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[Auto Addict]

I think I may have one in the junk box. Came with some towbar electrics but surplus to requirements.

 

[Andy B]

Why not use a conventional voltage sensing relay? ......

I've not had any if these problems yet so haven't really looked into it myself. But they've used VSRs for years now due to their ease of fitting/wiring & tend to think that if the answer was so simple there would never have been a problem in the first place 

[Marks DTM Calib]

Why not use a conventional voltage sensing relay? That will connect the caravan battery when the vehicle electrical system is at a high voltage and disconnect it when the voltage drops. You'll get some charging of the caravan battery when the vehicle battery is being charged and it will protect the caravan battery from being discharged.

http://www.12voltplanet.co.uk/voltage-sensitive-relay-12v-140a.html

Would this do the trick?

Your issue is that under normal operation the output volts from the battery changes considerably and such a relay will keep dropping out, potentially for long periods.

Hence why I suggested the solar charging type, this will take the varying alternator output and convert it into a controlled charging supply for the leisure battery, couple this with an (ideally adjustable voltage sensing or ignition controlled relay) and you will get a nicely topped up pikey supply.

[Auto Addict]

Why not use a conventional voltage sensing relay? That will connect the caravan battery when the vehicle electrical system is at a high voltage and disconnect it when the voltage drops. You'll get some charging of the caravan battery when the vehicle battery is being charged and it will protect the caravan battery from being discharged.

http://www.12voltplanet.co.uk/voltage-sensitive-relay-12v-140a.html

Would this do the trick?

Your issue is that under normal operation the output volts from the battery changes considerably and such a relay will keep dropping out, potentially for long periods.

Hence why I suggested the solar charging type, this will take the varying alternator output and convert it into a controlled charging supply for the leisure battery, couple this with an (ideally adjustable voltage sensing or ignition controlled relay) and you will get a nicely topped up pikey supply.

Not bothered about it being topped up when towing, as there will not be a drain on it if its isolated from the car battery

[Auto Addict]

Customer service!

I decided to phone the towbar electrics manufacturer to see if they could throw any light on the matter, in case they had something inbuilt to stop reverse drain.

Spoke to a technical bloke who said he hadn't heard of that, when I started to explain the different voltages of the two batteries he said 'Get a life' and hung up on me.

Perhaps I should

 

[Kevin Wood]

Customer service!

I decided to phone the towbar electrics manufacturer to see if they could throw any light on the matter, in case they had something inbuilt to stop reverse drain.

Spoke to a technical bloke who said he hadn't heard of that, when I started to explain the different voltages of the two batteries he said 'Get a life' and hung up on me.

Perhaps I should

 

Towbar fitters. If only their legs were long enough to stop their knuckles dragging on the ground.

[Auto Addict]

Well that definitely proves it, plugged the 'van into the car, and the leisure battery voltage started to drop like a stone!

Where to mount a split charge relay though, it's got to be on the caravan, after the Al-Ko ATC system.

No wiring diagram came with the caravan.

[Bigron]

I know that I suggested this earlier and I'm prepared to be shot down in flames, but why won't a suitably rated diode feeding the caravan's battery do the job? This will allow it to be charged and prevent reverse drain. It shouln't affect the "smart" alternator, as it won't be able to see it as anything other than extra capacity of the main battery. You would, of course, lose about 0.7 Volt across the diode - not a worry, surely?

Ron.

[Auto Addict]

I know that I suggested this earlier and I'm prepared to be shot down in flames, but why won't a suitably rated diode feeding the caravan's battery do the job? This will allow it to be charged and prevent reverse drain. It shouln't affect the "smart" alternator, as it won't be able to see it as anything other than extra capacity of the main battery. You would, of course, lose about 0.7 Volt across the diode - not a worry, surely?

Ron.

Or I could just disconnect the caravan battery whilst towing

[Marks DTM Calib]

I know that I suggested this earlier and I'm prepared to be shot down in flames, but why won't a suitably rated diode feeding the caravan's battery do the job? This will allow it to be charged and prevent reverse drain. It shouln't affect the "smart" alternator, as it won't be able to see it as anything other than extra capacity of the main battery. You would, of course, lose about 0.7 Volt across the diode - not a worry, surely?

Ron.

0.7V drop is quite a reasonable amount when considering lead acid battery charge voltages.....made worse with the Mercs operating system   

[Bigron]

Understood, Mr. Fuse, but it will still keep the caravan's battery relatively happy. If it really is a worry, spend a little more on a power Schottky diode which will have a 0.1 - 0.2 volt drop across it?
RS Components, Farnell, Rapid Electronics and Maplins for sources.

Ron.

[TD]

Why not use a conventional voltage sensing relay? That will connect the caravan battery when the vehicle electrical system is at a high voltage and disconnect it when the voltage drops. You'll get some charging of the caravan battery when the vehicle battery is being charged and it will protect the caravan battery from being discharged.

http://www.12voltplanet.co.uk/voltage-sensitive-relay-12v-140a.html

Would this do the trick?

Did you read this bit??

IMPORTANT NOTE: These relays are not suitable for vehicles with 'smart' or 'intelligent' alternators. As a guide this applies to vehicles having engines that comply with Euro5 emissions standards and onward, often also incorporating regenerative braking (if in doubt please consult your vehicle manufacturer to confirm). If your vehicle has a smart alternator then you will need to use a battery-to-battery charger instead of a VSR

[henryd]

I know that I suggested this earlier and I'm prepared to be shot down in flames, but why won't a suitably rated diode feeding the caravan's battery do the job? This will allow it to be charged and prevent reverse drain. It shouln't affect the "smart" alternator, as it won't be able to see it as anything other than extra capacity of the main battery. You would, of course, lose about 0.7 Volt across the diode - not a worry, surely?

Ron.

Or I could just disconnect the caravan battery whilst towing

You should have fuses for the charging electrics from the vehicle into the caravan,just pull the ones relevent to charging and battery to isolate car from van

[Marks DTM Calib]

Understood, Mr. Fuse, but it will still keep the caravan's battery relatively happy. If it really is a worry, spend a little more on a power Schottky diode which will have a 0.1 - 0.2 volt drop across it?
RS Components, Farnell, Rapid Electronics and Maplins for sources.

Ron.

So Schottky diodes have a habit of not lasting on what is considered 'the worst power supply in the world' (automotive) as they do not tolerate reverse voltage and the spikes seen tend to age them quickly and 'pop' them (hence why Schottky barriers are not used nearly as often as you may think)

What you have to think of is that the difference between a 90% charged battery and one that is 60% charged is about 0.6V so yes, the V drop has a big impact and any battery below 90% charged will tend to sulphate to.

The main issue is if the leisure battery will charge at all connected to the car due to the way the alternator output varies, the setup is totally focused on the state of the vehicle battery and nothing else plus, they have current sensing on the battery lead just to make matters worse.

Hence why I suggested the solar type charger, it prevents back feeding and takes what ever supply voltage is given to it and tailors it to what the leisure battery needs, plus it gives current limiting to.....

[Kevin Wood]

... Hence why I suggested the solar type charger, it prevents back feeding and takes what ever supply voltage is given to it and tailors it to what the leisure battery needs, plus it gives current limiting to.....

Yep, that would undoubtedly be the best solution if you actually need to deliver charge to the caravan battery reliably. It just becomes another load on the car's electrical system, regardless of voltage, until the battery is satisfied and it stops drawing significant current.

I suspect, for AA's scenario, just disconnecting the caravan battery from the towing connector and ensuring it is fully charged before a trip would probably be the simplest solution.