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.