I still have my old battery, replaced with a Bosch S5. I don't think it's all that knackered in fact it may not even be knackered at all, the Bosch still goes flat after a few cold days parked, 20mins with the FBH much the same as the old one. It's currently just sat in the garage and when I think on I connect the C-Tek to charge it. I also use it for camping, air bed inflation, iPod charging and wine cooler etc - I think it's called "glamping". So I think I can just leave it connected to the 12v socket in the boot (always live), assuming I start with it fully charged but I would disconcert it when starting as I know the 12v socket won't take that sort of current required to start the car if the main battery went flat. If this situation arose I would just cart it to the front and use jump leads

Thoughts/opinions
 Geoff

I think there is actually instructions on how to do it on the "new" rave joe90 found the other week. There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6

I'll have a look, I bought the hooky CD - less messing  Geoff

Out of interest I have a 110AH battery sitting in the boot permanently plugged into the boot socket.
The plug has a 10A fuse in it.
Been running round like this for months, never disconnect it for starting.
Certainly works in terms of battery capacity.
When it gets warmer I intend to fit a large second battery in the side compartment connected to the existing setup using 16mm2 cable, 100A rating.
I will put two Anderson connectors in the cables for disconnection purposes.
Nice and simple.

'nuff said ill order some bits and bang it in. Geoff

There's lots of people say you shouldn't leave them permanently connected as there's something called parasitic drain - when they are not under charge, 2 batteries will start to drain each other - I'm not sure how quickly that takes effect, but if your reason for doing it is to be able to leave the car for extended periods, then worth looking into.

I run dual batteries in my Jeep and have a 200A relay between them that is only engaged with the ignition - I also have solar chargers on both, so that I can leave it without using for extended periods.