CTEK Battery Conditioners
£75
If you end up leaving your car for a couple of weeks or more between drives, then it is highly recommended to plug in a battery conditioner.
This is plugged into the mains supply and then connects to your car battery, either directly or via the 12v power socket ('cigar lighter' socket). The conditioner constantly measures the charge in the battery, and when this sinks too low, it puts a charge back into the battery until the correct level is reached once more at which point it goes back into 'monitoring' mode.
CTEK battery conditioners are amongst the best on the market, and are not to be confused with cheap 'trickle chargers' which put a constant charge into the battery whether it needs it or not, which over time can 'cook' the battery.
There are different sizes and grades of conditioner, starting at £75.
Adaptors to plug the conditioner into a car's 12v power socket (often easier than using the supplied crocodile clips to connect straight to the battery) are £14.
All prices include VAT.