Redarc SBI12 & AGM?

SBI 12 is not a charger, all it is is a solenoid that cuts in a set voltage to allow the second battery to be charged. The cranking battery is therefore charged fully before the secondary starts to be charged.
The charge is from your alternator. If your fridge is connected off the secondary battery therefore won't flatten your cranking battery as it is isolated by the SBI 12

The SBI12 is an isolator, designed to separate the start battery and the aux battery to protect the start battery from external loads when the engine is not running. It doesn't charge batteries, it allows the alternator to charge them when the primary electrical system is ready to do so.

Whether or not that will work depends on a number of things, such as
- Alternator output voltage. Increasingly they are too low to fully charge an auxiliary AGM battery.
- Depending on the AGM battery you have chosen, it may (probably will ) have max charge current restrictions below what the alternator will deliver when the battery is at a low SOC. Exceed those and you risk reducing battery life.
- Cabling to the second battery.
Etc.

You might be lucky. If you get the right combination of alternator output, battery charge requirements, get the cable sizing right it might work.

To assess a particular case like yours, more info is needed.

Thanks for the replies folks. I haven't purchased a battery yet, still looking.

I have 2 SBI12's . I'm trying to save a few $$$s so I may just go with an old style wet, deep cycle battery, the SBI12 should be fine to recharge that.

I need as a minimum 100Ah. It's going to run a single fridge that draws at worst case 2Ah and a few LED lights. The battery needs to mount under bonnet and can't be longer than 310mm.

If I have to I could buy a cheapish DC DC charger, spotted this this morning, wondering if anyone has any experience with these, DC DC Charger from ABR:

https://www.sidewinder.com.au/page168aa.html

Thanks everyone for the help. The vehicle is a 2013 Patrol 3 litre.
I've found a few AGMs that say they can be fitred under bonnet. 1 is from SSB. The names of the others elude me at present.

I understand that you want to save money, but whilst going cheap on initial costs means less money spent now, it won’t save you money in the long run.

With any battery charged only via a VSR & under the bonnet expect a max battery life of up to 5 years. (But quite possibly less).

Taking the battery out of the heat & having it in the car may increase it’s life a bit, but if inside the car with you - ensure it is AGM, not a wet battery.

If using a VSR & under bonnet battery, I see no advantage in spending any more than you have to on a battery. A cheapie will get you a battery life much the same as a more expensive one.

A good AGM inside the car, looked after(ie. not regularly discharged below 50% & charged back to ‘float’ every 24 hours) with a smart charger should have a lifespan of 10 years+.

Batteries are expensive.

I don’t know the ABR dc to dc product, but offer a couple of considerations. A lot of cheaper dc to dc chargers are low amp outputs. Commonly 10amps or thereabouts. They will take a far longer driving period to recharge the battery so may not be effective, I recommend a minimum of a 25amp output. Also temperature compensation is a very desirable feature which is a significant aid to long battery life.

There are two commonly available types of smart chargers - dc to dc & solar regulators. If you have solar then the use of a VSR to charge whilst driving, & solar to top up the battery to 100% (which the alternator never will in day to day driving of a battery supplying regular loads) is a valid & workable solution for less initial cost than a dc to dc charger, providing you either stop driving early enough each day to set up portable solar to give it enough time to do the job, or if you have roof mounted solar which works all the time in conjunction with a VSR.

At the end of the day, a VSR alone will work, but will give you less available overnight battery capacity (from the same size battery which has been ‘smart charged’ but it may still be ‘enough’ to meet your needs). A valid path to take, but just buy a cheap battery - even just another starter battery - & expect to replace it far sooner.