An old rule of thumb says that the recommended size of the battery charger can be worked out by multiplying the capacity of the battery by 0.1 to give the recommended current in Ampere. However, with a modern CTEK battery charger you can charge with a much more powerful model as the current is extremely 'clean' and the voltage is very well controlled. Charging with a current which is four times greater than the old recommendation is normally no problem with a CTEK battery charger.

A battery charger that is too small will take far too long to charge of course, so much so that it sometimes becomes impractical. A motorhome or caravan owner may only want to spend a night at a campground to recharge batteries, do some washing, have a long shower etc. After this the user usually wants their battery back up at full capacity.

A battery charger that is too large is unnecessary for two reasons. Firstly, the battery will heat up more which is not ideal. A battery charger with high ripple (which is common in most older linear battery chargers) heats the battery more, which means the current must be kept down. A modern switch mode battery charger has a ripple around one quarter that of a linear charger and can be used for batteries which are smaller than you might expect. The CTEK MULTI XS 3600 has been used for a 1.2Ah battery with only 2 degree temperature rise. A normal maximum temperature rise is considered to be three degrees, which means that CTEK battery chargers can charge a wide range of battery sizes.