Those of you who run an S-Type will be well aware of how sensitive the car's electronics is to voltage drop.
In the old days, eventually your starter motor would begin to exhibit a distinct lack of torque to start the engine and that warned you to charge the battery.
In our cars, the system throws a wobbly at the slightest voltage drop, well before the starter has trouble. All you get is a plethora of fault codes and the immobilizer refuses to disengage.
I, along with others, make use of the superb CTEK charger to trickle charge the battery, in my case, once a week. I've now installed the CTEK panel mounted status indicator. Not only does it give you an immediate red - amber - green LED indication of battery state but it also has the fairly unique CTEK charging socket built into the device so you just plug the CTEK cable in without even having to lift the boot floor. The CTEK status indicator connects directly to the battery terminals via a fused cable.
I suppose you can mount the indicator anywhere in the car but I dislike having extras visible in the passenger compartment so I've mounted it in the side of the boot, alongside my auxiliary power socket.
Having a sealed battery, I close the boot-lid which doesn't damage the charger cable and refresh the battery overnight. You can't over-charge the battery with the CTEK charger and it automatically cycles if it detects the battery voltage falling below maximum.
In use, if the indicator displays the red LED, there is still sufficient power to run the car but a charge, either from a run or from the charger is required to bring the battery back to peak.