Today I checked my battery charger/ 18 amp booster output with four meters connected.

The inbuilt voltmeter indicated (MC) 14.9.

My Kingdom ETU multimeter (MC) 15v.

My Optilux (Auto specific) Multimeter (MC), 13.8

El-cheapo E06100 digital multimeter 13.11 volts.

Getting accurate base readings is going to be a problem.