solar charger is a decent idea,

not sure if they will work on the fattie as some have solar attenuation glass. Dont know if this will affect the performance of the panels. 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 have switched-in my 100ah rear battery on several occasions to provide starting current when the main battery is too flat. The 100A cable can withstand many times its rated current for the short period required to start the engine. I fitted a 250A resettable CB at the 2nd battery in case a short circuit occurs along the 100A cable yet withstands the short duration starting current. The CB doubles as a battery switch. The 2nd battery is charged via a VSR.

^ aggraa. You wouldn't happen to have a link to the solar panel you used by any chance?

Having an isolator on the starter battery enabling everything else to run on the aux battery is all well and good but what if one forgets the main battery has been isolated and attempts to start the car,wouldn't the wiring from the aux battery suffer as it attempts to feed the starter motor or would the 100amp fuse on it blow? 2008 L322 Vogue 3.6 TDV8 in Java Black with parchment and Navy interior. 125000mls.

I used to be indecisive and have a split personality... but we are ok now...I think?

I've had my second battery on the car for over 3 years now and switched off the front battery on numerous occasions and to date haven't forgotten to switch the front battery back on before starting the car. If you have a worry about this it would be a simple matter to clip or Velcro a small notice on the steering wheel or over the keyhole, assuming you have one, indicating the front battery is switched off.
The longest time to date I have had the battery switched off is 3 weeks, still had 12.4V on the rear battery and when the front battery was reconnected the car started as normal.
Never tried starting the car with the rear battery via the 100A fused cable to the rear fuse board but it's seems Graeme S above has on a number of occasions without a problem. I'm inclined to agree with him that for the duration the start current is applied the cable and fuse are adequate for the purpose. It may however be the case that repeatedly doing this would cause the fuse to gradually degrade and ultimately rupture.
Hence the reason I remember to switch the front battery back on before starting.

Thanks for your responses.I will use an isolator on the front battery.With regards to fitting a VSR,do I cut into the cars original positive red cable carrying the voltage to the rear fuse board Or to the new positive cable going from the rear battery connected to the same point as the first cable,or either? Many thanks.  2008 L322 Vogue 3.6 TDV8 in Java Black with parchment and Navy interior. 125000mls.

I used to be indecisive and have a split personality... but we are ok now...I think?

...OR having read that a VSR should be fitted as close as possible to the cranking battery,fit it onto the positive cable right next to the battery? Bloody confoosing this electickery!  2008 L322 Vogue 3.6 TDV8 in Java Black with parchment and Navy interior. 125000mls.

I used to be indecisive and have a split personality... but we are ok now...I think?

AJGalaxy2012 wrote:

millerj90 You actually said "The problem I cannot seem to resolve is how to protect the 100A cable from carrying "starter current" when the vehicle is cranked.

My thinking is a 20A auto-reset circuit breaker. This would allow the batteries to be connected for charging and the provision of extra capacity, but would trip as soon as cranking current was drawn. Then reset after cranking current has been removed"

So Im a little confused what you want. If the circuit breaker has tripped due to start current then the battery is disconnected, in your last reply you then said you DO want it to supply the cranking amps!

Being an offshore worker I had this problem on my Touareg. I fitted an extra battery as you are suggesting but left them in parallel, this gave enough capacity for 6 week duration trips i was doing. The immediate thoughts if 2 batteries are not enough are to leave the additional battery in the boot connected and fit a battery isolator on the main battery and when youre parked up switch it off. The boot battery will then power the accessories while youre away, when youre back, switch the main battery back on and start up, both batteries then charging from the alternator.