i'm going to mount a small voltmeter under the dash near the center console. can i just tap into the radio power, or would it be better to use the cigar lighter plug?

a proper answer requires the year and model vehicle in question, but on 123's the cigar power and radio signal wire are one and the same...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Really doesn't matter as long as it is switched power so the voltmeter doesn't draw current when the car isn't running. The radio/lighter wire is fine.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

oops! i've made a sig so that doesnt happen again. thanks for the replies!

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1981 NA 300D 310k miles

A voltmeter shouldnt take appreciable current, so shouldnt draw down a battery.

Always smart to check though!

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

True, if the car is driven every week or more often. The problem comes when the car isn't driven for weeks or more, due to vacation, illness, or whatever. The slow constant drain is the worst thing that can happen to a battery. In that case, a trickle charger is an invaluable aid.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

Absolutely. But we have to separate a few different phenomena.

The battery has its own self discharge rate where it naturally discharges.

The car has its own parasitic loads, ranging from small ones like the clock, to big ones that some electronics or a bad alternator diode can provide.

Then third there is the loss from any small load like a voltmeter.

I know I can keep my digital voltmeter in parallel with the battery I deck itself and it will not meaningfully effect battery charge.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)