Possible short
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Tuned
Joined: Sep 2005
Posts: 437
Likes: 4
From: SE Missouri
Possible short
Back in the old days before all this electronics stuff came along we would take the positive cable off battery use a test light hooked to cable and battery post and if it would light up you had a short. I didn't have a test light handy so I used my volt meter and did same thing and it read 12.5 volts....my question is does this mean I have a short since it reads full voltage. Also when I put cable back on it sparks pretty good.
Mountain Pass
Joined: Aug 2014
Posts: 137
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There is always some current draw on today's vehicles. Since the voltmeter draws no current, it will just show you the battery voltage. You would need to use an ammeter to measure the current draw, and unless you knew what the normal draw was, it would not tell you much. Most of us worry about current draw when the battery goes down over a period of time that seems short. Odds are that the battery is on it's way out, but there are things that can make for a high parasitic draw. I am not an expert on this, but I would unplug or disable items one at a time to see if the current is reduced. I would start by unplugging the alternator as I have had bad diodes cause a high drain.
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Tuned
Joined: Sep 2005
Posts: 437
Likes: 4
From: SE Missouri
Nelbur can these diodes be intermittently bad truck for now has quit draining battery over night. Also battery is good i unhooked one night to make sure it wasn't battery and that it was definitely a draw on it we then had snow hit and 3 days of negative degree nights and 7+ below freezing days when snow finally melted and could work on truck battery had 12.5 volts after that. So fairly sure battery ok
Freshman User
Joined: Feb 2021
Posts: 29
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From: NorCal/Seal Rock,OR
Just curious if you have an aftermarket alarm system or head unit? The only way to measure parasitic load is with an ammeter, battery voltage alone won't really tell you much. I have owned so many German cars that I installed a shunt on the positive cable since they plagued with electrical issues. Bosch is the ******* child of Lucas, The Prince of Darkness for those unfortunate to have owned old British sports cars. Aftermarket stereos and alarms commonly played havoc with them. Then again you might just have a failing battery that shows normal voltage but is on the way out and can't pass a load test.
Mountain Pass
Joined: Aug 2014
Posts: 137
Likes: 12
I don't know the nature of diode failure, but I doubt they would be intermittent. If they were bad you should see lower than normal charging voltage. If your charging voltage is around 14 volts they are likely fine.
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