What happens between a smart car battery charger and a b2b

[Derekoak]

With lockdown we did not drive far in our doblo xl everyday use camper. In January with cold weather and not thinking about it the starter battery died due to being at low voltage. A common story. So I got a new starter battery 60ah lead acid.
So that the new battery did not die too I bought a noco genius 5 , a smart battery charger 5amp maximum charge. By the time it arrived my voltmeter showed 12.3 volts at the starter battery (sat for hours) so I clipped on the noco. It did what it was supposed to do read above 25% charge and charged through 50 and 75 to 100% and did some sort of battery recondition. Whilst it stayed cold I charged the battery weekly so it did not drop so low. Recently it has warmed up and the starter battery has had higher voltages.
BUT My leisure battery is connected through a ring rscdc30. There is solar on the roof 120watt and even in winter the leisure battery with only tiny loads I think stays full.
What happens if I need to charge the starter battery when the leisure battery is not full? The leisure battery is AGM so the ring is set to 14.4v absorbtion, 13.6 float.
I imagine the ring starts taking current when the alternator puts the voltage above full battery 12.7? Volts. I also guess the noco must push the voltage above full lead acid voltage to finish charging the starter battery. I guess the ring might pull current from the noco and charge the leisure battery before letting the noco fully charge the starter battery. Will any harm happen? Will it make a difference if the sun is shining?

 

[wildebus]

With lockdown we did not drive far in our doblo xl everyday use camper. In January with cold weather and not thinking about it the starter battery died due to being at low voltage. A common story. So I got a new starter battery 60ah lead acid.
So that the new battery did not die too I bought a noco genius 5 , a smart battery charger 5amp maximum charge. By the time it arrived my voltmeter showed 12.3 volts at the starter battery (sat for hours) so I clipped on the noco. It did what it was supposed to do read above 25% charge and charged through 50 and 75 to 100% and did some sort of battery recondition. Whilst it stayed cold I charged the battery weekly so it did not drop so low. Recently it has warmed up and the starter battery has had higher voltages.
BUT My leisure battery is connected through a ring rscdc30. There is solar on the roof 120watt and even in winter the leisure battery with only tiny loads I think stays full.
What happens if I need to charge the starter battery when the leisure battery is not full? The leisure battery is AGM so the ring is set to 14.4v absorbtion, 13.6 float.
I imagine the ring starts taking current when the alternator puts the voltage above full battery 12.7? Volts. I also guess the noco must push the voltage above full lead acid voltage to finish charging the starter battery. I guess the ring might pull current from the noco and charge the leisure battery before letting the noco fully charge the starter battery. Will any harm happen? Will it make a difference if the sun is shining?

Short Answer:
You ideally need to disable the B2B when charging the Starter Battery from a Mains Charger otherwise the B2B, if it is voltage controlled, thinks the engine is running and turn on.
In your case, you have a B2B that might try to take 30A out the Starter Battery and a Mains Charger only able to put 5A in, and the end result may not be what you want

Here is a little graph of what happened in this scenario in my camper when I connected a 20A mains charger to the Starter Battery when I had a Redarc 40A B2B installed....
You can probably spot the point when the mains charger was connected to "charge" the Starter Battery!

RED_SB-Voltage-Ext by David, on Flickr
Eventually it recovered as the Leisure Battery got fully charged - but that charge was mostly from the Starter Battery and not the mains charger for many hours

 

[Admin]

With lockdown we did not drive far in our doblo xl everyday use camper. In January with cold weather and not thinking about it the starter battery died due to being at low voltage. A common story. So I got a new starter battery 60ah lead acid.
So that the new battery did not die too I bought a noco genius 5 , a smart battery charger 5amp maximum charge. By the time it arrived my voltmeter showed 12.3 volts at the starter battery (sat for hours) so I clipped on the noco. It did what it was supposed to do read above 25% charge and charged through 50 and 75 to 100% and did some sort of battery recondition. Whilst it stayed cold I charged the battery weekly so it did not drop so low. Recently it has warmed up and the starter battery has had higher voltages.
BUT My leisure battery is connected through a ring rscdc30. There is solar on the roof 120watt and even in winter the leisure battery with only tiny loads I think stays full.
What happens if I need to charge the starter battery when the leisure battery is not full? The leisure battery is AGM so the ring is set to 14.4v absorbtion, 13.6 float.
I imagine the ring starts taking current when the alternator puts the voltage above full battery 12.7? Volts. I also guess the noco must push the voltage above full lead acid voltage to finish charging the starter battery. I guess the ring might pull current from the noco and charge the leisure battery before letting the noco fully charge the starter battery. Will any harm happen? Will it make a difference if the sun is shining?

The starter battery trigger and recover voltages can be set on the ring B2B, it should be set to at least 13.2v if not 13.8v.

 

[Derekoak]

The starter battery trigger and recover voltages can be set on the ring B2B, it should be set to at least 13.2v if not 13.8v.

Thanks Phil I will look into that.
However I imagine the noco will top even 13.8 volt and still charge the LB although I suppose the oscillations in Wildebus's graph will be less dramatic? I suppose the starter battery will not be drawn into the charge and we may be straight into the smooth upper part of Wildebus's graph.

 

[ScoTTyBEEE]

There is an option on the ring to disable the b2b part and operate as a solar charge controller only. Just do that.

 

[Nabsim]

I don’t think it would make any difference to my set up, if I remember right I took the d+ from the ignition switch, only switches the b2b in when key is turned. Am I remembering wrongly or does that sound right?

 

[wildebus]

I don’t think it would make any difference to my set up, if I remember right I took the d+ from the ignition switch, only switches the b2b in when key is turned. Am I remembering wrongly or does that sound right?

Sounds like a pretty typical factory setup
the Voltage controlled B2Bs are popular because they make it very easy to self-install or quick for van converters (time is money!)

 

[Nabsim]

Sounds like a pretty typical factory setup
the Voltage controlled B2Bs are popular because they make it very easy to self-install or quick for van converters (time is money!)

Oooh, that may be a compliment as I installed it ha ha