It probably isn't overcharging as that will kill the Cranker as well. A huge percentage of Patrol Aux batteries mounted on the LHS guard die because of sulphation (under charging) or heat or a combination of both.
First thing to know is... Solenoid/Relay style Isolators (Redarc, Projecta, TJM etc ) WILL NOT (except in exceptional and very rare circumstances) control the charging of the vehicle batteries when the Alternator is running.
They WILL NOT ensure the Cranker is fully charged first
They WILL NOT ensure you have a fully charged aux
They can only be relied upon to do 1 thing and that is prevent the Cranker being drained below 12.5 volts with the Alternator off.
If you want your Aux battery charged correctly then the best alternatives are a multi-stage Battery Charger either 240 Volt (at home or in camp) or Onboard. The Onboard jobbies are also called DC DC converters or similar and are a voltage step up device and electronics to charge the Aux Battery.
Ctek, Redarc and many others make units such as the Redarc BCDC1220
Once you have your charging sorted out then it is time to address the heat issue and also the type of battery. Low Charge current AGM batteries such as Full River etc are not ideal for the LHS engine in a Patrol. Look at the battery specs and if the manuf reccomends a max charge current of say 20 amps select a different battery. The battery may be top notch brand and work gang busters as an Aux (I have 2 in my camper) but it is not suited to high engine bay temps and higher charge rates from an Alternator. There are many Deep Cycle AGM's suited for the application but not all of them by a long shot.
Anything you can do to reduce the heat soak into the LHS battery will greatly extend it's life regardless of battery type.
To sumarise...
1. the Solenoid/Relay Isolators will only isolate a Cranker from being drained by an Aux load and are good for bugger all else
2. the DC/DC chargers are the best solution to controlling the charging of the Aux (choose a manuf and capacity that suits you)
3. anything that protects the Aux from heat soak is a very good thing
4. if you don't use an onboard charger then regularly and also after every trip pop the Aux on a multistage charger at home and top it off
Here is a link to get you started, purely FYI http://redarc.com.au/products-and-se...tart-bcdc-1220
Hope that helps a bit
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