Alternator Draining Battery?

[Ed Martin]

Something is causing my battery (new in December 2019) to slowly lose voltage when sitting for a week at a time without being driven. This has happened three times (not every time) in the last 2-3 months. When I go to start it, I find the voltage has dropped to 12.0V. The only thing I know that might be causing this is a bad alternator diode(s). I took it and had the alternator bench-checked at a rebuild shop and they said it tested good. It charges good when driving at 14+V. No dome light or other known drains. I measured the amps across the disconnected battery negative cable with the alternator battery terminal connected at 2.2 on 200µ DCA scale of meter (see picture) and measured the amps with the battery alternator terminal disconnected at 0.0 on the same scale. Does this reading mean it's 2.2µ amps? That doesn't seem like enough to hurt anything if that's what it is. What else could cause this?

 

[70chall440]

Do you have an aftermarket radio? I have seen this often and I cannot say what it is precisely but pretty much all of my cars (like 5 of them) do pretty much the same thing. The new cars sip power via the stereo/computer but the older vehicles seem to lose electrical power through the general system. One of the vehicles (my 52 B3B) has an exceptionally simple electrical system but it will lose power over time as they all do. I run battery tenders on pretty much everything.

 

[Don Frelier]

An aftermarket radio would take months to drain the battery down.
Does the horn work?

 

[Ed Martin]

Do you have an aftermarket radio? I have seen this often and I cannot say what it is precisely but pretty much all of my cars (like 5 of them) do pretty much the same thing. The new cars sip power via the stereo/computer but the older vehicles seem to lose electrical power through the general system. One of the vehicles (my 52 B3B) has an exceptionally simple electrical system but it will lose power over time as they all do. I run battery tenders on pretty much everything.

Thanks. No, I don't have an aftermarket radio. Only the stock AM. If I'm reading the multimeter right, there's 0.0 micro amps when alternator disconnected. I never had this problem before for decades!

 

[Ed Martin]

An aftermarket radio would take months to drain the battery down.
Does the horn work?

Yes, the horn does work (I believe). Thanks

 

[69a100]

An aftermarket radio would take months to drain the battery down.

B.S.! Between the radio and clock, it kills my battery within a month! Just disconnect battery when not in use. Good Luck

 

[Don Frelier]

What is the clock? Motorized?

 

[JimKueneman]

Look for the trunk or glove box light staying on or barely turning off when closing. May need an adjustment

 

[Ed Martin]

B.S.! Between the radio and clock, it kills my battery within a month! Just disconnect battery when not in use. Good Luck

That may be the best solution --- to disconnect. I don't have a clock either. Thanks

 

[Ed Martin]

What is the clock? Motorized?

No clock. What's the connection to the horn?

 

[Don Frelier]

The clock question was for @69a100 apparently my take on the car stereo parasitic draw upset him.
I've seen where the horn pad in the wheel goes bad, pulls in the horn relay all day long.
If the horn doesn't work then it's easy to miss/find.
Sorry should have just wrote that one out rather than ask a random question.

 

[RJRENTON]

What type of alternator are you using? An original Mopar (square back or round back) OR Powermaster unit with the "one wire" connection? IF its this type, the internal voltage regulator is always connected to the alternators output, so it can sense the battery voltage. There is a very small parasitic loss associated with this arrangement, similar to GM's 10SI and 12 SI designs with the internal voltage regulator.
IF you have a square back alternator, and a diode is shorted, it will drain the battery over time, thru the stator windings to ground, likewise a round back unit will as well.
With the engine stopped, disconnect the alternators output wire and measure the current losses, if any, between it and the disconnected wire to see if there is current flowing back to the alternator. If so, there is a bad diode, either in the positive diode group or the negative group. These style of diodes can develop reverse leakage but still conduct full current in the positive direction and exhibit full charging capabilities. IMO, test by substitution of the alternator...a relatively easy thing to do.
BOB RENTON

 

[70chall440]

I will say that I believe that some batteries begin to exhibit this behavior especially as they get older. I realize your battery isn't that old but it still could be the issue. I had this same problem in my 01 1500 and eventually put a smart battery disconnect in it that would disconnect that battery if it went below 11.8V. The problem was that the battery would go out randomly, I replaced the alternator and checked every connection and ground but no joy. Eventually I replaced the battery and it has been good since. I still have the offending battery which still will hold a charge and use it for my trailer winch.

 

[69a100]

What is the clock? Motorized?

No, I wind it up once a week! WTF do you think, or do you think? Yeah, it runs anytime electricity is applies to it! You're a prime example of idiots on the I-net!

 

[Ed Martin]

The clock question was for @69a100 apparently my take on the car stereo parasitic draw upset him.
I've seen where the horn pad in the wheel goes bad, pulls in the horn relay all day long.
If the horn doesn't work then it's easy to miss/find.
Sorry should have just wrote that one out rather than ask a random question.

I gatcha. I will have to check the horn next drive. It's been a while since I remember using it. Seems like there was some issue with it to get it to work right after replacing my steering wheel a few years back.

