Although I charge via the mains every night it just shows you how much the split system can put back into the leisure batteries to keep them topped up,I can’t remember the last time I switched off the van to start work and it read 13-7 and held it for a fair while before it started to drop.
End of day it was still reading 13v most of the time only dropping to 12-8 briefly over the course of the day and then back up to 13-7 after a short drive.
I thought about this yesterday. When you switch your engine off both starter and leisure batteries will show the same voltage of 13.7v.
If your split charge relay is an intelligent one then it relies on voltage from the starter battery to trigger it and disconnect it.
Depending on how that relay is programmed it could be sometime before the circuit is disconnected after the engine is switched off.
In other words, if your relay is set to disconnect at 12.8v then the relay will still connect both batteries together until the starter battery voltage drops to 12.8v.
I believe most 'intelligent' split chargers are connected to the charge signal wire of the alternator or an ignition feed. So when the alternator is not producing a charge or the ignition is not on the leisure battery is disconnected from starter battery. The split charger should only connect leisure battery to starter batt/alternator when starter batt V is high enough and alternator is producing electricity. That is why on some modern (Euro 6) engines you need a more sophisticated battery to battery charger as the ecu stops the alternator producing electricity when starter batter is full to save fuel! Some of these euro6 vehicles have a shunt resistor in so they can monitor the load on the battery and work the alternator according to the need. The alternator is only about 60% efficient ie. it pulls almost double the power off the engine than it generates. So being able to stop the alternator drawing power as much as possible reduces fuel consumption and emissions.
If both batteries are fully charged they should be at the same voltage when you stop engine.
If they are only sensing the starter battery voltage then you could have a drain on the leisure battery that would take the starter battery lower and that could compromise starting!
My first split charge relay had a trigger wire that either went to the ignition switch (or accessories powered by the ignition switch) or the alternator field wire. I chose the alternator.
The last split charge relay was a smart (intelligent) one that just operated on voltage sensing. It was an M Power, a Durite copy, which was bi directional. They call them VSR's or voltage sensing relays. The relay is only activated when it sees a voltage of 13.7v. That voltage will only come from the alternator when its charging so current will flow from the starter battery side to the leisure battery.
The relay deactivates when either side sees a voltage of 12.8v.
The B2B charger I now have is only 1 directional. But the cut out of the relay is much lower at 12.6v which I don't like. When I switch the engine off the B2B charger still remains connected to the starter battery and draws current from the starter battery to add a little more charge to the leisure battery. I can see what's happening on a Victron battery monitor app on my phone.(12.6V is more than sufficient to start the engine so maybe I'm knit picking as its worked fine for the past 18 months.) The B2B charger does have provision for an ignition trigger but this isn't ideal. An alternator trigger would be better but its a long way to run a trigger wire.
I will probably wire in the old VSR from my old van and use that as a trigger and disconnect at 12.8v which I would prefer.
I'm going to stick my neck out here and suggest that the Sterling B2B charger I have has been designed around the motor home leisure market rather than us windies who require high power draw but do limited mileage.
With our industry the general thing is that the starter battery will always have a higher charge than our well used leisure batteries.