A lengthy description of the telecom system says that that error message occurs when the embedded SIM card
cannot connect with the cellular 2G system which is what is in our card. Low or dead battery gives a different message. There is a rechargeable AA and a regular AA battery welded together. The rechargeable battery is constantly recharged from the main 12v battery when the engine is running. If the engine is not started for four days the entire system goes to sleep. You can set a timer on the phone app that will wake the system up on the set day. This saves the rechargeable battery from discharging. If that were to happen then the regular battery would discharge and you would lose the emergency backup power until replaced. Replacing the batteries requires some dealer reprogramming so it says.
Some cars have a theft protection that works off the TCU system that has to be turned off from the app before towing the car or it will be reported stolen. When 2G cellular is gone, none of these systems will work. For now it works great. We would need a new TCU system with 4 or 5 G SIM cards. Can’t imagine how much that will cost.

 

The part was through a 3rd party, LRBritishparts. They contacted me asking for my VIN and then said NOPE, mine is on back order.

 

I probably got the last one they had. I bought from the same supplier but through eBay.

 

Where did you find the lengthy description? I replaced the battery pack and still had the warning come up multiple times when I just took it out for some errands. I got it several times in succession immediately but then less frequently and it was not showing when I got home. I'm hoping it figures things out and I don't need to visit the dealer.

It's two AA-sized batteries, but they're not standard cells. One is a lithium ion rechargeable and the other a lithium metal oxide; both are nominally 4V. They're shrink-wrapped together. Each has its own wire to the connector with a common ground. The cells are made by Tadiran, TLI-1550 and TLM-1550.

 

 

That's a better description than any I recall seeing before but it's for a newer version than I've got (Defender, 2020-2022). It notes that the battery is a single rechargeable cell and "When the backup battery has been replaced, the system must be reset, setting the backup battery level back to 100%, using the JLR approved diagnostic equipment." I'm not sure that applies to the twin cell battery pack, but I'm also not sure it doesn't.

EDIT:

 

Do not expect your dealer to diagnose any esoteric electronic problem. My dealer spent two days trying to figure out why my touchscreen only came on randomly. They never had a clue. But they did tell me there were no codes generated. So I took the car back, pulled he touchscreen and checked every capacitor. They were all perfect. While doing this, a small chip called a thyristor fell off the touchscreen inverter board. The solder joints had become brittle. This was too minute for me to replace so I bought a touchscreen on eBay for $50 with a cracked glass and swapped out the inverter board. Now everything is perfect. I asked the dealer why they didn’t just swap out the touchscreen with a working one. They said they are forbidden from doing that.

 

I’ve been talking to my old/new boss (long story) about this. He was a JLR tech at the corporate level and then a service manager up until 2015. He called his old level 4 tech to see what the current poop is.

There is a specific code for a dead telemetric battery. Usomething. The first thing the JLR overlord has the dealership tech do is leave the car running at idle for an hour. This should bring back the charge state if it’s going to come back. If it's still dead, then change the battery, if they had any.

 

I couldn't get a code the one time I tried. It seemed to come on but disappear before I could complete a rescan. Considering the battery module reset issue I didn't try again, but my hope that it would perk up was answered. Not once in my errands today did I get a warning. The manufacture date on my original was MAR 15, the replacement is DEC 19, so it had been sitting on the shelf for a while.

It seems the unit with the dual-cell battery pack does not require any reset, just enough driving to get a good charge.

 

You must have checked to old battery with a multimeter. Was it discharged?

 

Both are rated at 4V (according to their labeling). Right now, sitting on my desk, the rechargeable is 3.82V and the non-rechargeable is 3.89V. How much would they drop under even minimal load, and how long would the rechargeable keep a charge? I don't know. I did not check voltages before ordering the replacement but did check the manufacture date. That was enough to set me on the quest for a replacement.

It wouldn't be difficult at all to fit the harness to a new pair of cells except that one of the cells themselves seem to be unavailable. The rechargeable (TLI) I found for about $20 but the other (TLM)? I gave up looking.

 

I hardly think that’s enough of a drop to cause and error message but who knows. I’ve tried to connect two batteries for a pack for a different appliance and could not solder the leads on. They were originally welded. I gave up. We bought our cars the same year and it’s interesting that the TCU system has never been an issue until now.

 

Those voltages are open circuit (no load). I expect that under even light load they'd drop considerably.

I've replaced cells in various things, like rechargeable clippers, and soldering to the cell can be a challenge. It takes a lot of heat applied for the minimum time necessary. A soldering pencil will probably not work and just end up heating one end of the cell above it's rated temperature but not enough on the contact to get a joint. These cells can be had in three configurations: no tabs, welded tabs for wire, welded tabs with pins for a printed circuit board. The tabs would be the way to go.

Mine never had an issue until very recently, and then it was warnings every few minutes when driving. It will be interesting to see when yours hits the EOL threshold.

 

Had my emissions inspection today. Got the car back with the SOS light out. 🤷‍♂️ And, of course, the red triangle light on.

 

I went for a short drive today to exercise my franchise and had no recurrence of the warning. It seems that the replacement battery pack has charged up and the the system is happy.

 

Just bought both batteries with solder tabs already attached. When it goes bad, just gonna tape them together and solder the connector on.

 

That's what I'd do now, after taking apart the original and getting the numbers. I didn't want to have issues with no pack at all installed while I figured that out. I found supply issues when researching the cells. Where did you end up getting them?

 

 

Outstanding Buckingham, I just ordered the described batteries because I am having the problem and would like to get rid of it.

Lizzardo, you indicated that you accessed the TCM where the batteries are by "The Telematics Control Module is behind the bolster between the seats (coupe)". Can you elaborate any more on the that explanation of how you gained access the the compartment? You, me and Buckingham have pretty much the same cars except for color. Thanks to both of you.

 

I have some pictures somewhere (phone? camera?) that I can put up, but it's easy enough. The panel in question is between the seats below the cubby. Pop the top part out (towards the front of the car) about an inch then lift straight up. The module is right there and has a small access cover. It takes a small torx bit. Don't drop the screw or it might disappear forever. It's small but is magnetic. (Ask how I know!) There's a bracket that is slightly in the way but not enough to prevent a little careful force from getting the cover off.