Batteries

More that 99.9 per cent of the time batteries are just big, heavy lumps that fill the empty recesses of alarm panels and smaller access control panels. But without the guarantee they provide of operation during power loss, security systems...

Is there a rule of thumb for 12V batteries that allows a basic voltage measurement to work out exactly the state of charge? Also, at what point is battery completely dead? A battery with a voltage reading of 12.9V or...