Our '07 XK is my wife's daily driver, used mostly on very short trips with monthly 100 mi round trips to Portland. My rule of thumb has been to replace batteries every four years. I am revising that to every three years with the Jaguar. Jaguar has a lot of low voltage circuits that do not always work well with less than optimum power. We were having little glitches like a key that was acting dull, even with new key battery; and an intermittent airbag warning light.

The new battery has "fixed" this, I think/hope. "Time will tell...."

I bought the battery at BatteriesPlus at $219. It was their most expensive Duracell, which is a bit taller than standard battery size. It fits nicely, plenty of room. I have a ScanGauge velcroed to my steering wheel pad, and I monitor voltage. Old battery system was in high 13s. New in low 14s.

Most folks install a new battery "as is" and it is "good to go." Batteries from the store are not fully charged, but are usually charged "good enough" to start the car. Better to install the battery fully charged at a trickle rate. Whatever, charging a new battery on a trickle charger is not harmful. In my case it took almost 12 hours on a Battery Tender to bring it to full charge.

 

There are a lot varied opions on battery and issues surrounding them. I know when the battery gets some age it can start to give glitches if not fully up on charge. I have no idea how old my current one is as I just picked the vechicle up a few months back but after being at a paint shop for a couple weeks it started up with no problems. I'm hoping that is an indication it is very healthy.

My previous Xk was always on a tender when not being used for a while or between very short drives.

I'm thinking of picking up a 12v power point gage just to monitor voltage but am not sure if that is the better to test there vs through OBD2. I'm sure there are discussions on that topic somewhere here.

 

I think a good all-around "gauge" is the ScanGauge. It is quite small and easily fits on top of the steering wheel pad. It connects to the OBDII port. It monitors a lot of functions including voltage, coolant temp, fuel pressure, miles per gallon, speed, and, of course many default codes (sic). It can also clear codes temporarily or permanently, depending upon the severity of the fault. It does not read oil pressure - wish it did.
Its readings reflect exactly what the motor and drive train sees; and these readings may or may not be totally accurate. But for our purposes, they work well enough.

The point I was trying to make above is that a battery may well have enough to start a car, but that does not necessarily mean it is "healthy." The three year old battery I took off had no problems whatsoever starting the car. Yet, it was not fully charged through daily driving. I put it on my bench with a Battery Tender and it took 10 hours for it to reach full charge (as per the Battery Tender).

 

It's a cheap investment but tells you what you want to know. Just to bad we need to buy it.

 

Seems like I hear less and less people recommending AGM batteries. There are a couple out there that are direct replacements for the standard battery, and they charge much (6X) faster during daily driving and supposedly have much less voltage loss when sitting for periods of time.