How smart is the isolator??

Doesn't sound right. Depends on the isolator, but I wouldn't expect it to know that charging voltage is available at the auxilliary and try to charge the cranking battery.

I would NOT connect the charger's positive to one battery and negative to the other - both the negatives should already be tied together via the chassis anyway (so that connecting to either negative is in fact connecting to both), but the charger positive should be connected to the battery you intend charging.

If it's a solid state isolator, perhaps it was triggered on a once only basis by some voltage spike (such as could occur if the charger was running when connected to the aux battery, or perhaps during installation of the new battery?). Could also be that it's died and is connecting the batteries permanently together, something you may not notice in normal use.

John

Hi Spade

If the isolator is still engaged from recently switching the engine off then it will stay on and charge both batteries.

Perhaps switch the headlights on and wait for the isolator to switch off before connecting the charger.

Also some isolators are bi-directional and will charge in both directions.

Perhaps give us more info in the isolator - make and model - also battery voltages etc.

Regards

Derek from ABR

Spade,

Interesting post mate.
In my opinion there are "dumb" isolators and "smart" isolators.
In fact, I wouldn't even call a "dumb" isolator an isolator at all.

I once put a query to the technical department at Piranha re the DBE150S Isolator I owned and received a reply that this (smart) isolator kept the primary and secondary batteries electrically separated from each other at all times.
This is exactly what I would expect from an ISOLATOR and not "sometimes" having them connected to each other and sometimes not.


Bill.

Every car is fitted with an isolator the Sand Man describes. Fit a battery anywhere and it remains isolated at all times - exactly as per "the primary and secondary batteries electrically separated from each other at all times".
Yep - very smart indeed.

But many prefer practical isolators that keep batteries separated but allow both/all to be charged.
Lots of ways to do this. Some even purport to involve "microprocessors" - now that's hi-tech - way different to normal analog electronics or circuits that others use. They may even be using 32-but architecture now.... just for that ultra-fast intelligent response.

Alas I have my cheap simple isolator (relay) that doesn't have the above problems.
But as per J&V and Derek wrote, most isolators do not "sense" the 2nd battery(s) unless they are still connected.
If those isolators are connected the wrong way around, the they would probably never connect except when being externally charged.

But there are dual-ended sensing isolators - though they tend to be equalisers and converters (dc-dc) whether with or without isolation.

And chargers should be charging across one battery (except maybe for "balanced" matching parallel connections with diagonal power feeds).
Remember - systems are designed with battery voltages in mind (ie, 14.4V, 13.8V etc) - not dash or load or alternator voltages (within reason of coarse!).