Thoughts on charging caravan battery from car!

[ericmark]

Back in the late 60's I set up the car to charge the caravan battery, I used many methods, blocking diodes, relays, and even second three diodes connected to alternators three phases, latter had some success, but the volt drop on cables, and diodes resulted in only really working when battery in car boot to charge, and swapping with one in caravan.

However at that time very little 12 volt in caravan, gas lights, gas fridge, water pump was rubber done in floor, so battery was for TV so if discharged it did not really matter.

Today however no gas lights, water pump needs 12 volt, and motor mover needed to get caravan in/out of drive, so caravan battery now important. The old split charging was never very good, OK on starting engine 15 amp going into second battery, but after 15 minutes lucky to get 5 amp, and after 1/2 an hour down to one amp.

So avoiding having second battery in boot, and getting a reasonable charge rate, only option is DC to DC inverter, this way likely to get 10 amp charge for an hour or more using stage charging, so 2 to 3 hours towing and 20 to 30 AH into battery, however that is not much for a 110 AH battery, even when charging as home with as much power as battery can take looking at 8 hours to fully charge, and I will never tow for that long.

Split charging needs a method to stop second battery supplying car when cranking engine, but blocking diode drops voltage so reduced charging, and relay will be in/out like yoyo as car switches alternator off when engine under load, and if there is an electric anti snake device don't want 12 volt switching off while ignition is on, even if not charging. So in spite of volt drop, safety of supply means diode, unless no second battery charging or using an inverter.

Now most inverters have a cut off voltage so it will not over discharge the doner battery, so if using inverter better not to have diode in car, but if no inverter then diode needed to stop cranking current being drawn from caravan battery.

Clearly with one car and one caravan you can select inverter and no blocking diode, or no inverter and have blocking diode. But diode fitted on car, and inverter fitted on caravan, so how do make sure car and caravan matches.

 

[Keithmac]

What car have you got?.

Our Kuga has a "SMART" alternator controlled by the Body Control Module. Battery voltage can drop below 12v at times.

Caravan battery is only charged with engine running and lights on..

 

[Harry Bloomfield]

Split charging needs a method to stop second battery supplying car when cranking engine, but blocking diode drops voltage so reduced charging, and relay will be in/out like yoyo as car switches alternator off when engine under load, and if there is an electric anti snake device don't want 12 volt switching off while ignition is on, even if not charging. So in spite of volt drop, safety of supply means diode, unless no second battery charging or using an inverter.

I don't quite follow what your problem is - there should be no diodes in series with the charging supply because a diodes will drop 0.7V - hence too much voltage lost to charge.

A modern split charge system uses what the call a voltage sensing relay. The relay makes when the voltage rises, applying charging volts to the trailer battery and opens when the voltage falls.

The older split charge system used the dash warning light to trigger the relay.

Some installers use too small a 12v cable, which causes significant volts drop along the length of a car, which results in the voltage controlled relay oscillating. Voltage rises as engine is started, relay closes, load then causes relay to open. I use a 6mm main 12v feed or 2x 4mm in parallel to minimise volts drop. Minimising volts drop also means more voltage available to charge the second battery and to run the fridge.

I have towed for decades with numerous cars, caravans, setups for thousands of miles and all have worked absolutely fine.

 

[ericmark]

I don't quite follow what your problem is - there should be no diodes in series with the charging supply because a diodes will drop 0.7V - hence too much voltage lost to charge.

A modern split charge system uses what the call a voltage sensing relay. The relay makes when the voltage rises, applying charging volts to the trailer battery and opens when the voltage falls.

The older split charge system used the dash warning light to trigger the relay.

Some installers use too small a 12v cable, which causes significant volts drop along the length of a car, which results in the voltage controlled relay oscillating. Voltage rises as engine is started, relay closes, load then causes relay to open. I use a 6mm main 12v feed or 2x 4mm in parallel to minimise volts drop. Minimising volts drop also means more voltage available to charge the second battery and to run the fridge.

I have towed for decades with numerous cars, caravans, setups for thousands of miles and all have worked absolutely fine.

BS7671 only says two suppliers, one all the time and one when ignition on, it does stipulate cable size, but there is nothing about a device to stop cranking taking current from caravan battery.

As said one car, with one caravan either inverter in caravan which means no need to switch on lights to make it charge second battery, or relay/diode to stop reverse feed, however voltage triggured relay has a problem if engine management turns off charging, as then anti snake will not work, so to ensure supply, diode is only option, and as said most have a 0.6 volt drop although the schottky diode is not as bad, with 0.15 to 0.46 volt, but still a volt drop.

A diode and inverter is belt and braces but again inverters are set to drop out at around 11.8 volt and when car alternator switches off it could be switching on/off repeatedly with even a 10 amp load.

 

[Harry Bloomfield]

BS7671 only says two suppliers, one all the time and one when ignition on, it does stipulate cable size, but there is nothing about a device to stop cranking taking current from caravan battery.

I really don't understand your issue, or why you are quoting BS7671, or involving diodes. The viable way to charge a caravan battery either in your boot or in the trailer is a voltage switched relay and adequately sized cables, not for current rating, but to limit volts drop.

 

[ericmark]

I really don't understand your issue, or why you are quoting BS7671, or involving diodes. The viable way to charge a caravan battery either in your boot or in the trailer is a voltage switched relay and adequately sized cables, not for current rating, but to limit volts drop.

Although one may get limited split charging with a voltage switched relay, with a stage charger one needs around 10 hours to charge a battery from 10% to 100% so time is a rather important factor, and with a modern car not charging 100% of the time the engine is running unless one gets the voltage below a set threshold, to stand a chance of transferring a reasonable amount of energy the voltage dependent relay is really a non starter today.

