Batteries drain to flat?

[meterreader]

Hello people.
Havn,t been on this site for some time as everything has been running smoothly. I have a 1996 2.4 turbo transit. I have utilised the second battery space to add a standard 12 volt battery with isolator between as I occasionally go camping and use 2nd battery for internal lights only. After my last trip the van was stood for a couple of weeks and when I went to start, the battery was dead flat. Fitted a brand new battery and now this has done the same thing. Both batteries show 14.3 volts when engine running. Isolator is supposed to prevent "main" battery from being drained. Obviously a Drain from somewhere anyone got idea? System has ran for a number of years without any problem.Only electricl issue I have ever had is starter solenoid occasionaly, twice in 12 months, stuck and required tap to clear. Suggestion please !

[dumper]

When you park up do you isolate the second battery if it’s weak it can drain the main battery down on early vans we had at work with the twin setup if one of the batteries was done for if it was disconnected the remaining battery would be all right and start the van with no problems.
I’ve also had batteries going flat when parked overnight due to a internal fault in the alternator draining the battery.

[mk3_wrc]

Are you using a 12v relay with connection made from a live feed from the alternator? Or have you fitted a "smart sensing" split charge relay?

What does the 2nd battery voltage show when its not charging? Could be arseholed and killing the 1st.

When you say running lights do you mean normal van lights of have you got a conversion/partial conversion?

[bruce.edlin3001]

It doesn't take much of a parasitic leek in the existing system to drain the battery over a week or two

Sent from my SM-A156E using Tapatalk

[meterreader]

Thanks for the replies. I have a splitter unit between the two batteries that "should" have stopped the 2nd battery from draining the first. Currently having both batteries checked out and have disconnected the second from the first. Starter battery is holding charge. But the world is not treating me fair. On the way home I noticed that the rev counter wasnt working and from past experience knew this waas likely to be an alternator problem. Lifted bonnet and alternator was red hot and the front fan was spinning freely. It was too hot to check anything so will check out tomorrow.b Two issues not related as I had checked charging voltage day before at 14.6v.

[mk3_wrc]

When we had a lead acid in the engine bay as the 2nd battery for the habitation element of the van we juat had a basic high amperage relay to charge from the starter battery, this only made the connection when there was a live from the alternator (i.e. engine running).
The engine battery ran flat when we didn't move for 3 weeks, so we fitted a votronic battery maintainer which charges the vehicle battery ar about 3 amps when the solar is giving juice to the leisure battery. No issues since.