Do I have a B-B or Relay or a VSR charging my Aux battery ? (1 Viewer)

[Affinite]

I'm trying to establish how my Aux/Leisure battery is being charged when engine is running on my (new to me) MH
Is there a simple way to establish this ?
I haven't even found the start battery yet let alone a relay or VSR
I've noticed that when the engine is running the batteries very quickly hit exactly the same voltage (14.1V) which suggests that they are being charged in parallel.
Presumably this means that I have a relay of some sort and I don't have a SMART alternator ?


MH is Compass Avant Garde 194 (Similar to Elddis Autoquest 194)

 

[GeoffnDee]

The starter battery will be either under the bonnet (pre 2006) or under a cover in the cab floor (2006 onwards) . The split charging (which appears to be working ok) may be through the PSU , depending on year of Motorhome and make/model of PSU

 

[Affinite]

The starter battery will be either under the bonnet (pre 2006) or under a cover in the cab floor (2006 onwards) . The split charging (which appears to be working ok) may be through the PSU , depending on year of Motorhome and make/model of PSU

Thanks - its a 2020 model with a BCA PDU so I'll have a look around the cab for the battery tomorrow. Do some have the split charge relay in the PDU ? - I imagined that there would be an external device. I have the PDU schematic (courtesy of Two on Tour ) so I'll take a closer look at that.

 

[GeoffnDee]

Yes the split charging can be through the PDU relays , both leisure and starter batteries are connected to it , but you may also have a separate fuse/relay module linked to the D+ signal somewhere (possibly at the base of the drivers side B pillar.)
Do you have a reason to locate it , or is it out of interest ?
The starter battery is under the cab floor/carpet towards the passenger side.

 

[autorouter]

I have the PDU schematic (courtesy of Two on Tour ) so I'll take a closer look at that.

There will be two power cables from the leisure battery and the starter battery, going to two terminals on the circuit board. The relay will be a small rectangular box between those two terminals. Usually they try to make the distance between the two terminals and the relay as short as possible.

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[headlight]

Thanks - its a 2020 model with a BCA PDU so I'll have a look around the cab for the battery tomorrow. Do some have the split charge relay in the PDU ? - I imagined that there would be an external device. I have the PDU schematic (courtesy of Two on Tour ) so I'll take a closer look at that.

Being a 2020 model won't it have a smart alternator as they were introduced in 2016 or 2017 ?
If it does have a smart alternator it may also have a B2B.

 

[autorouter]

If the battery voltages are always the same then it has a split charge relay - either VSR or a D+ activated. If the battery voltages are sometimes different, and maybe the same by coincidence, then it has a B2B.

As to whether it has a smart alternator, that can be hard to tell. For a few minutes after starting, a smart alternator behaves just like a standard alternator. But eventually the voltage to the starter battery drops to 13V or less. If the leisure battery voltage stays up at over 13V, then it has a B2B.

Usually if it has a smart alternator, it has a B2B. But some motorhome manufacturers went through the learning curve: fit split charge relay, customer complaints, upgrade to B2B. And there are still some unfixed ones out there.

 

[Affinite]

Thanks for the very helpful replies.
Confession time ...
I am measuring the start battery voltage using the Aux input of my Victron shunt but I am now worried that I may have spliced into a Leisure battery feed to the PDU instead . (That would of course explain the identical charging voltages I observed) I should have thought to check voltages when the engine was not running but with the MPPT charger enabled as I assume the battery voltages would then have been quite different.
As for my reasons ... I eventually want to improve my off-grid capability and I want to understand what Ive got first

 

[GeoffnDee]

If you want to measure the starter battery voltage without lifting the floor covering , the jump start point under the bonnet is useful for this.
www.apuljackelectronics.co.uk have a technical library with info. regarding going to lithium batteries and your BCA system which may be helpful.

 

[LostInSpace]

If you want to measure the starter battery voltage without lifting the floor covering ......

...... plug a cigarette voltmeter into the cigarette lighter socket (or 12v power socket) and whilst driving you will see the voltage going up and down if you have a smart alternaator.
- - 12.something whilst driving
- - 13.something on engine overun
- - 14.something when breaking.

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[Raul]

It is not hard to tell if you have a smart alternator, just look at the starter battery negative post; if it has a shunt gizmo with a small cable out of it, then it has a smart alternator. If it has a smart alternator, it may have a B2B.

 

[Affinite]

It is not hard to tell if you have a smart alternator, just look at the starter battery negative post; if it has a shunt gizmo with a small cable out of it, then it has a smart alternator. If it has a smart alternator, it may have a B2B.

This is what I found on the negative post. Looks like a relay to me. What do you think ?

 

[Raul]

It’s a relay controlled by BCM battery control module, I think. BCM is usually needed where smart alternators are fitted.
Another think to look for, if you have a voltage display plugged into a cab socket, see if the voltage goes up on engine braking. That’s a regenerative charging on smart alternators.

 

[autorouter]

Yes the black box with A and B on it is a relay, for isolating the battery. They are also fitted on some non-smart alternator vans.

There is also a shunt, just below it - that shaped piece of metal with two screw heads in it. That's what tells you it's likely to be a smart alternator, as Raul said.