split charging problem

[Geoff Cole]

Hi fellow techies,

 

Interesting problem with electrics I thought I would share with you.

 

I have had a problem with the battery charging system fitted to my 2012 Vauxhall Vivaro campervan, which I recently acquired.

 

It became apparent that the leisure battery was not charging when the engine was running, this was traced to a blown fuse on the interconnecting battery cable, I replaced this and all seemed ok. After leaving the van for a week or so I noticed that the both the cab battery and the leisure battery voltages were down to about 12.2v and were both reading exactly the same voltage. I took the van for a run and both batteries were reading only 12.5v so I plugged the van in on hook up so the onboard charger would bring the leisure battery up. However the charger did not appear to be putting in sufficient charge;

 

investigating further revealed that both cab and leisure batteries were connected together and there was current flowing between them (about 3 amps). The charging system uses a heavy duty Voltage sensitive relay (VSR) which is connected between the 2 batteries, the cab battery being the voltage sensing battery, with the engine running, the voltage on the cab battery rises to 13.4v the relay switches connecting the 2 batteries together for charging, the relay is supposed to switch off when the voltage of both batteries drops below 12.8v. This means that both batteries will draw current from each other until both battery voltages drop. This is not good for the batteries, in my case the battery voltages were always 12.5v or less, and the VSR was not dropping out.

 

Hereby lies the problem if you hook up after parking with the on board charger switched on, the batteries will still be above 12.8v, the VSR relay would not have dropped out so both batteries are joined together through the VSR, and the on board charger will keep the voltage above 12.8v and will be trying to charge on both batteries, there is also the possibility of the habitation equipment interfering with the vehicle systems through feedback from the leisure battery through to the cab battery. When the hook up cable is unplugged both batteries will be fully charged and connected together so when the vehicle is started, current is being drawn from both batteries, hence the blown fuse. The same scenario would happen if parked for short period, both batteries would be interconnected if the voltage on either was high enough to keep the VSR switched on, I also noticed that the VSR was getting very hot it did not switch off until both batteries dropped below 12.5v.

 

A VSR does not require a connection to the vehicle alternator so is a lot easier than fitting the traditional Lucas split charge relay, which is connected to the ignition light connection on the vehicle alternator, the advantage of the Lucas relay is that it is switched off when the engine stops, disconnecting the batteries.

 

The system has serious flaws; the use of VSR relays on campers has not been thought out, keeping the habitation and cab batteries connected when the vehicle ignition is switched off allowing current to flow between the 2 batteries is bound to lead to problems. The VSR fitted to my camper had also developed a fault in the voltage sensing circuit keeping the batteries joined together, which is why it was overheating, there was also a constant drain through the relay electronics, draining the batteries when not on charge. To diagnose this fault and overcome this battery problem without interfering with the vehicle electronics took some serious head scratching.

 

My criterion is to have the batteries separated when the vehicle ignition is switched off so that there is no conflict between the habitation equipment and the vehicle electronics. I have solved the problem the following way:

 

I have replaced the heavy duty VSR with a Lucas split charge relay connected through a small VSR making it voltage sensitive so that it only switches when the engine is running and the voltage is more than 13.4v this is connected to an ignition switched supply, I have taken a supply from the aux socket fuse this does not interfere with the vehicle canbus system, when the ignition is turned off the two systems are isolated, this was my preferred option.

 

The other simpler way of isolating the systems, if using a heavy duty VSR would be to fit a switching relay, again powered from an ignition switched supply but switching the neutral relay return wire from the VSR this would switch the relay off isolating the batteries.

 

Having completed the re- wiring, and fitting suitable fuses in the relay wiring, checking voltages and current flows, I now find that the leisure battery is charging and maintaining its charge correctly both on the onboard charger and when charging from the engine, there is no drain between the batteries anymore, the onboard charger is not overworked the cab battery is not being drained and there is complete isolation between vehicle electrics and the habitation systems, problem solved.

Geoff C

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;-)

[Alanb]

Hi Geoff,

 

May I assume the duty of thanking you for a very interesting post. I can follow your logic.

 

I do not suffer from the problem as the "cbe" unit in my motorhome functions in the same way as your solution. (Voltage sensing on ignition key controlled source.) You may wish to note that the cbe unit offers the resulting supply as "simulated D+". This can be used for functions such as automatic step retraction control relay.

 

I first met VSR's on while abroad in a hired vehicle some years ago. They were made by BEP Marine, and the Durite model looks remarkably similar. If you are interested a pdf data sheet can be downloaded from the web, but perhaps you already have it.

 

I have subsequently hired similar vehicles abroad in subsequent years, and I have not noticed the problems that you describe, but I had wondered once or twice. Perhaps I should check the indicator LED when on EHU next time.

 

Just a thought though, the vehicle in question had a compressor fridge drawing about 3,5A when running. Perhaps that pulled the battery voltage down before EHU connection made.

 

I do like your punch line about incorrect assembly. On my vehicle the cbe unit was incorrectly connected to D+ on delivery. The step retraction control and power was via an over fused ignition switch controlled source. If I switched on the ignition whlle on site (to check odometer reading) the step retracted. I resolved to correct this and connected the control relay with 2200MF capacitor to the above simulated D+. ( Step should stay out until engine started.) However on starting the engine, the rpm started to fluctuate, due to the capacitor across the step relay becoming connected to real D+ and pulling the field voltage down so that the voltage dropped, simulated D+ switched off engine speed rose, and cycle repeated. The effect was rather alarming.