Dual Battery Dilemma

Yes Michael, something is not right.
A battery isolator will isolate when the system voltage falls to typically 12.7 volts.
So as your faulty battery pulled the main battery down it should have disconnected and prevented the main battery from being discharged any further, leaving sufficient to crank the vehicle.
The isolator needs to be checked out, and it would be a good idea to have the health of the main battery checked also.

Crook isolator dragged main battery down

Get yourself a basic solenoid (about $45) with a dashboard switch so that you can manually isolate the batteries.

I have been down the path with two brand name isolators and they both failed before having been 12 months in use.

In providing balance to one reply, a new isolator might be the go - there are hoards of happy long term users of such devices - my current Surepower model is still quite happy after 6 years service - A Rotronics unit I had in my previous conveyance was about the same age and still going fine when traded.

Thanks for all your replies.

Is it possible that the alternator could have been directing all the charge to the faulty 2nd battery, at the same time forgetting about charging the main one and allowing it to go flat??

Or is that a dumb non-sparkie question?

Hi Michael

It is best to have the main battery and isolator tested.

It is also possible that the contacts in the isolator have melted / welded together. I have also seen a popular solenoid full of dust and not allowing it to disengage. (Mounted horizontal did not help and the thin cork gasket was in pieces).

Do a few voltage drop tests across the solenoid or send it to me and I will give you a report.

Regards

Derek from ABR

Michael,

Are you sure you have an isolator between the batteries?
If no isolator is present, a fault in one battery will drage the other one down until both are flat.

Until you ensure an isolator is present and working properly, I would disconnect the deep cycle marine battery to protect your new investment.

The best isolator to use is a "smart" one, that does all the switching for you.
Some, like the Piranha DBE150 or DBE180 series electronic isolators have an indicator to show when the isolator has switched over to allow charging of the second battery, once the starting battery has reached a satisfactory voltage.

Have had a redarc isolator for several years. Wired up so that if the main battery is flat, I push ina button, hold and both batteries will be connected...this allows me to start the vehicle. Relaese button and both batteries isolated and charging as before( naturally will investigate why main battery was flat).
Supershure have had a recall because several of them started fires. True...my works nissan,s unit has to be replaced due to this recall.
cheers

Michael
Might pay to get your charging system checked as well, if both batteries were flat there is a good [maybe bad] chance that it's an alternator problem and not the isolator.
Murray

Thanks to all who replied!

Off to the auto electrician's today for a check-up.