I have a car trailer with a battery operated winch and currently have it wired so that the battery is charged from pin 9 - constant supply - in the 13 pin socket. This is the conventional way of charging caravan batteries which use pin 10 for fridge supply when being towed. But in order to use pin 9 to charge the battery I need to use two relays on the trailer: the first is a voltage sensing one connected to pin 10 to ensure that it doesn't become live until the engine is running and then this relay is in turn connected to a changeover one in order to connect pin 9 to the battery once the engine is running.

It seems to me that since I don't have a need for anything else to be connected to pin 10 I could simply use it to charge the battery and dispense with the changeover relay which connects pin 9 to the battery. However, I want to be sure that, if the trailer was being towed by another car, using pin 10 for charging purposes wouldn't cause any problems.

Can anyone see any problem with using pin 10 for charging instead of pin 9?

Related to my question above about using pin 10 for battery charging, I see from the LR wiring diagram that there is a relay on the 'accessory feed for fridge' wire operated by the trailer ECU: anyone any idea what the purpose of this?

It's to ensure the 12v feed to a caravan for running the fridge and charging caravan battery when towing and is only live when the engine is running. This is to prevent the cars battery being run flat.

Permanent live is on pin 9. see attached.

 Andy
Freelander2 HSE (59 Plate, MY10 spec)

Last edited by andyk159 on 5th Mar 2020 7:34 pm. Edited 1 time in total

Interesting question but I don't know the answer re. the said relay. I did have a thought however, as different batteries do require different charges; what are the specs of you battery and what is the suggested charging method or device recommended? I did a google search that left me thinking of multiple possibilities, I don't know the viability however.

Andy
Thank you for your suggestion. However, I don’t think the trailer ECU relay is for ensuring that pin 10 is only live when the engine is running for two reasons. The first is that there is another relay on that wire that I didn’t mention called the ignition relay that ensures that pin 10 is live when the ignition is switched on - note, not necessarily the engine running - and, second, when I check the voltage at pin 10 on my tow bar socket, it is live with ignition on, but without the engine running. Ideally it should only come live once the engine is running as otherwise starting the car could draw current from the trailer battery. (Pin 10, of course, isn’t ordinarily connected to the caravan/trailer battery to charge it, but to the fridge. However, the conventional practice on caravans is for pin 10, once it’s live, to trigger a relay to connect the battery to pin 9, the permanent live, for charging purposes. But this would allow a starter motor to draw current via pin 9 if pin 10 was live as soon as the ignition was switched on.)

MotionInc
The battery on my trailer is a small 22Ah VRLA golf cart type battery which is sufficient for powering my small winch. It doesn’t need any particular charging method as far as I can determine - it should be easily charged by the alternator.

Simon, I missread\understood what you were saying in first post. apologies.

Looking at wiring diagrams you are right that power for pin 10 is from an ignition switched relay, backed up with second relay operated from trailer ECU, presumably to check trailer (caravan) is connected before making pin live. Don't know how this may be done, other than the ECU detecting lighting connected from trailer. There is no other indication to the ECU as far as I can see.

This then is your problem. How to tell the trailer ECU to switch pin 10 relay.

As previously said, pin 10 is always live. Andy
Freelander2 HSE (59 Plate, MY10 spec)

Thanks again, Andy. The problem I am trying to resolve is that, if pin 10 is live as soon as the ignition is switched on, not just once the engine is running, then the normal relay that connects the battery to the permanent live pin 9 will make that connection while the engine is being started and thus potentially draw starter current down the relatively light wire connecting pin 9. Ideally pin 10 wouldn’t become live until the engine is running but that isn’t the case.

I’ll double check this weekend whether pin 10 is live without a trailer being connected. I’m pretty sure it is but worth double checking.

How about a marine isolator/regulator switch that will only allow the voltage flow one way?

Here is an interesting article:
https://www.12voltplanet.co.uk/split-charging.html

Indeed, that article from 12 Volt Planet is interesting but I’m sceptical of any advice provided by someone who says

“Split charging is the term used to describe the simultaneous charging of the vehicle starter battery and the leisure battery (or batteries) from a common charging source.”

There’s a basic contradiction here -‘simultaneous' isn’t 'split'. Split charging dates from the days when cars had dynamos with limited output, perhaps less than 30 amps, and so if you wanted to charge two batteries the charging system was set up to split the charge between them, i.e. once the starter battery was fully charged the dynamo was switched over to charge the leisure battery, and then switched back if the car's battery needed topped up. So the charging from the dynamo was literally 'split' between the two batteries.

Today, with alternators producing 150 amps or more there is no need to split the charge - you simply need to connect the two batteries together in parallel and the alternator just sees them as a single battery and charges them jointly. The challenge is in how to connect the batteries so that using the leisure battery doesn’t flatten the car battery and vice versa - hence the variety of relays, usually voltage sensing of some sort or another, available.

Your voltage will be less by the time it reaches the trailer so you probably need a DC-DC charger near the trailer's battery.

You really need to check voltage & easiest is to use a DC clamp meter to check current.

If testing very small currents loop the wire through the clamp meter if there is enough slack say 4 times. You get 4 times the current then divide that by 4 to get the actual current. Procrastination, mankind's greatest labour saving device!

Acoustic insulation ARB TPMS 3xARB air compressors After cooler Air tank On-board OCD pressure air/water cleaning Additional 50L fuel Carpet in doors ABE 2x1kg Waeco 28L modified fridge Battery 4x26ah Solar 120w Victron MPPT 100/20 DC-DC 18amps 175amp jumper plug Awning 6x255/60R18

Yes, that’s the route I’m heading down although simply hooking up the leisure battery to be charged from pin 9 (constant feed) is the normal way of charging caravan batteries in the UK. The problem I’m trying to resolve is how to prevent the two batteries being connected until after the tow car engine has started. As far as I can tell, on the FL2, and many other cars, they are connected as soon as the ignition is switched on. This could result in the tow car drawing current from the leisure battery while the engine is being cranked. Not an ideal situation given the relatively light wiring between the two. The simple solution, which I’m currently experimenting with, is to use a voltage sensing relay on the trailer to switch in the second battery once the engine is running and the system voltage has risen. But the voltage drop on the long cable length between the two batteries may make a VSR unreliable.
P.S. Good tip about using a clamp meter with low currents.

The current will not go backwards through a DC-DC charger. Just stick in a small Victron 9amp 12-12volt, cheap enough & no further thinking required. I have 2 of these in parallel in my car for my fridge batteries (all within the car).

https://www.victronenergy.com/dc-dc-conver...s-isolated Procrastination, mankind's greatest labour saving device!

Acoustic insulation ARB TPMS 3xARB air compressors After cooler Air tank On-board OCD pressure air/water cleaning Additional 50L fuel Carpet in doors ABE 2x1kg Waeco 28L modified fridge Battery 4x26ah Solar 120w Victron MPPT 100/20 DC-DC 18amps 175amp jumper plug Awning 6x255/60R18

Those look very interesting - thank you!

I’ve had a look at the data sheets but I can’t work out how the Victron units would be connected. If you use the constant supply, then it’s always connected when the trailer is plugged in; and if you use the switched supply, then you end up using pin 10 as the charging supply. Which brings me back to my original question - is there any reason not to use pin 10 for charging?

Simon J wrote:

I’ll double check this weekend whether pin 10 is live without a trailer being connected. I’m pretty sure it is but worth double checking.