battery charging

Do you have a redarc or similar setup to charge the agm.

As long as you have this it will be fine.

The different types of battery do matter, and the redarc has nothing to do with it

Yes I agree get a redarc but for the other reasons.

Simply put, the two batteries, when connected, are in parralele and charges in parralel., Best then for the two batteries to be similar, or at least of similar size/capacity

But it will work, like many systems not quite 100% but hay, it is still better than one.

CJ

Yes, they will charge fine together.
Make sure you have good cables to the agm, and when you finish your trip/ use, top up the batteries with a good battery charger again.

Great idea to have more amp hrs with another battery.

Trains

sounds like it's going to work then :)

Will go for the Remco 105a/h which fits under the seats quite nicely. Agree it's not perfect but I can't get an AGM to fit under the bonnet and it's too hot in there for that type anyway.
this will give me a 150a/h solution rather than my current 50 which just isn't enough unless you are driving every day.

thanks for everyones advice.

Maybe a bit late on this (with apologies), but I've been dealing with these issues a lot recently and found some good sources....
One clarified the difference between needing matched batteries when paralleling full time (which is not recommended anyhow) as opposed to during charging.

The most common implementation seems to be a standard vehicle battery system with added "auxiliary" battery(s) for PCs, audio, fridges, etc.
These require an isolator to avoid the "not in use" parallel issue as well as provide independence to ensure the cranking ability of the original battery (engine etc).

BUT, instead of costly isolators, a relay is used.
The relay's coil is simply connected from earth/chassis to the alternator's or voltage regulator's charge lamp circuit.
The relay is normally de-energised, but when charging begins thereby switching the charge-lamp's earth to +12V and extinguishing it, the relay energises and connects the aux battery +12V to the main system.

Hence when the vehicle is charging, the batteries are connected in parallel.
This is NOT a problem whilst they are charging - irrespective of capacities and type. (Other than must be lead-acid type - ie, 12V wet, flooded, sealed SLA, Gel-cel, AGM etc.)
The batteries charge at their own rates based on whatever the alternator can output which should up to 14.4V and usually at least 13.8V.
Rarely will batteries take the full output current of an alternator, but if they do, they are still charged at the maximum rate the alternator can handle which is the fastest recharge rate available (in terms of combined system charge).


There are some other issues like what limit relay current the alternator can supply (0.5A to 1A seems typical which is more than most relays) but this can be easily overcome.
And the inter-battery cabling and fusing at each end (2 fuses!), but this is standard anyhow for non-adjacent parallel batteries.


I may not be explaining or describing this well enough, but I could link the source(s) I got my info from if anyone is interested.

I've used the same system on my truck and it's been great.
I use circuit breakers instead of fuses for the battery interlink as well as a low-voltage cutout from the second battery. I too had fuses blow due to some typical transient and then flattened the second battery so my fridge failed.
Not that I've had a failed interlink since the self-resetting circuit beakers were fitted, but if I do, at least I won't wreck my aux battery!
Costs were approximately $10 for a 60A relay, 2x$7 for 2x50A circuit breakers, and $20 for the low-voltage cutout, plus cabling, Anderson connectors etc.