FWIW....CTEK are very good, though I bought this one (motorcareltd on ebay- £33.99) (not because I think it's better...I just liked the look of it !) :
Ring RSCA4 Intelligent Battery Charger 8 Stage charging process Suitable for all types of battery including batteries for stop/start vehicles 8 stage smart battery charger to deliver optimum battery charging. This smart charger has a built in microprocessor that analyses the battery condition and regulates the battery charge to ensure maximum battery performance The charger has specific charging modes for different types of battery. The modes suit lead acid, Gel and AGM batteries. It is also suited to charging more complex batteries fitted to vehicles with Start/Stop technology Includes a power supply mode to maintain a constant battery voltage when devices are connected to a vehicle and draining battery power, this mode enables you to leave the charger connected permanently to the battery to maintain the voltage in a safe and efficient way The RSCA4 features LED charge indicators to show what stage you are at in the charging process, giving you clear indication to when the battery will be fully charged. Temperature compensation is also a clever feature on the RSCA4 which adjusts the charging voltage depending on the ambient temperature. The RSCA4 is supplied with battery clamp leads and also ring terminal leads for a more permanent connection Specification Input Voltage: 220 - 240v AC Output: 7.2v - 1a 14.4v - 1a 14.4v - 4a 14.7v - 4a 13.8v - 4a Types of battery: Lead Acid, Gel, EFB, Calcium, AGM Battery Size: 10aH - 120aH Protection: IP65
Being forgetful and stupid), I've had to use it 3 times on a FLAT, FLAT battery...seems to have done the trick...it's on permanently now...