Rx400h trailer electrics

welland · May 27, 2019

Hi All,

I have an issue with my trailer electrics when my rx400h is attached to a caravan. I don't seem to be able to charge the leisure Battery or run the fridge on 12v when connected to the car and travelling. I believe that the car is outputting 12v on the correct pins, and I believe that the caravan senses this supply, but the fridge and charging of the leisure Battery don't seem to work.

I'm wondering if the outputted voltage might not be high enough to trip the caravan relay to switch these functions on. Has anybody else had such an issue with their hybrid?

On non-hybrids, the Battery voltage can rise when the engine revs, but I don't know if this happens on hybrids.

Is there a way to keep the engine running and or blip the throttle on the rx400h? Often when stopped, the engjne cuts out (as it's designed to), but this isn't always helpful for testing, or maintenance.

Delphius1 · May 27, 2019

An interesting problem because the 12v Battery is maintained by the DC/DC invertor from the hybrid Battery and engine revs have no effect on the Battery voltage. There won't be enough voltage change to trigger a voltage sensing relay.

You may have to check the towing socket on the car and see if you get a 12v feed on one of the towing socket pins when the hybrid system is activated. Then use that 12v feed as a signal for the caravan relay to switch between house mode and towing mode in the caravan.

Check the thickness of the cables to the pins on the socket though, as the "sensing" pin maybe the pin that feeds 12v to the fridge and/or Battery and may need to supply a fair bit of current.

It may need a decent electrician to check the car's installation (don't want any fires!) and modify the caravan wiring as necessary.

Herbie · May 28, 2019

I don't know how a caravan decides to switch and/or charge, but what I do know is that a 'standard' car alternator output will vary with engine speed.

Because a hybrid system uses a DC/DC inverter to do the job of the alternator, I don't think there is any voltage variation and, unless my car is faulty in some way, the two photos below seem to confirm this.

First one shows a 'standing' voltage of 12.532V with the tailgate open (meaning that the car's interior lights are on).

Second one shows 14.139V with the car in the READY state, after which I switched on full-beam headlamps, heated rear window, heated seats etc., etc., until the hybrid Battery drained to the point where engine fired up. Even with the engine running the voltage measured was 14.147V, an increase of only 0.008V (should have been a third photo to prove that but my phone Battery died!).

Delphius1 · May 30, 2019

Exactly what I was saying. The hybrid Battery maintains the 12v Battery and the only thing that you can detect is when the hybrid system is activated.

But then that's the hybrid version of engine running on a normal ICE-only car. The hybrid Battery will maintain the 12v Battery in the car and supply 12v to the caravan. If the hybrid Battery can't maintain a charge then the engine will come on.

But the process of starting the engine is entirely controlled by the car's electronics.

The big thing you have to avoid is the caravan draining both the 12v and the hybrid batteries. So there has to be some way of disconnecting the caravan electrics from the car when the hybrid system is deactivated.

welland · May 31, 2019

On 5/28/2019 at 4:02 PM, Herbie said:

I don't know how a caravan decides to switch and/or charge, but what I do know is that a 'standard' car alternator output will vary with engine speed.

Because a hybrid system uses a DC/DC inverter to do the job of the alternator, I don't think there is any voltage variation and, unless my car is faulty in some way, the two photos below seem to confirm this.

First one shows a 'standing' voltage of 12.532V with the tailgate open (meaning that the car's interior lights are on).

Second one shows 14.139V with the car in the READY state, after which I switched on full-beam headlamps, heated rear window, heated seats etc., etc., until the hybrid battery drained to the point where engine fired up. Even with the engine running the voltage measured was 14.147V, an increase of only 0.008V (should have been a third photo to prove that but my phone battery died!).

Herbie,

I think that you have demonstrated behaviour in the hybrid that is akin to a non-hybrid. When a non-hybrid engine revs above a certain engine speed, the charging circuit activates and the voltage at the Battery terminals rises ( in my experience to 13.xx volts). In your demonstration, you showed the Battery voltage rise to 14.xx volts when the inverter became active.

I would think this would be sufficient to activate circuits in a caravan.

In my testing earlier this week, I was only measuring the voltage on my leisure Battery in the caravan: the voltage did not rise when the car was in the Ready state nor when the engine started or the engine revved. I must test again to see how the voltage at the car Battery alters, and also the voltage on the output pins on the trailer socket.

Rutlandlex · June 4, 2019

Could the issue be inside the caravan? When you flick the switch inside the caravan to car Battery do the other electrics work like the interior lights? It's been a while since I've had a caravan hooked up to mine so I can't remember how it all works but in theory then charging of the leisure Battery and powering the fridge as well as operating other electrics should work apart from mains stuff.

Delphius1 · June 4, 2019

Welland, the reason I mentioned the gauge of wire in my previous post is because the wire that supplies current to the trailer socket (and then on to the fridge and the Battery in the caravan) may not be chunky enough. If the wire is too thin, then there will be a voltage drop along the wire, which means that although you get a voltage change at the Battery terminals on the car, the voltage at the trailer hitch socket and then the caravan is a lot less or there is none at all. What you end up seeing is just the Battery voltage of the caravan Battery.

In bad installations, even if the wiring has been installed specially, it can still be under-spec and dangerously thin and risk catching fire if it's trying to supply current to the caravan fridge and a discharged caravan battery at the same time.

In some really bad cases I've seen caravan wiring tapped into wires in the boot, relying on the car's wiring harness to supply several amps to the caravan socket, something it was never designed to do.

You need to work logically on this and go back to basics. Don't assume anything, including the fact that the wiring for charging is actually installed in the car!

Check first you need to check that the car Battery voltage goes from 12.5v or thereabouts when the hybrid system is inactive to 14v approx. when you press the start button and the hybrid system is active. That then gives you the base to work from because you can see the voltage jump is happening. A tired Battery may not reach the required voltage.

Then check you have decent gauge wire installed from the car Battery terminals to the trailer socket (through a fuse at the car Battery terminal of course). You then need to make sure the wire goes to the correct pin on the socket and that the voltage is exactly the same at the trailer hitch as the Battery terminal.

Then check the wiring on the caravan plug corresponds to the wiring on the caravan socket. I've seen installations where the pins are not matched so you can plug the caravan socket into the car, but you'd never get voltage to the caravan because the car is supplying a different pin on the socket than the caravan is wired to on the plug. It's more common than you think, because different people wired up the car socket and the caravan socket.

Once you are sure you've got a voltage step at hybrid switch on, decent gauge wiring to the socket, no voltage drop, the car and the caravan connect to the same pins and you have a good connection from the car Battery terminals to the caravan, then you can measure the voltage at the changeover relay in the caravan.

When the caravan is plugged into the car, the nominal 12.5 volts of the car Battery should not cause the relay to switch from house mode to charge mode.

When the hybrid system is activated on the car (the same as starting the engine on a normal car), the voltage on the Battery terminals on the car should raise to 14v-ish. The same voltage should appear at the caravan relay and it should detect this and switch over to charge mode.

Pressing the button to stop the hybrid system should drop the voltage back to 12.5-ish and the caravan relay should drop out and disconnect the fridge and Battery. If it doesn't then you are in danger of flattening the 12v Battery which comes with it's own issues on a hybrid. Just see the number of posts on here relating to dead 12v batteries on hybrids. :-)

Hopefully that should cover most of everything.

I'm a bit rusty with trailer and caravan electrics, haven't done it for a few years (pre 13-pin socket era) and it changes constantly. That's another reason why the plug and socket might be wired differently: because the standards can change year-to-year.