I received an email from a gentleman who is a motor home owner and tows a car trailer that wanted a bit of help. Here’s what I received:
Dear Caravan Chronicles I hope you can help. I have a motorhome which I use to pull a car trailer to classic car racing events. My trailer currently has a Warn winch to help load and unload my sports car but I have recently had a caravan motor mover fitted to help with manoeuvring the trailer with the car loaded. The trailer has one 110Ah battery fitted for the winch, which I have to charge by plugging in the trailer to a mains socket in the motorhome. I asked the company installing the mover if they could install a second 110 Ah battery for the mover and make it so I could charge both batteries while towing. Unfortunately they said it could not be done. I also asked if they could make it so I could link the two batteries if required with a switch, which they also said couldn’t be done. They did advise me to talk to the people that fitted the tow bar. To cut a long story short they also said what I wanted could not be done. Can you come up with something that would work? Many thanks XXXXXXXXAfter a couple of email exchanges to confirm a few things, here’s what I came up with:
Click on image to see full size
Using the Leisure battery charging circuit to charge the 110Ah motor mover battery and the Fridge circuit to charge the winch battery was a simple task. However, to link the two batteries and make it so they could not be linked while connected to the tow vehicle so as not to cause any issues with equipment installed in the motor home took at least one cup of coffee’s thinking.
When the batteries are linked (it’s incase one or the other is flat so that either mover or winch can be used) there would be a potential for a large current to be drawn… around 80 amps for either the winch or mover so I sourced a 100 Amp DPDT (Double Pole Double Throw) relay. To stop this relay being operated a second 30 Amp relay was used that when the leisure battery charging circuit was live – i.e. plugged into the motor home, it would be impossible for the main relay to operate. A second level of safety was also added in the form of a key switch. Only the person with a key can actually link the batteries and the key would be on the motor home ignition key ring so the engine of the motor home would have to be off and therefore even if the trailer was still connected to the motor home the fridge circuit would be isolated.
Electrical protection would be via three fuses. If any fuse failed the circuit would ‘fail safe’. If F3 failed (protecting the relays) it would be impossible to link the two batteries. The key switch was one that you could only remove the key on the ‘off’ position to stop the batteries being linked, removing the key and starting the engine of the motor home.
The gentleman gave the drawing to his local tow bar auto electrician who upgraded the motor home towing electrics from a 12N to a 13 Pin socket and installed all the wiring on the trailer. He had it all installed and working within a day. The heavy-duty wiring to the winch and motor mover was completed in 16mm cable and the rest was in 2.5mm cable. Additionally, two C-TEK chargers were installed one for each battery so that the trailer could be left plugged in over winter to keep the batteries in good condition. The trailer has been used on five trips and has worked perfectly I have been informed.
It’s a bit of an unusual one and not generally something you would come across, but it might be of use to some of the motor home owners that tow cars on trailers that read Caravan Chronicles.
S
Caravan Chronicles 
HI Simon, when customers go to a garage or dealer etc and ask for something unusual to be undertaken, I hate it when they say to the customer “It cannot be done” this is a defeatist attitude and proves that they are not experts within their own field. Most dealers etc can only undertake run of the mill jobs that are undertaken parrot fashion. I may come across a bit strong on this but it annoy’s me. Colin
Hi Simon, To be honest, I don’t think the loss will be more than 0.6 volt, if even that, but you do need the diode in circuit to stop the tow vehicle drawing power from the mover battery while cranking. Fuse 2 would not blow when using the mover, as you would not be using the mover when connected to the towcar. With reference to using said diode, if there should be a significant difference with the charging level of the mover battery (as said, I don’t think there will be) , then there is a way around it. Fitting a Voltage Sensing Relay (VSR) across the winch battery, this controlling yet another automotive relay, which is connected in place of the diode, but doing this is making everything even more complicated for the installer and the adjustment of the VSR would be very criticle in order to function correctly. Colin
Hi Colin
I see you spotted it about the diode stopping ‘feedback’ when the Motor Home engine was being cranked!
My original sketch had the equivalent of a habitation relay as well until the lightbulb moment and I realised the diode would do the trick. I also had a habitation relay in for the winch battery until the auto electrician confirmed that he could have an output from the engine that only came on once the engine was running.
I did think of using a VSR, but dismissed it for the same reasons you picked up on.
S
Yes Simon, I cannot find fault with your design, when first studying the diagram, I wondered why you have used the large current diode but without going into great detail, I can see why as you would not be able to link the mover battery to the towcar via a relay controlled by the fridge circuit as with a caravan system, the said relay would be powered permanently by the winch battery. The only weakness in the system is the mover battery would always be fractionally undercharged compared to the winch battery due to the slight loss through the diode, but this is unavoidable. Kind regards, Colin
Hi Colin
I did tell the gentleman that there could be a possibility that the mover battery charging could be compromised… I can’t remember what the forward voltage drop was for the diode, but the motor home is a rear engine unit, and the batteries for the trailer are in a aluminium tool box mounted on the trailer “A” frame and apparently are as close to the engine alternator as the motorhome ‘house’ batteries which are half way along the coach. I suspect that the voltage drop won’t really matter too much.
I’m not too sure of the set up, its an American motorhome, the only thing I know about American Motorhomes are what I have learnt from Jason & Nikki Wynn.
I’ve been promised some pictures when the chap gets back home to Germany at the end of the racing season.
S
May be worth considering using a portable solar panel.
Hi
The chap seems happy enough… the generator in the motor home is a diesel ONAN RV QD 10000, so 10Kw, I don’t think power is too much of an issue.
S
Been having a think.
What about using a battery charger through an inverter to charge the trailer/caravan batts.
That way will always be trickle charging & not parallel to tow vehicle
Hi Charlie
You could use an inverter to power a battery charger…. but that would involve quite a lot of loss. Inverters are only usually about 80% efficient and chargers have losses, so maybe only 80% efficient as well. If I was going to spend that sort of money I’d opt for a 12 volt DC to DC charger to get the best charging performance for the batteries.