A voltmeter will really only give you information about charging, and unless
it is a high-sensitivity digital unit it probably won't show the difference
between enough charge to start and not enough charge to start until you turn
the key, by which time you will know anyway.
Removing the battery ground strap and connecting a voltmeter (on its 12v
scale) in its place it a good way of detecting drains, however I have only
used an analogue meter for this, a digital meter may not give the same
indications. On a dynamo equipped car you should see zero volts registered.
If you see any volts registered there is a drain. Anything around 5 or 6v or
so can be ignored as it represents micro-amps (this is what a typical
alternator registers from the normal reverse leakage of its diodes), a full
12v is a significant drain. Alarm systems in particular, some sound systems
if they have a 'keep alive' feed to keep the tuning, and some clocks will
exhibit a small drain. In my experience only alarm systems will drain enough
to make a difference (unless not used for many months), it is the repeated
partial draining when not used daily and recharging that causes batteries to
fail prematurely, I was having to replace the V8 with alarm every couple of
years or so whereas in the roadster without any extras I'm still on my 2nd set
of replacement batteries after 18 years. I solved the problem on the V8 by
fitting a battery cut-off switch (without a bypass fuse), since then the
current (ho ho) battery is still going strong after several years.
(to be continued ...)
----- Original Message -----
I have a 1957 MGA 1500 with generator and pos ground.
My battery has been loosing voltage.
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