The other day I was reading an article about the stop/start feature in a GM car. It appears that in order to keep all of the electrical functions working when the car engine shuts down at a stop light they have a supplemental battery. I know Jag has the same feature and was wondering if they use 2 batteries. So just for fun I was wondering if this would have been useful in the XK. I know it's to late for the model but just thinking out loud if this would have made any difference. I'm just posting because I don't want to go out and shovel last nights SNOW.

 

When I was originally shopping for a new car, and I knew nothing about Jags, the salesman implied that the XJ (or maybe it was an XF) had 2 batteries to handle all the electronics. I could see an extra battery in the XK providing more reserve for the electron sucker, but eventually you'd have the same fate with two dead batteries (unless the electrical system was segmented to isolate critical functions to a separate battery).

 

My 2013 XJ 5.0L SC has the stop-start feature with 2 batteries in the trunk. The second one is much smaller. At first, I didn't like it and disabled it using the switch on the dash. But after sitting at some long traffic lights, I decided to try it and now I like it and use it all the time. It's fairly unobtrusive and I think it really improves city driving mpg. Pressure on the brake pedal determines when it activates; heavy turns the engine off and light restarts it. If you're listening to music, you can't tell when it shuts off (unless you're watching the tach), but there is a slight vibration when it restarts. The system is computer controlled, so if the battery is low stop-start won't activate. A/C, radio, etc., continue to work with the engine off. I use a CTEK battery maintainer to keep them fully charged.

I was surprised that my 5.0L SC XJ gets better city mpg than my 4.2L SC XKR by more than 1 mpg. Maybe it's the 8-speed in the XJ compared to the XKR's 6-speed, but I'd like to think it's the stop-start. I didn't buy either one for their fuel economy, so I'm just mentioning this.

Love both of them. Too bad I can't put the top down on the XJ.

Stuart

 

I am interested in a dual battery system since mine seems to die in 3 days of sitting, do you have any info that might help? 2011 XF.

 

I was actually thinking about adding a second battery to my car when I get it. I am a big fan of having dual batteries in a vehicle. I'm also a fan of using deep cycle batteries instead of standard starting batteries. They just seem to work better in my experience.

 

It will be interesting to see where you find a place to locate it. Also if you are getting a 2010 or newer car You should start to look into how the battery monitoring system will work with the 2 batteries. You may want to check into the later model XJ/XF on how they have twin batteries and the circuit they use. Then there is the AGM vs flooded batteries and which one comes in your car. Start doing the research now so you'll be ready when the car comes.

 

I think that finding a place to physically mount the batteries will be the hard part.

I think the monitoring system will work just fine with multiple batteries. The only time that should be an issue is if one of the two batteries goes bad. That would obviously be an issue with a single battery as well. Using two batteries is, from an electrical standpoint, the same as using a single, double sized battery. Twice the cranking amps available, and twice the reserve capacity. The cranking amps from two deep cycles would be significantly higher than a single starting battery, so each battery would have a smaller cranking draw on it. The doubled reserve capacity of the deep cycles is what would really shine. That should curtail battery drain issues as long as the vehicle is driven with any regularity. There would be so much reserve there that the electronics would have to burn through before the voltage drops. That's my thinking on it anyway. I might find out differently down the road.

I'm definitely going with AGM batteries. I've had bad luck with the flooded ones in vehicles.

 

You might want to read this article as it is pretty much the same system that is being use in the 2010-2015 XK.
Aux Battery Charging With Smart Alternators | 12 Volt Planet

 

Dual battery circuits are common vehicles pulling travel trailers, so you should have no problem finding a system.
A Lithium motorcycle battery is spendy, but has a small footprint, and can deliver almost 300CCA. I'd find a way to charge it, and hold it in reserve for times you absolutely need the car to light.
(thinkin' outloud, and maybe way outside the box)
V