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post #1 of 13 (permalink) Old 05-04-2016, 04:34 PM Thread Starter
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Battery voltage

Drove 140 miles on Saturday.
Checked battery voltage on Sunday, 12.59 volts.
Monday, 12.53 volts.
Tuesday 12.49 volts.
Wednesday 12.44 volts.
Battery is new.
Is this normal?
I don't drive the car much, maybe 1500 miles per year.
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post #2 of 13 (permalink) Old 05-04-2016, 05:49 PM
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Quote:
Originally Posted by laminarflow View Post
Drove 140 miles on Saturday.
Checked battery voltage on Sunday, 12.59 volts.
Monday, 12.53 volts.
Tuesday 12.49 volts.
Wednesday 12.44 volts.
Battery is new.
Is this normal?
I don't drive the car much, maybe 1500 miles per year.
Seems like an excessive amount of drain to me, .15V over 3 days.

Check for parasitic loss, trace it down, there's a recent thread here with a good video on how to troubleshoot parasitic loss.

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post #3 of 13 (permalink) Old 05-04-2016, 07:42 PM
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These figures are with the car off, and not having been started at any point in the interim? If so, I'd say they are fine. There are certain things on the car that still draw small amounts of current when the car is off, so the voltage will go down over a few days.

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post #4 of 13 (permalink) Old 05-04-2016, 09:07 PM
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Originally Posted by MadMikeyL View Post
These figures are with the car off, and not having been started at any point in the interim? If so, I'd say they are fine. There are certain things on the car that still draw small amounts of current when the car is off, so the voltage will go down over a few days.
At that rate, he'll lose an entire volt over 20 days. I think that's a little extreme.

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post #5 of 13 (permalink) Old 05-04-2016, 11:03 PM
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That is not extreme at all. 50mA is what is considered an acceptable draw. Most cars are in the 15-30mA range. A 1000CCA battery has a capacity of around 60-80 amp-hours. If we assume the higher 80 amp-hour capacity, if he is losing .05v per day, it would take 240 days, or 5760 hours, to drain the 12V battery completely, so 80/5760 gives us a draw of .014A, or 14mA, which is at the lower end of what is typical. Of course this is not exact since the discharge rate is not constant, but you can see from this basic calculation that he is nowhere near having an issue.

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-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
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post #6 of 13 (permalink) Old 05-06-2016, 03:33 PM
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So, you say you drove the car 140 miles on Saturday but what about the other days? If you aren't driving the car much then you will see what you are seeing unless you take it out on the highway for more than a few miles each day. I make sure to take extended trips, a minimum of 30 miles away, at least once per week to keep my battery charged but you could buy yourself a battery tender and be better off.

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post #7 of 13 (permalink) Old 05-06-2016, 08:33 PM
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So, you say you drove the car 140 miles on Saturday but what about the other days? If you aren't driving the car much then you will see what you are seeing unless you take it out on the highway for more than a few miles each day. I make sure to take extended trips, a minimum of 30 miles away, at least once per week to keep my battery charged but you could buy yourself a battery tender and be better off.
According to my voltage gauge, my car is back up to 12.5V by the time I get to work on Fridays, usually with my only driving it on Tuesday before to pick my daughter up from her visit with her mom. I drive it all weekend, but she charges up quickly.

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post #8 of 13 (permalink) Old 05-07-2016, 02:10 AM
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The only voltage I worry about is the voltage while the engine is running because even with the alarm armed my battery still had juice weeks later when the car did sit still for lengths of time. When my voltage became more RPM dependent and I had almost not enough cranking amps is when I figured out I had bad diodes in the alternator. I expect my alternator to put out a fairly consistent 13.4-13.9v with the occasional 14.4 highs and 12v lows depending on what I'm running. When my voltage stayed closer to 12v even when on the highway I knew something was wrong.

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post #9 of 13 (permalink) Old 05-09-2016, 03:31 PM Thread Starter
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update

The car has not been driven since I did the voltage readings.

Thursday - 12.41 volts
Friday - 12.39 volts
Saturday - 12.37 volts
Sunday - 12.36 volts
Today - 12.34 volts

I don't understand why the drop from day to day is less than my initial reporting.
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post #10 of 13 (permalink) Old 05-09-2016, 07:50 PM
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V=amps/ohms. As voltage goes down and resistance stays the same, the current must also go down. Less current flow means less voltage is used, and so your current draw will actually reduce as the battery voltage drops.

-91 Cougar LS, coming soon, complete overhaul with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
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post #11 of 13 (permalink) Old 05-10-2016, 03:15 PM Thread Starter
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So your saying that the parasitic draw goes down?
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post #12 of 13 (permalink) Old 05-10-2016, 03:42 PM
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Depends how you think of it. The load, or resistance is going to be constant, but that means that as voltage goes down, necessarily the current flow must go down.

-91 Cougar LS, coming soon, complete overhaul with a 427" Windsor.
-90 XR7 5-speed black on black w/sunroof, MP2, coated rotors, double intercooler, 15%OD, ported heads, comp stage 1 cam, 85mm TB, 90MM LMAF, 80# injectors, and ported big valve heads
-98 Mark VIII LSC, Procharger P600b, TR3650 swap and 3.73s.
-90 SC Automatic rustbucket winter beater
-97 Tbird Sport 4.6 Nice weather daily driver
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post #13 of 13 (permalink) Old 05-10-2016, 04:56 PM
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Quote:
Originally Posted by laminarflow View Post
The car has not been driven since I did the voltage readings.

Thursday - 12.41 volts
Friday - 12.39 volts
Saturday - 12.37 volts
Sunday - 12.36 volts
Today - 12.34 volts

I don't understand why the drop from day to day is less than my initial reporting.
You charged the battery.

There is a function of terminal voltage vs % capacity or "state of charge", a table of these readings are on the table on page 3 of this PDF:

http://www.arttec.net/Solar_Mower/4_...20Charging.pdf

The current with the key off is less than a C/100 discharge rate, for our batteries, so use that curve as a reference; it shouldn't be far off.

This voltage is fairly independent of the size of the battery; it's a function of the chemistry.

I would do an equalizing charge to the battery, and see if it will hit a 100% charge level.

If you don't know that term, look up how to do it here:
Charging Information For Lead Acid Batteries ? Battery University

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