I have spent most of the day reading the various threads on this great forum to see if my solutions exits, but unfortunately have not come up with a definitive answer.

I leave my car at the airport for extended periods of time (3 weeks) and on my return this time my battery was completely flat (4v).

I'm going to start the investigation into current drawn as soon as my clamp meter arrives. But I've been thinking about adding additional capacity for a while now.

If I connect an additional battery, similar in capacity to the one in the front, to the 100A cable in the back, this would provide the additional capacity and hopefully leave enough in the main battery to start the car on my return.

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.

I'm sure its not as easy as that, but I have "faith in the forum"

have you read this from the wiki,

http://www.fullfatrr.com/forum/topic26065.html

Just fit a split charging relay as used for caravans, that would sort out the charging. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

Thanks Stan;

I did read that thread, the bit that prompted my question was Joe90's comment "The cables on the second battery and the wiring back from the rear end is not really capable of coping with cranking amperages, hence the fuse to prevent melting cables and/or fire!! Might work in an emergency, though....."

AJGalaxy
From what I've read a split charge system will isolate the additional battery when the main battery voltage drops. This is exactly when I need the extra capacity.

Never had a problem with the cables or the fuse.

Your best bet is to fit the second battery, but only connect it up when you get to the airport (unless you have two "new" batteries or equal worth, otherwise you need fancy regulators and switches), and then disconnect the main battery.

Then all you have to do is reverse the above on your return, in the hope there is enough juice left to open the doors! .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

I have been trying to find out how the 2 battery system on the L405 works, but without the workshop Manual I am not getting very far.
Mike. G reg 2.5VM Vogue Portofino red 1991- 1999
V reg 2.5td P38 Rioja red 1999- 2006
53 reg td6 Vogue Oslo blue 2006- 2015
11 reg 4.4 TdV8 Vogue SE. Baltic blue 2015- date.
https://www.fullfatrr.com/forum/topic56162...tty+affair

I believe dan_1984_uk did this ages ago (with write up?) - I have his car now and it is a lovely job.

http://www.fullfatrr.com/forum/topic5721.h...ra+battery

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. BMW i3 Electric Car
2012 Full Fat RR 4.4 TDV8 (now gone)
2006 VW Touareg 3.0 TDi V6

Thanks both. I did read that thread and Dan_UK_1984 describes pretty much the same idea i.e. the need for extra capacity. Which is what got me thinking in the first place.

I sounds to me like that is my solution.

Quote
Initially it will just be extra capacity, the battery that's in there is about 50% of the power of the main battery. I bought a gel battery but unfortunately was a smidge too big to fit in the hole, I think Optima's are too big too Sad

I'm aware of the 100Amp cable limitation, that's why there will be an inline fuse rated to 40A and I'll see how often it blows. In reality, it just means I can run the preheater that little bit longer, or forget to turn the lights off or longer etc. As instead of discharging the main battery 50% and then not being able to start, it will discharge only about 75% and should still have enough power to start the car.

I have a second 110Ah battery fitted in the rear of my car connected to the 100A cable that serves the rear fuse board.
This cable is protected by the 100A fusible link as per original figment.
I also have fitted a battery isolating switch to the -ve terminal of the original starter battery.
My arrangement for leaving the car at an airport etc on parking the car up is to switch the original starter battery off via the isolating switch and then leave the car running for the duration on the rear battery.
This then insures that the starter battery is still full charged on my return to the car.
Returning to the car I simply switch the starter battery on and then start the car normally.

Done this many a time without any problem.

I have to admit that my car doesn't have a battery drain problem and that on returning to the car having been away the second battery usually still has 12.6V on it. I have a permanent voltmeter installed.
I see no reason to worry about starting the car with this arrangement even if the second battery is full discharged.
I did in fact test this by deliberately discharging the second battery, switching on the starting battery and then starting the car as normal. Nothing happened and the second battery simply charged back up as normal.

You really need to resolve the battery drain problem, I afraid a voltage of 4V will kill the battery if repeated very often.
Although the second battery on my car runs the car quite happily when parked up, I do insure that the manual locking on the drivers door is fully functioning as obviously if that battery did deplete as yours does the electric locking will be inoperative.

There is no real need to fully fit a second battery for intermittent use, my arrangement above can be accomplished by simply putting a second battery in the boot area as required and connecting it to the car by a fused 10mm² cable to the rear fuse board supply and a 10mm² earth cable to a local earth point.
These two cables could be run into the boot space on a permanent basis terminating in Anderson connectors to allow
connection to the temporary battery.

Hope this gives you some ideas

I think you are skirting the real problem, what is causing the battery drain, my car often gets left for a couple of weeks between runs and I've never had an issue starting it.

Have you had your original battery tested, what is it? if its the original LR battery then after this length if time it could be a prime suspect for being a bit duff or losing its charge over time.

Personally I would be trying to fix the drain.



edit, if its getting that flat and you actually have a drain which is taking that much out of the battery, then adding another will just result in 2 flat batteries in the car, unless you isolate one as said above, 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

Many thanks for the speedy and informative replies. The wealth and depth of knowledge on this forum is priceless.

To fill in some of the blanks. This is the first time that I’ve had a completely flat battery, I’ve always managed to start the car (without jump pack assistance) on previous occasions.

The previous battery was a Land Rover item and fitted last December. Unfortunately I no longer have it as the RAC man took it away with him.

The thinking from the RAC man is that the interior lights had been left on by the Valet service.

I’m paranoid about the battery on these vehicles, and would relish the extra capacity for FBH, radio when parked and of course; starting after a prolonged period.

Clamp meter is on order so that I can investigate if I do in fact have a drain.

Once again, many thanks.

i had this problem with my BMW M3 , leaving it parked at airports/train stations whilst abroad. In the owners manual it said the car will drain the battery @17% per week whilst parked . I had no room to fit an extra battery so I got a solar charger and hard wired it to the battery and bought one of those little jump starter packs as back up . This worked well after I replaced the battery as it struggled to hold a charge after going flat 3-4 times .I bought a C-Tek charger but that couldnt save the battery.