Where to locate my D+ signal

[autorouter]

Thanks. I knew that box had the fuses for the fridge and step (took me ages to locate the step fuse when it failed). Didn't know it also had a D+ connection, but it now seems likely that I have a dumb alternator in which case I don't need to connect to D+.
Could be I've been seeking a solution to a problem that I don't have

It's worth sorting out a D+ trigger for the B2B (DC-DC charger) even if you have a dumb alternator. If you use voltage sensing instead, then anything that raises the starter battery voltage will turn on the B2B. Such as trickle-charging the starter battery, or charging the starter battery from solar. Voltage sensing is fine for simple systems, but can cause problems with more complex systems. D+ sensing just works.

 

[ForeignShaws]

Yes indeed. Looks like you need to find that D+ after all.

Found both the Fridge D+ and Towing D+ signal on the EM40 Interface just below the OS door pillar, below the Converters Interface PWB as described by HarryML7. I have no towbar so I'll use this for the charger.

 

[ForeignShaws]

It's worth sorting out a D+ trigger for the B2B (DC-DC charger) even if you have a dumb alternator. If you use voltage sensing instead, then anything that raises the starter battery voltage will turn on the B2B. Such as trickle-charging the starter battery, or charging the starter battery from solar. Voltage sensing is fine for simple systems, but can cause problems with more complex systems. D+ sensing just works.

Thanks autorouter, that makes sense.

 

[MisterB]

There will be an existing arrangement to charge your leisure battery. Either a split charge relay or a small DC-DC charger. Whatever it is, it is very likely turned on and off by a D+ signal. So the D+ signal will be present at the distribution box, or at the existing DC-DC charger if there is one. There should be no need to go all the way back to the alternator.

If the existing arrangement is a split charge relay then as jwells says it needs to be disabled, if not it will simply short the DC-DC charger input and output, so it does nothing. If the existing arrangement is another DC-DC charger, then that probably needs to be disabled too, but may be OK to leave it connected.

Could the isolation be achieved by just isolating the installed control unit from the leisure battery by using a rotary isolator switch? Then when not driving, switch it back on again. Not ideal, but would it work ?

 

[autorouter]

Could the isolation be achieved by just isolating the installed control unit from the leisure battery by using a rotary isolator switch? Then when not driving, switch it back on again. Not ideal, but would it work ?

Yes, that would work. A common workaround is to install a relay that does exactly that when the engine starts, and reconnects when the engine stops. So a manual switch should do the job just the same.

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[MisterB]

Yes, that would work. A common workaround is to install a relay that does exactly that when the engine starts, and reconnects when the engine stops. So a manual switch should do the job just the same.

Thanks, it's been running around in my mind the last couple of days whilst touring in the EU with very little solar, I thought it was too simple but the more I thought about it the more it made sense as the only connections then from the battery would be to the inverter and from the 60A Renogy B2B. It's all wired up ready to go but I just didn't get chance to sort a relay out as per your previous suggestion.

I also realise that the fridge prob won't work on 12v but I can run it on gas whilst driving as we have crash sensors etc and will NOT be going anywhere near petrol stations etc, just motorway/A road driving.


The other question is does it matter if I have a smart alternator or not? What difference would it make either way ?

 

[autorouter]

I also realise that the fridge prob won't work on 12v but I can run it on gas whilst driving as we have crash sensors etc and will NOT be going anywhere near petrol stations etc, just motorway/A road driving.

To power the fridge on 12V, you could use a battery changeover switch instead of an isolator switch. That has a common (COM) connection, a Battery 1 (B1) connection and a Battery 2 (B2) connection. When it's 'OFF', the COM is isolated from both B1 and B2. In the B1 position, COM is connected to B1. In the B2 position, the COM is connected to B2. Basically it switches the COM between the B1, OFF and B2.

Connect the starter battery to the COM, and the control unit to the B1. If the switch is at B1, they are connected, if it's at OFF or B2 they will be disconnected. Just like an isolator switch.

The difference is, when the switch is at B2, the starter battery is connected to whatever is wired to the B2. So if you wired the fridge to the B2 it would connect to the starter battery when the switch was at B2.

So to use it, switch to OFF while actually starting the engine. When the engine is running, switch to B2, so the fridge is powered. When the engine is stopped, switch to B1, so the control unit is powered.

 

[autorouter]

The other question is does it matter if I have a smart alternator or not? What difference would it make either way ?

A B2B (DC-DC charger) works just as well with either a smart alternator or a non-smart alternator. The difference is basically it's a nice-to-have for a non-smart alternator, but it's an essential for a smart alternator.

Also a B2B is more or less essential for a lithium battery, whether the alternator is smart or not.