presumably even with a very flat trailer battery, the charge from the alternator wouldn't exceed this for very long and 2.5mm² cable should be sufficient? (The trailer battery will be used for powering a winch so could be quite low in charge if used for any length of time.)

[Simon J]

OK, I've checked the voltage at pin 10 (the 'fridge' circuit) and it is live once the ignition is switched on and isn't disconnected while the engine is cranking. Can this be right as it would mean that if the car battery was low on charge, starting the car would attempt to draw on the trailer battery via pin 9 once pin 10 was live (assuming standard caravan wiring with a habitation relay).