Caravan battery charging?

[ericmark]

Please do not move to Cars, I want electrical opinions.

There have been two methods to charge a second battery, split charging diode, and the relay, main idea of both is to allow current from car to caravan but not from caravan to car.

However both methods are rather useless, on start yes may get 15A but 15 minutes latter luck if charging at 1A.

So today we have a third method, the DC to DC inverter, I have been looking at the Ring RSCD30 a bit bigger than I wanted, 15A would be ample, but it will also take the output from solar panels, however then comes the likely problem, it has a safety system to switch it off if the donor battery voltage drops to 12.6 volt.

Not exact as cars can use chassis return but with an alternator producing 14 volt then using 2.5 mm² cable your only permitted around 1 meter of cable at 30A.

Now using a DC to DC inverter I will only need to fit fuses in the car BS AU 177a wiring, so I have to decide before I wire the new tow car what I want to do. But £175 is great if the Ring RSCD30 works, but it is a lot of money if the unit keeps disconnecting on under voltage. And attempts to buy 2.5 mm² 7 core at local supplies have not been successful they only seem to stock 1.5 mm² cable.

The Ctek D250SA is a lower output, but £220 price tag is some what off putting. The Ablemail AMC12-12-30 seems to be OK, but can't find a price.

What I want is a device which if I leave a caravan site with a discharged battery two hours from home will put enough in the battery so I can use motor mover to get caravan to back of the house. The clear alternative is a second battery which if kept charged at home which I can swap when I get home.

So I would guess the device would need to put around 15 Ah of charge into the 75 Ah battery, so 8 amp average is minimum charge.

If caravan charging is out, then I wire car with 1.5 mm² cable and remove the battery to battery fuse in the caravan, if charging is an option even at a latter date, then need at least 2.5 mm² cable to wire car. So don't need to do job of fitting a DC to DC inverter now, but need to work out if it's a viable option before wiring the BS AU 177a to the car. BS AU 149a already fitted.

 

[ban-all-sheds]

https://www.practicalcaravan.com/reviews/accessory/model/4684-battery-chargers

 

[ericmark]

Thank you however all but one are mains powered and odd one is solar powered non are car to caravan. I have been directed to sterling-wildside-caravan-battery-to-battery-charger-12v12v which it seems does have a lower switch off voltage, but at £192.78 I am now leaning to just using 3 core cable and only wiring for fridge and reversing light. And buy a new 110 Ah battery and keep the 75 Ah for emergency to swap at home if the 110 Ah is flat on return.

 

[Keithmac]

If you have to use the mover to get the caravan up the drive I'd say a 2nd fresh battery is your best bet, the battery takes a big hit from the movers.

For us we always have sites with EHU so the battery charges away merrily and mover is ready when we get home.

You could alternate the stay at home battery to get most use out of both.

Be carefull if you have a newish car as the ECU/ ECM monitors and estimates battery life and also does load shedding in certain circumstances. The dedicated towing electrics will talk via canbus to keep it all happy.

Have you tried your car with the headlights on if you are having problems as this has "solved" many SMART charging issues with caravans. The alternator drops off to eek out more fuel economy but lightin load will force the ECU to keep alternator at 14v.

 

[SimonH2]

If you are charging the battery in line with "sensible" practice (and most manufacturers' instructions) then the charging current WILL drop off quite quickly regardless of the current capability of the charging arrangement.
Dad used to have a connecter in the boot of the car wired in parallel with the charging line to the caravan - and when out and about on holiday would put the battery in the boot so it could charge up (it also had a cradle and straps to keep it in place). This was back when they used to go off for a couple of weeks at a time and didn't have all that new fangled mains lectrics in the van - so the battery would need charging up every few days.

 

[SUNRAY]

When I was towing a PA booth I fitted an adverk charging system which inserted a diode in the charging line to each battery. I fitted a 70mm² cable from diode pack under bonnet to a 100A anderson connector by tow bar and a 10mm² twin flex on trailer, this was duplicated for 2 batteries in trailer. This allowed the alternator to charge at a high current for as long as any battery required it. the long run from pack to trailer battery restricted it to 80A which is a bit OTT for the 10mm² but it had to be very very flat for that to occur for more than a couple of minutes.

 

[ericmark]

In the past I have done things to charge two batteries which were not standard. With the old Lucas 18ACR alternator Durite did a replacement diode pack where the field diodes were same size and main ones, I would add an extra 3 diodes for the field 1A was enough, then use the original field diodes to feed second battery. And yes it worked well. I have also mounted three fork lift diodes behind grill and taken the three phases out of the alternator.

Using 65/0.012 cable it would charge a battery in the boot well, but the 7 pin plug would not take that size of cable and as the length increases so the charging dropped off, so it was a case of two caravan batteries and swap them, charging them in car. This was likely a better method as also charging on days out even when not towing the caravan. This was before 1980 as once I started to work abroad we did not use caravan much. And at that time inverters were very expensive. Also electric hook up was rare.

I did consider the DIY approach and use an inverter in car then transfer power to caravan as 230 volt then a charger in caravan. However the charger I have needs the user to press a button and the output is rather low, so not worth the effort, I was rather surprised to see how quickly the battery hits the voltage which reduces charge rate, all well and good with narrow boat where batteries are 400 Ah and being charged for at least 6 hours a day, but we are unlikely to tow for 3 hours a day, and not every day, and battery only 75 Ah so after considering the options the best option seems to be, swap caravan battery to 110 Ah and keep the 75 Ah at home in garage so we can swap on return to ensure motor mover gets caravan up the drive, and forget all ideas of charging while towing.

Unless we decide to do something daft like take the caravan to Turkey to visit daughter, I will not be charging battery while I tow, yes cheaper sites without hook up, but that means no micro wave, coffee maker, induction hob etc. So no hook up would be in emergency only, we like out creature comforts.

I may connect up fridge supply and reversing light, but that is it.