Had the smaller battery go bad on my 2015 XF. The indication was only that the annoying engine stop feature quit working. When I tested the batteries, the large battery had good voltage and impedence with a Battery Tester, but the small battery had only 10 volts.
After research, I was able to find a replacement from eBay by searching for Exide EK151. Tough battery to source, due to unique top posts, AGM, physical size etc.
It was an exact fit, and a fairly easy DIY project.
Although I dislike the start stop feature, I felt it was important to return the vehicle to the design intent. I knew that I could not leave a discharged battery in our midwest climate since a dead battery can freeze and leak.

Original Jag part number CX23-10C655-AC

Battery box after removal of old dead battery. Two 10mm bolts allow the metal top bracket to be removed. Note the vent tube.

New battery in place, showing good voltage and 100% on the digital battery tester.

 

Some good content on this topic also in this thread: https://www.jaguarforums.com/forum/x...-issue-234819/

 

BTW, the local dealer wants $346 for this battery and they are on back order.
The eBay Exide battery was $159 with free shipping!
Hope this helps.

 

The small battery should give a minimum reading of 14v , so may need charged up over nite at some points , And no it don't feed from the larger battery

 

Thanks George for your reply. I agree with your line of reasoning, and had done this but not included that verbiage in the post.
I did attempt to put a charge in the small battery, and the charger indicated an error, so it must have been shot. The car had been driven daily, and the alternator was not able to keep the small battery charged either.

 

Crazy cars jaguars with battery's

 

Thank you!

 

Did that one have a vent on it or not? When the one went in or 13 I put a motorcycle battery in there which didn't have a vent but of course no issues. I am sure the battery in the 15 is probably not long for this world either. Interestingly I replaced the heavy plate OE big battery with a Dekka for a non start/stop car and it has survived in there longer than any of the Jag heavy plate batteries ever were.

 

You do realize that you can simply REMOVE the stop/start battery with no effect whatever on any system or functioning of the car? Unless you spend a lot of time in stop-and-go traffic the benefits of this irritating system are absolutely minimal.

 

Agreed.
Also the anecdotal evidence is mounting that removing the stop/start battery can prolong the life of the main battery.

 

Having had my car since late 2018, the stop/start function has never worked & I've never bothered to replace the secondary battery. To this day, main battery puts out 14.4 -14.6v when running & 12.4v with ignition on. Happy for it to be the way it is.

 

Thanks for your reply. Yes, I understand that I could have removed the aux battery. My personal preference was to maintain the car as factory/stock with the design intent.