[Please note that the socket must be powered even when the ignition is in the 'Off' position.]

I have just bought one of these to keep an eye on my battery state over the winter, £16.39 from Amazon.

The CTEK INDICATOR CIG PLUG is an easy to use LED indicator that lets you quickly check the state of your battery. Designed to give you quick and easy access to the battery, just plug it into your vehicle's 12V cigarette lighter or accessory socket. The simple flashing LED 'traffic light' system will give you an immediate indication of your battery charge level, so you know when it needs charging. The CTEK INDICATOR CIG PLUG features CTEK's unique CONNECT system so it can be attached to CTEK chargers up to 7 A to charge your battery using the 12V socket. This unit is a universal model that fits both 12 mm and 21 mm sockets. Please note that the socket must be powered even when the ignition is in the 'Off' position.

Features

Red: means the battery charge is below 12.4 V. It's definitely time to charge the battery.
Yellow: means the battery charge is between 12.4 and 12.65 V. It's time to start thinking about charging the battery.
Green: means the battery charge is more than 12.65 V. OK! No need for charging.



Chris

I have always used a CTECK charger on my buggy.   I have leads from the battery to the rubber CTECK plug that are permanently installed. Over the winter I used to just leave it on but over the past couple of years (and doing little mileage) I have had to connect it to my daily as there are some much high tech electronics on it which eat power and if you are not doing the miles then the battery just does not get enough power back.

I really rate CTECK (though a bit expensive).   

I have a battery isolator on my Buggy and with it switched so the battery is off, I have found very little drop in power from the battery over winter.