Accessories battery replacement

[Aussie al]

Hi all.
I have a 2015 mk8 2.2 RWD, with twin batteries, no stop start enabled
I have no problems starting the van atm
The accessories battery has a resting voltage of 12.1, and drops further when under load, with engine off.
This was a national grid workshop van, that ran a small inverter from the accessories battery via the Customer Connection Point, and has after market wiring within the battery area as well.
Everything I've read says replace both batteries not just one.
I believe there is some form of voltage sensing relay in place, that is protecting the start battery from draining. So replacing the accessories battery only shouldn't be a problem
Has anyone got any experience or knowledge of this or a similar system?

Thanks Al

[dumper]

From personal experience every time at work they got their tight head on and would only charge one battery in a 12 v 24 v system the old battery would kill the new one sometimes in weeks or longer as they were connected together all the time but with the transit disconnecting them when ignition is turned off you may be all right but personally I’ve always charged mine as a pair and that includes the leisure batteries in the rear .

[bortaf]

i can only speak for my set up but i changed the acc battery (2nd time) 3 years ago and no issues yet, my reasoning was the only time the 2 are connected is with the engine running and then they are being charged by the alt so no draw from one to the other BUT,,, if the acc battery is dead-dead IE under 12v it will sag the starter battery and pull it down during GP pre heat cycle then you can get issues starting.
IME 2 batterys need to be in perminant parallel or series connection for one to drain the other, i'm on my 3rd acc battery in 7 years but still the same starter battery, this time i added a solar pannel to the acc battery, hopefully it will last more than the usually 3 years ??

[Aussie al]

Thanks for the replies.
I'll also need to check the chemistry of the batteries, my gut feeling is they'll both be agm.
I to forgot mention that when the alternator is charging them, it's putting in 14.9v with no additional loads on them. But 15.1v when under load. I seriously do no not like the 15v+ figure, as from prior experience it'll cook the alternator eventually

[Aussie al]

Also this is purely a vehicle battery, not a leisure/auxillary battery set up.

[bortaf]

Also this is purely a vehicle battery, not a leisure/auxillary battery set up.

That's what i thought and was answering too, leasure setups are a whole differant matter