I have been having intermittent trouble codes. Although nothing caused the check engine light to luminate. A suggestion from a mechanic at a local european auto repair shop was to check the auxiliary battery.
Dead (knackered). VOM meter showed 0 volts.
I remembered a UTuber that managed to revive a dead battery by connecting a good battery to the dead one, which also showed 0 volts, then connecting a charger to the two connected together. It worked and the dead battery was able to be recharged.
So I gave it a try.
I left the two connected for over an hour with the charger also connected.
Not a single volt showed on the battery after disconnecting the two.
The auxiliary battery is truly dead.
I had searched online for a replacement. I know the Exide EK151 is a suitable replacement, but I don't like the cost. I found a replacement with Walmart but it was a straight lead/acid type. Not AGM. It took me several more days of searching before I found another on partsgeeks.com. This one is AGM.
The Exide price in the US was over $200 USD. The partsgeek.com one was $101 USD with shipping.
I will report how things progress once I receive/install the new battery.

 

The only thing that battery is used for is for the start/stop system. If you don't like that feature, there is no harm (and no codes) in unplugging it permanently (and removing it to save weight). Several members here have done just that.

 

Consider yourself lucky. As Matt has pointed out, the secondary battery has only one (useless) function: powering the annoying stop/start system. Disconnecting it was Job 1 on my F-Type years ago. Zero negative issues resulted.
The amount of fuel saved by this irritating bit of electronic manipulation is very, very minor.

 

I know that it is only for the stop/start. But I am chasing whatever is causing the intermittent codes that keep showing up during OBD scans. This is just a step in that direction. It was a suggestion from a certified technician.
If the replacement of the battery doesn't help, then I go to something else.

 

If you're chasing intermittent codes, you really need to know what those codes are. I don't have the auxiliary battery so can't say for sure, but I don't think it throws codes. As noted, many have removed it to permanently disable the eco start/stop feature with no (reported) adverse effects.

 

As per every Forum member I disconnected it after second failure years ago with no problems.

 

1: Certified doesn’t mean anything unless they are JLR proficient and by him suggesting that the aux battery might be your problem suggests he doesn’t know JLR’s. It could be that your main battery isn’t happy and letting the gremlins come out to play. Did he test that battery with a proper battery tester? If it’s down around 500 amps then you need a new AGM main battery.

2: Don’t waist money throwing parts at the problem. If you don’t have a good JLR mechanic then send it to the dealership for diagnosis.

 

I never looked for codes, but I never got any sort of error or warning when my aux battery died. I ran it like that for a year or two before I finally removed it.

 

Like Dave, I ran with a dead aux battery for years. No problems. No codes. Your problem lies elsewhere.

Intermittent codes are a common sign of a battery issue - but the main battery, not the aux. The first/easiest action would be to put the main battery on a CTEK and make sure it has a proper charge. If the main battery isn't recent, it may be the culprit. At Costco, a regular Interstate battery was a bit over $200 (probably up since I got mine). Past that, the next, most likely cause would a grounding issue.

 

I know what the codes are. I had them before a new main battery was put in last year and also immediately after. Same codes with a random code for 'something' else thrown in once in a while. Including an intermittent code for the secondary battery. Once there was a wheel speed sensor code. Again intermittent. Cleared that and it never came back. Then there was the time I started the car and only 4 cylinders were firing. Started it up the next day to take it to my mechanic and all 6 cylinders were firing. Still working a month later.
The codes are for communication errors for various modules. But everything works like it should. (With the exception of the cylinders that one time.)
I do have a permanent code for the misting sensors, but they also work as they were designed.
I could just let it all be as it is since it is all intermittent and everything works, but I am obsessive/compulsive when it comes to things not be correct with my vehicles.

 

All these random/intermittent codes certainly makes the battery/electrical suspicious. Remember, a new battery is rarely fully charged and should be charged before or shortly after being installed in the vehicle. It could also be a bad ground (earth). There are several grounding points on the vehicle and if one is loose/corroded/bad it can cause the problems you see. Double-check the battery terminals--we've had members reporting similar issues only to discover the cable on the negative battery post was loose and would break connection while driving over rough road and then be fine when checking everything stopped at home. Ground on the transmission housing can corrode and come loose as well.

These cars are SO sensitive to voltage that you need to completely rule out the battery and its cabling before looking further. Plus, it's the cheapest fix!