#428 Aux Battery Supply Low 13.x V

[fsmouterr]

My 12V battery is "charged" when car is active/driving only at 13.x V.
My other cars show with the same device measured 1 Volt more.
TR Vaps 13.1V Vsb12.4V when on/driving
How to bring 12V charging level back to normal ?
What do you guys measure as "12V" when car is switched on ?

DETAILS:
I'm getting that alert "Aux Battery Supply Low"
I've read several threads with that, all about replacing the 12V battery.
That was done. 3 times now with 2 years, warning kept popping up, usually same day or so after replacing. (And yes, all new 12V batteries placed were new and good).
Note that I can still drive the car - I just need to wait 5 seconds (Annoying, well thought of by the TR designers to keep bothering you to fix it !) after pressing the "unlock" middle button of the key before the drivers door listens to the door opening switch/button. Or when I have not locked it, as normal use the "ignition" key and get rid of #888 by pressing the "unlock" middle button of the key.
But it looks as if the "12V charging" level which must be done by some unit based on the 400V pack, has a defect.

 

[gregd]

I did a bit of analysis a few years ago when my 12v battery died, taking some time to understand how the system worked. 12v battery time (sigh) Note that this is a US car, so it does not have the immobilizer system common to the European models; I don't know if that substantially changes things.

The 12v "charging system" on the 2.x Roadsters is indeed just a 13.8v supply, derived from the DC-DC inverter within the main battery pack. It's limited to a trickle charge rate (about 100 ma) when the car is asleep, and an amp or so when awake. There is no charging cycle (bulk / absorption / float), just the constant voltage, and the battery is not powering stuff in the car under normal conditions. It just sits there, held at 13.8v by the DC-DC supply, with that DC-DC supply keeping the rest of the 12v systems powered.

Given that there's a constant 13.8v charge applied to the battery, the detection of a "bad" battery appears to be done when the Unlock button is pressed. The car does a momentary (few seconds) "load test" on the battery, pulling a significant current from it (17+ amps) and watching how the battery reacts. If it dips too far the test is considered to have failed and the #428 alert is generated. If your car is still failing that test, I would check the wiring for loose or corroded connections, for example at the battery terminals or the fuse (in the front of the car on the right, just in front of the firewall near the HVAC cover). 17+ amps needs good connections in order to not have a significant voltage sag.