jdogg Posted May 16, 2015 Share Posted May 16, 2015 i've got an old motorhome that's had many owners in the past and i made a discovery today. to cut a long and boring story short i've realised that my leisure battery and the starter battery are connected in parallel. the leisure battery is 85A and the starter battery is 110A. i realised something might be wrong when my solar charger had cut off this morning and the voltmeter that i though was just connected to the leisure battery was reading 14.6 volts and rising. i installed the 80 watt solar panel a few days ago when i thought the batteries were isolated but now i'm thinking that having 2 different capacity batteries in parallel charging is going to cause overcharging in the lower capacity battery. for now i've disconnected the solar panel until i can get my head around it. am i right about the overcharging? and would someone who knows about this kind of stuff recommend that i disconnect the batteries from each other? i can't imagine why they were connected together in the first place but for someone to do that i can only assume that they knew what they were doing. maybe not. i'd love to be able to throw a load of money at an auto electrician and have them sort out all the botched wiring that is going on in here but i can't. basically. does anyone else here have both batteries connected in parrallel? is that normal given the different capacities? cheers. / john Link to comment Share on other sites More sharing options...
MagHal Posted May 16, 2015 Share Posted May 16, 2015 My Avantgarde came with a solar panel fitted. As far as I can tell the vehicle and leisure batteries are separate. When I switch from aux to veh I get nothing (so I assume the vehicle battery is not available to run interior 12V). With a solar panel fitted and constantly charging the leisure battery there doesn't seem to be any point in having the vehicle battery as a back up and risk having a flat battery and no way to start your bus. I do have 2 batteries in parallel however charged from the solar panel. Link to comment Share on other sites More sharing options...
colin Posted May 16, 2015 Share Posted May 16, 2015 Are you sure there's not a split charge relay between them, if not instal one. Link to comment Share on other sites More sharing options...
Guest Joe90 Posted May 16, 2015 Share Posted May 16, 2015 If the batteries are wired in parallel you'd need some pretty beefy cabling to handle the current draw for the starter motor, sounds like an accident waiting to happen, or some seriously compromised cabling that as yet hasn't been discovered, I'd keep a fire extinguisher handy if I was you, or seperate them pronto :-S Link to comment Share on other sites More sharing options...
jdogg Posted May 16, 2015 Author Share Posted May 16, 2015 Thanks for the replies people. You've all given me some good info to go off. Turns out my control board does have a main switch to go between starter and leisure. It's called a hauptschalter - my MH is a German import, I have a manual but that's in German too. It does have a split charger too and I think it's a relay activated one. So it looks like I was wrong to assume that the batteries were wired in parallel - I just couldn't make sense of the readings I was getting on my voltmeter when I'd disconnected my leisure battery. Still can't but I'm happier about the situation than I was a few hours ago. It's a big learning curve going from complete novice to full time motorhomer. I'll be doing some more experimenting tomorrow to understand it a bit better but I'm very happy right now to see the voltage on my leisure battery drop when the water pump is on. My main switch must have just been in the wrong position all this time - in had no idea what that switch was for until now. :-D Link to comment Share on other sites More sharing options...
aandncaravan Posted May 16, 2015 Share Posted May 16, 2015 Yes you are correct that the batteries will be either overcharged or undercharged, depending on the charging technology. Some charging systems 'monitor' the battery voltage and charge according to the voltage it 'sees'. So your smaller battery will recharge more quickly and reach 12.9 v well before the other battery. The charger doesn't know there are two batteries connected together, it just sees the voltage of the most charged (smaller) battery. It thinks it's fully charged and goes into 'trickle' charge mode. Obviously leaving the bigger battery only partially charged. It is more complex than this, but explains the point. Another scenario might be that the 'Charger' (either Alternator, 240v or Solar dependent on technology) will 'see' that the bigger battery is discharged and pump out a big current which both batteries will get as they are connected together. The smaller battery may get more than ideal. That is why batteries connected together should always be the same age, size and technology (you obviously don't want to pair a Gel with a Wet Acid battery). You also have the issue that if you run down the habitation battery, you run down the Starter battery as well and risk not being able to start the engine. In the event of a low Starter battery, as Joe90 points out, the Starter Motor will try to draw its current from the habitation battery, possibly via very thin wires? Hence the risk of burnt wiring/fire. The obvious question is why was the original split charge mechanism de-activated?. Our advice would be to find out and fix the issue as in almost all cases the same mechanism that joins the batteries together for charging also 'tells the Fridge' that the Alternator is spinning so power is available to run on 12v. However, assuming that whoever bodged connecting the batteries together also put in a bodge to get around the Fridge issue, then the cheapest solution would be to fit a split charge relay between the batteries that is activated by the D+ output on the alternator. The D+ is the output that puts out the ignition warning light, most Split chargers use this feature to connect the batteries. However, the best/easiest way, is to fit a Voltage sensitive relay that monitors the Starter battery voltage and connects the batteries together if the Starter battery voltage rises above 13.5v'ish. Under normal circumstances the voltage will only rise above 13.5v if the Alternator has started spinning. This type of relay is just wired between the two batteries so very simple to connect up. Durite make a very good VS relay. However, we suggest you ensure you understand exactly what has been done to get the Fridge working to make sure your charging 'fix' doesn't cause an issue elsewhere? As we say above, the best long term solution (for any future owners as well) is to put it back working the way it should be. For example if you took the MH as it is now to a garage to have work done they may well disconnect the Starter battery. They then know that doing this will isolate the power while they play with the ECU/whatever. However in your case the wires will still be live, powered by the Habitation battery with potentially disastrous/expensive results. When we get in MH's that have had a 'cheap fix' applied it complicates any repair. It can take us hours to sort out what is working and what isn't and why. That is obviously expensive for the owner. If the electrics break when you abroad and are 'none standard' that can be VERY expensive, not just in terms of longer time to fix but also the consequences of the wiring not being as expected causing additional damage. We have read advice on the Forums that connecting the batteries together as a bodge is ok, but it really, really isn't a good idea. The long term cost effective solution is to fix it they way it was intended to work. Link to comment Share on other sites More sharing options...
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