As you have pointed out golf drop is an important factor, so the closer the DC to DC inverter is to the second battery the better, so best location is in the caravan, and if the caravan has a DC to DC inverter fitted there is no requirement for any relay or diode to be fitted to the car, all it needs is an ignition and non ignition supply, the caravan will ensure no current can transfer caravan to car.

But, and this is the main problem, what protects to wiring from heavy reverse currents if that car is used to tow an old caravan which does not have a DC to DC inverter fitted?

You clearly don't want a voltage dependent relay fitted to car as this may cut the supply to the anti snake device, it could be dangerous, OK with old cars where the alternator not controlled by engine management, but not modern cars.

 

[Harry Bloomfield]

Right, I understand now and no, I don't know the answer. I thought the anti-snake (which I don't have) would be wired to the always live supply from the car?

I have my relay adjusted so as soon as the voltage rises above 13.4v, it makes and drops out at around 13v. Checking the (previously chilled) fridge with the engine running it showed 13.50v and the (fully charged already) battery showed 14.20.

When I initially installed the system, I had an issue with the relay holding in at too low a voltage and discharging the battery, until I tweaked it.

 

[Keithmac]

On a 13 pin there is more than one +ve supply. You should never have an issue with ATC regardless of charging battery or not.

Iirc there's Battery charging, Fridge pin (ours requires engine running and lights on) and Switched 12v for ATC?.

 

[ericmark]

At the moment I don't have an electric anti snake either, what I am looking at is how to make the two tow cars so if I tow a caravan with it fitted it will work. The old voltage dependent relays had double contacts and you had two ignition supplies, one to charge battery, and one for fridge.

Latter we had relays in the caravan so the ignition supply switched to non ignition supply, so even if fridge supplied from non ignition supply it would not use the supply until ignition supply was seen. So two supplies to caravan became three.

I know one can cheat the system and turn on head lights so alternator does not switch off, but relying on that does not seem a safe method.

So schottey diode is considered. However could use a simple fuse, if there is a warning should it rupture. Also seems to be an error with system, there is a buzzer to tell you trailer indicators are working but no visible indication and since hearing not part of driving requirement there should be a visible indication to show they are working, this seems to have been missed.

I am told trailer is now part of MOT so guess I need to correct this, so may as well do charging at the same time.

 

[Harry Bloomfield]

My car's OEM towing electrics doesn't use a buzzer for the turn signals, rather it constantly monitors all of the lighting circuits, including a trailer, but only flags up issues on the display and generates a bong, when there is some sort of failure. It obviates the need to check the lighting before setting off on a trip, but I still do a walk round anyway.

I think the law actually requires that there is either a buzzer or a light which operates to confirm the trailer turn lights are functioning properly. My car works contrary to that, only indicates a failure.

 

[ericmark]

On a 13 pin there is more than one +ve supply. You should never have an issue with ATC regardless of charging battery or not.

Iirc there's Battery charging, Fridge pin (ours requires engine running and lights on) and Switched 12v for ATC?.

View attachment 163651

The wiring diagram you show is not to BS7671, one pin is for the brake warning lamp, can't of hand remember which, one of higher numbers, and you should not need to switch on head lights, BS7671 gives what pin is used, and cable min size, and states fridge and charging supplies should have independent live and return supplies, but not much else.

 

[bernardgreen]

My choice would be a DC to DC converter but one designed to be used for charging a battery.

By providing galvanic separation between the two batteries a DC to DC convertor elimates all problems that might arise when the 0 v ( negatives ) of the two batteries are connected to each other.

 

[Harry Bloomfield]

My choice would be a DC to DC converter but one designed to be used for charging a battery.

By providing galvanic separation between the two batteries a DC to DC convertor elimates all problems that might arise when the 0 v ( negatives ) of the two batteries are connected to each other.

I cannot think of anything which I have come across to do that, but why not a 30amp (20 amp might do it) DC to DC, followed by a separate voltage regulator?

 

[ericmark]

My choice would be a DC to DC converter but one designed to be used for charging a battery.

By providing galvanic separation between the two batteries a DC to DC converter eliminates all problems that might arise when the 0 v ( negatives ) of the two batteries are connected to each other.

I would agree Sterling do 10, 20, 30, 40 and 50 amp versions, the problem is the inverter is fitted in the caravan, where the schottky diode or voltage dependent relay is fitted in the car.

Belt and braces with schottky diode in car and DC to DC inverter seems the way forward, as standard they turn on at 13.6 volt and off at 13.3 volt, in regenerative braking mode the drop off voltage drops to 12.2 volt. So the big question is with around 10 meters of cable feed and return to caravan plus a schottky diode when the car goes into regenerative braking mode will the volts at the inverter remain above 12.2 volt?

Removing the schottky diode from car will reduce volt drop, but then if that car is used to tow an old caravan will there be a problem when engine is cranked?

I know my caravan has 16A fuse to battery so worse case the fuse will rupture, but have all caravans got that fuse?

With old voltage dependent relay on start up one would see 10 amp charge, but within minutes this dropped to less than 2 amp so really useless, but by that time no longer measuring, so with the DC to DC inverter we have same problem, by time it reaches regenerative braking mode we don't have an ammeter connected so not a clue if charging or not.

 

[bernardgreen]

I will look to see if I still have the rough notes from 1990's about the three battery set up in a mobile first aid unit. A towed caravan converted into a treatment centre

Battery 1 was vehicle battery in the towing vehicle
Battery 2 was cabin supply in the carvan
Battery 3 was intended as a back up to start the vehicle if battery 1 had been drained or supply cabin if battery 2 had been drained.

There was the added complication of being able to run on a hotch potch mains supply when deployed on duty for several hours.

As far as I recall no satisfactory solution was found for that unit.