ALDI. There's nothing wrong with the £12.99 Automatic Charger from ALDI. They do the same job as Branded ones at 5 times the price, or more. They do 6 Volt, 12 Volt, Cold Batteries, Dead Batteries, Incorrect Polarity Protection. The problem is you can only get them when they have them in.
Talking about Batteries now, the VARTA one on my 2003 A2 1.4 SE Petrol is still the Original Battery from new, so approaching it's 13th Birthday next month ! Never been any problem with it. The A2 lives outside and always starts even in the coldest weather, like -18C