So my FBH works both manually from the LCD and from the remote.

The main battery is a meaty AGM that is about a year old. When it was brand new it would run the FBH for the full period but, as I understand, the BMS manages the charge level so that the battery is actually never charged to full capacity.

The net effect is that on most occasions, despite a good battery and regular use, the battery management system intervenes to shut off the FBH prematurely when the main battery voltage drops to a threshold with the effect that the cabins never gets warm. Very frustrating!

I have considered plugging in my CTEK and putting it in supply mode but I mostly park on street so it means running a power cable across the pavement which is both inconvenient and a bit unpractical.

So I’m now thinking of installing a dedicated AGM in the boot solely to power the FBH. My initial thought is to run a new cable directly to the FBH and disconnect the original power supply by removing the fuse. ( which I recall from another post on here is in the engine bay fuse box ). I would then install a new DC-DC charger in the boot to keep the new battery charged having selected a power source in the rear fuse box that would avoid extra load on the charging system during starting.

As long as the new installation is safely fused and earthed to a suitable ground terminal dose anyone foresee any issues that may undermine the viability of this approach ?

All thoughts and suggestions welcome ! TDV8 4.4 2012

Isn't it simpler and easier to just run a standard aux battery to give you more capacity to run the FBH? I never had an issue with getting the the FBH to warm the cabin for the max time. 2012 TDV8 Vogue (L322)

Hi @JayGee, I’ve found that I’m not able to charge using the big power cable going to the boot fuse box with my CTEK. I guessed this was down to the BMS. That put me off when I was considering the standard second battery set up because if the CTEK won’t work as a supply there then why would a second battery.

P.S. - I see we have the same MY, does yours charge from the boot fuse box? May be there’s something weird with my BMS. TDV8 4.4 2012

Its not the BMS that shuts down the FBH, its the low voltage threshold setting in the FBH itself.
I've had several cases where the battery terminal voltage is high enough but there is volt drop in the circuit to the FBH, firstly, check the fuse and ground point for corrosion, if they're OK, then I'd suggest running a heavier fused connection direct to the battery.

Thanks for the tip @Phoenix ; I’ll take a look at the unit and the fuse box over the next week or two. That’s certainly step 1…. Much cheaper than a DC-DC smart charger and an additional lithium battery. TDV8 4.4 2012

If you are able to backprobe the FBH power connector, put a voltmeter between the FBH +ve and the battery +ve, then fire up the FBH, you'll be able to see what the volt drop is, and if it's the cause of the issue before you get b@lls-deep in the wrong direction... It'd be worth repeating the measurement on the -ve side as well.
FWIW, I wouldn't go with lithium, they're not great for the type of load a FBH will present, AGM or EF+ (ideally autostop rated) is your best option if required.

Ramblin Man wrote:

Hi @JayGee, I’ve found that I’m not able to charge using the big power cable going to the boot fuse box with my CTEK. I guessed this was down to the BMS. That put me off when I was considering the standard second battery set up because if the CTEK won’t work as a supply there then why would a second battery.

P.S. - I see we have the same MY, does yours charge from the boot fuse box? May be there’s something weird with my BMS.

I had the same issue of too early shut off due short trips.
I rebuild the FBH system compleatly, make it fully manual, without any smart controls.
Of course I install 230V charger to avoid real battery inflation (this option may not be suitable for everyone due no 230V plug near car parking place).
Now I can operate FBH from remote control and start it for unlimited time. I also install the switch in the cabin to keep FBH running while driving.