Excellent advice from davey8oy,
just to add that systems with CALCIUM batteries run up to 16 volts, this is normal, but the charge system is a lot more accurate to suit.
If originally fitted with a calcium battery it absolutely MUST only be fitted with a calcium replacement, a std lead acid one can explode due to the higher charge rate, but it is perfectly fine to put a calcium one in place of a lead acid with no harm at all.
tabetha