I regulary check the voltage of my battery, especially so now the nights are getting darker and more load is put onto the battery.
If mine drops below 13.5v, I put it on a trickle charger immediately.
Isnt that a bit of a pain tho AA, assuming you disconnect the -ve from the battery first,
having to reset windows/radio code etc :-/
I have it on good authority, i.e. from the experts, that if you only trickle charge the battery, you don't need to disconnect it.
I find that during the winter months, especially if you only do short journeys, the battery is hardly ever fully charged, and that's when you get deteriation.
I much prefered my Sennie and my Carlton, they were both fitted with voltmeters, and you could see at a glance the state of your battery.
Ah rite, cheers AA summat i didnt know.....didnt think it was a good idea to even trickle charge the batt with it connected.
I once foobarred a Vectra jump starting another car with it.....immediately afterwards the vectra would only run on 3 cylinders and its EML came on when you turned the ignition off!! Vx dealer diagnosed ECU knackered, which i could well believe in this circumstance. Never did find out if it was the case, as it was a lease car, so gave it back to leasing comp and asked for another.
You could always get a blingy voltmeter from Halfauds and stick it on your dash 
As I understand it, if you trickle charge the battery, at no more than max 4 amps, you will not do any harm.
I was given this info by Vx service manager, and responses off the other site.
It's when you boost charge the battery at more than 4 amps you can cause damage.
I would be interested though, to have Marks comments on this.
Any thoughts Mark?