Query re 12S Supply to Battery and Fridge.

Jun 7, 2012
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My daughter’s Volvo V50 and my Volvo 940 are both equipped with twin 12N/12S towing electrics with “smart” relays for the 12S.

Our caravan is a 1994 Bailey Pageant CD Auvergne fitted with a ZIG X2 battery charger and an Electrolux RM2260 fridge.

When I recently checked the output of pins 2 (battery) and 6 (fridge) of the relevant sockets on each car I found that:
The 940 – with engine on regulates at 13.5 volts.
when engine switched off drops to 12.3 volts then abruptly to 0 volts.
This is what I expected.

The V50 – with engine on regulates at 14.5 volts.
when engine switched off drops to 12,5 volts then abruptly to 0 volts.
Again this is what I expected, although the regulating voltage is rather
higher than I expected.

When the caravan 12S is connected to each of the respective cars and the voltage checked at the caravan battery connections (but with battery disconnected)
with engine on they each regulate at the same voltages as above BUT
when engine switched off the measured voltage reduces gradually past 12.3
volts and continues to reduce gradually (with the rate of reduction slowing
as the voltage reduces) until 0 volts.

I’ve checked the caravan wiring diagram in the owner’s handbook but cannot see any
reason for this.

Could someone explain this to me please?
 
Jul 31, 2009
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You are charging the capacitors in the PSU/Battery charger when connected & are seeing their relatively slow discharge.
Turn one of the 12V lights on in the caravan & you will see the discharge fall much quicker.
 
Oct 30, 2009
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hi Ian,
I read your post twice last night and could not work out what the problem was totally confused
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this morning, I read it again with Nick's answer and finaly the penny dropped
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sorry mate I don't think there is a problem exept in the way you have tested the circuits,
because running the charge circuit without the battery connected will not allow the residual voltage to dissapate as it normally would in use, the battery gives a reverse current to the charge circuit of 12v so the operating range of the smart relay is (13.8v charging) to (12v not charging) a range of only 1.8v not 13.8v to nothing!! as Nick suggested if you had turned on one or 2 of the 12v lights while doing the test you would seen the residual voltage disapate very quickly "almost instantly".
the normal way to test the circuits is "with battery connected" check the battery voltage across the terminals should read about 12v ish dependant of state of battery charge, then start the car engine the voltage should rise to 13.8v and stay there while the engine is running when the engine is turned off the voltage should reduce back down to 12v, by the time you have got back to the van in order to retest the voltage.

2 other points first the differance in the charge voltage between the 2 cars could be down to the difference in the smart relay used or possibly "if one is the type that can be adjusted" it may be set a little high or low but as long as the charge rate to the van battery is within the range as to not boil the battery or give it less than 13.8v this should be ok.
also the pin configuration of the 12s plug was changed post 1998 from pin2 to pin4 for the charge circuit, as your van is 1996 the configuration you have is correct however if you changed the van to a post 1998 on, the wiring may have to be changed in order to charge the battery.
 
Jun 7, 2012
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Thanks guys you've satisfied my curiousity. I like to know why certain things happen, not just that they do happen.
Colin-yorkshire, re you comment as to the different regulated voltage on each car.
I fitted the electrics to my 940 myself using a TEC3M smart relay, which, so far as I am aware, is not adustable but my daughter had her V50 fitted with Witter tow bar and dedicated electrics ordered from Witter and fitted by their local fitting depot so I don't know which relays they used and whether they are adjustable. I'm not duly concerned as long as the regulated voltage is within safe limits for the caravan battery but 14.5 volts seems rather high to me.
Also, thanks for the reminder about the change of pin configuration.
 
Jun 7, 2012
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Thanks Dustydog.
That's an interesting fix which makes a lot of sense.
I've heard of this problem before, but thankfully, never experienced it. My previous van was an '82 Sprite Musketeer which did not have a particularly power hungry fridge, although it was adequate for our purposes. My present van is a 1994. I fitted the car electrics myself and have earthed both pins 3 and 7 to the car chassis although I realise that if there is no earth connection from the van on pin 7 then earthing 7 is superflous. I'll obviously monitor the situation and deal with it if necessary.
 

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