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Split charge

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I have a Swift Sundance 520 based on a 2000 Ducato 1.9 base. I have just taken the van on a 10 mile run and parked up on the drive with the engine running. I have put my volt meter on the cab battery and it reads 13.8v I then put the meter on the leisure battery and it reads 11.3v. When hooked up to the 240v mains and the charger is switched to the leisure battery it reads 13.5v. 

Q does the engine have to reach a certain rpm before the relay kicks in or is it goosed.?

i cant find where the relay is located or what it looks like.

Any help would be welcome .

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Hi Jezz,

I am not familiar with the charging circuit of your motorhome but as I understand it there are at least three variations of circuit used for split charging.


A simple make/break relay (Lucas 6RA relay?) can be energised from a 12Volt signal that is only present when the engine alternator is running, so it is driven from the "charge signal" wire on the alternator. Thus when the engine is running, the engine battery and the leisure battery are connected in parallel and share the available charge current. This system ensures that the leisure battery is isolated from the engine battery while the engine is cranking but is connected once the engine starts. It ids disconnected again when the engine stops, so the leisure battery can be used to power the equipment in the habitation section of the motorhome but will slowly discharge as used (unless topped up from an alternative source, such as from a 240V mains charger.


An alternative system is to use a voltage sensing relay to monitor the engine battery volts. The VSR will close the contacts to connect the leisure battery in parallel with the engine battery at a pre-set engine battery voltage (typically 13.3Volts) and the contacts will open again if the engine battery volts drop below another pre-set voltage (typically 12.8V). This system gives charging priority to the engine battery and only charges the leisure battery once the charge used from the engine battery to start the engine has been replaced from the alternator. When the engine is stopped, the two batteries remain connected in parallel for a short time until the engine battery terminal volts drop below 12.8V, whereupon the relay opens and isolates the leisure battery. The ralay may be dual-sensing so that if the terminal volts of the leisure battery are raised by using an external charger, a charge can then also be passed back to the engine battery. 


Instead of an electro-mechanical relay, a third system relies upon a solid state diodes. This  system allows current to flow in one direction only from a charging circuit to both batteries, but blocks current flow between the two batteries, thus preventing current taken from one battery from depleting the charge in the other. The disadvantage of this is that owing to the volt drop across the diodes, it may not be possible to achieve a full charge of the batteries from the alternator. I believe it is possible to update the alternator charge regulator circuit using after market equipment to overcome this but I have no experience of these systems.


In addition our motorhome has an "Aux Start" switch that if held down, bypasses the split charge circuit and allows the three leisure batteries to be used in parallel to start the engine in an emergency, although I have never put this into use.


I hope this is some help.



RV Aux-Start switch.jpg

Aux-Start switch circled on the left, adjacent tot the mirror heater switch.

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Thanks gordon

electrics have always been a " black art" to me, i always thought, in a nut shell, once the cab battery was full the relay topped up the hab battery. 

Not knowing what system i am running at the moment iam in the dark. Think i should use the hab battery more and check that its topped up on my return to base by the alternator. If its not i will look at replacing the relay

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