Mmm…battery chargers,a lot of people have lots of varying ideas of how they work.I was a generator engineer for about half of my working life and the chargers are powered permanently on emergency gensets,but…an Optimate will charge up to around 14.2 v.and then back down to 13.8volts when it goes into “float” mode.Emergency genset chargers are set to 13.6 on float as this is just below the gassing level,or 27.2v on a 24volt set of batteries.I say a “set” of batteries as some very large gensets have a set of 12 x 2volt batteries,these can be as big as 2ft high and about 9 ins.square each.I built a charger a couple of years ago using an old Optimate in a plastic box with an interposing relay.This is run on a timer so it comes on for 30 minutes every week,when the timer energises the charger it switches on the 4pole relay which then makes the contact to feed the charger mains input and also another set of contacts in the relay connects the 12volt charger output to the battery.So when the power is off,the 12volt is also isolated from the charger and will never discharge the battery.So if the charger is set up correctly when it is on,the batteries just stay in what is called a “float” condition,neither charging nor discharging.Another bonus I have fitted is a cheap Chinese programmable timer that has a built in battery so if the mains goes off it keeps its timer settings for about 100 hrs.so you don’t normally have to reset it.They are as cheap as chips only a couple of quid each from memory.