Voltmeters on lighter socket

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.

Reply to
newshound
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It may be wise to define the term 'battery condition' as it is often misused. Off load, a DVM will give an indication only of the state of charge, not the capacity or internal resistance, for which a proper battery worrier is required.

How? Unless you monitor the terminals of the battery directly (which a socket meter doesn't), there is no way to tell if an excessive volt drop is due to excessive internal resistance of the battery, or resistance at the terminals.

Reply to
Arty Effem

Off load, a DVM will give an indication only of the state of charge, not the capacity or internal resistance, for which a proper battery worrier is required.

meter doesn't), there is no way to tell if an excessive volt drop is due to excessive internal resistance of the battery, or resistance at the terminals.

Because a battery with a dead cell will have a low voltage when there is

*no* load. Which, as my experiments show, is adaquately measured from the cigarette lighter socket.
Reply to
newshound

A low voltage could be caused by uniform degradation of all the cells, or a load caused by a fault, the latter indicated eventually by smoke rather than a voltmeter that can't tell the difference.

Reply to
Arty Effem

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