As follow up to previous thread, I just measured the battery voltage with various loads, and compared it with the reading on one of the cheap "lighter socket" DVMs.
I had previously done a "calibration" using a "lab" power supply which showed that the instrument on the socket was generally reading about 0.3 volts higher than the meter I used on the battery, over the range 5 to
15 volts. These are the results in "meter" volts since I assume this will be more accurate. (When I get round to it I'll cross-check all my DVMs).Ignition only Battery 12.2 Socket 12.3 Headlights Battery 11.9 Socket 11.9 Headlights + rear screen Battery 11.7 Socket 11.8 Engine running (idle) Battery 14.1 Socket 14.1
Obviously, you would not really expect the socket volts to be higher than the battery, but the "0.1" difference is in the calibration noise.
Conclusion: it *is* reasonable to use one of these socket meters to check battery condition as long as you recognise that its absolute calibration may not be highly accurate. If there are starting problems, they should certainly help to differentiate between a battery with a dead cell and a battery with dirty terminals.