The Digital Voltmeter seen below required a PP9 battery to operate.
So, I have installed one which does not require a battery, it also is able to with a flick of the switch measure the current of the leisure battery. Picture above shows in Voltmeter Mode, 12.18 Volts. Picture immediately below shows in Current mode, 1.5 Amps..
DIGITAL meter. This gives a more accurate reading of the battery.