 

[sam dupont]

I'm not trained, not always the sharpest knife, so my diagnostic method is usually physical. I would hope to get lucky by putting a test light between the battery post and cable, then start unplugging fuses to see when the light goes out. That sometimes gets a guy in the right neighborhood.

 

[Ranger16]

Something is causing my battery (new in December 2019) to slowly lose voltage when sitting for a week at a time without being driven.

Never assume that the battery is not the fault just because it is new or relatively new. I've had them fail in 1 or 2 years, and have had 2 with a dead cell right off the shelf.

 

[Ed Martin]

What type of alternator are you using? An original Mopar (square back or round back) OR Powermaster unit with the "one wire" connection? IF its this type, the internal voltage regulator is always connected to the alternators output, so it can sense the battery voltage. There is a very small parasitic loss associated with this arrangement, similar to GM's 10SI and 12 SI designs with the internal voltage regulator.
IF you have a square back alternator, and a diode is shorted, it will drain the battery over time, thru the stator windings to ground, likewise a round back unit will as well.
With the engine stopped, disconnect the alternators output wire and measure the current losses, if any, between it and the disconnected wire to see if there is current flowing back to the alternator. If so, there is a bad diode, either in the positive diode group or the negative group. These style of diodes can develop reverse leakage but still conduct full current in the positive direction and exhibit full charging capabilities. IMO, test by substitution of the alternator...a relatively easy thing to do.
BOB RENTON

Thanks Bob. The alternator is a round back with external regulator. I removed the negative cable from the battery post and measured the (reverse) current from the negative battery post to the disconnected cable with the alternator battery terminal connected and got 2.2 on the 200µ DCA scale of meter (see picture at #1 above). When I disconnect the alternator wire to the battery and measured the amps, I get 0.0 on the same scale. I assume this 2.2 reading means it's 2.2µ amps (or would it be 2.2 amps) when using the 200µ scale? Do you think I am measuring this right? It seems the only drain is from the alternator but 2.2µ amps is not much. Is this what you're talking about doing?

 

[RJRENTON]

Thanks Bob. The alternator is a round back with external regulator. I removed the negative cable from the battery post and measured the (reverse) current from the negative battery post to the disconnected cable with the alternator battery terminal connected and got 2.2 on the 200µ DCA scale of meter (see picture at #1 above). When I disconnect the alternator wire to the battery and measured the amps, I get 0.0 on the same scale. I assume this 2.2 reading means it's 2.2µ amps (or would it be 2.2 amps) when using the 200µ scale? Do you think I am measuring this right? It seems the only drain is from the alternator but 2.2µ amps is not much. Is this what you're talking about doing?

Ed,
YES.....Since your meter is on the 200 uA scale and you are reading 2.2 uA that is the current loss back thru the alternator. Over time this small loss will drain the battery enough to cause starting difficulties. The battery has several bench mark measuring points. Almost everyone quotes the CA (cranking amps) or CCA (cold cranking amps - amps produced at a specified temperature...important for cold weather operation) as the most important factor BUT...the overlooked factor is Amp Hour rating....which is how much energy can be supplied (usually 20 amps) for a specified number of time (minutes or hours), without charging, b4 the terminal voltage drops to less than ~ 10.5 volts. The larger the AMP HOUR rating is the reserve capacity, and is determined by the number of plates per cell and the surface area of the plates. The chemistry of the battery (lead, lead dioxide and sulfuric acid) results in 2.1 volts per cell, but the number plates and their area determine the CA, CCA, and AMP HOUR rating of the battery.
Another POSSIBILITY for the loss is a high resistance ground in one of the stator windings insulator strips. This loss can be measured by performing a MEG OHM test, using a special test device, best performed by a reputable shop. Most shops would not do this test because of overall system voltage is low (14.4 volts), saying its not needed. But you are chasing a very low loss factor....micro amps....and this test may help in finding the loss. Others may say: ...don't worry about this small loss..., but any loss over time will drain the battery to a no start condition. The greater the loss, the faster the battery will be discharged. Just my opinion of course.
BOB RENTON

 

[Ed Martin]

Ed,
YES.....Since your meter is on the 200 uA scale and you are reading 2.2 uA that is the current loss back thru the alternator. Over time this small loss will drain the battery enough to cause starting difficulties. The battery has several bench mark measuring points. Almost everyone quotes the CA (cranking amps) or CCA (cold cranking amps - amps produced at a specified temperature...important for cold weather operation) as the most important factor BUT...the overlooked factor is Amp Hour rating....which is how much energy can be supplied (usually 20 amps) for a specified number of time (minutes or hours), without charging, b4 the terminal voltage drops to less than ~ 10.5 volts. The larger the AMP HOUR rating is the reserve capacity, and is determined by the number of plates per cell and the surface area of the plates. The chemistry of the battery (lead, lead dioxide and sulfuric acid) results in 2.1 volts per cell, but the number plates and their area determine the CA, CCA, and AMP HOUR rating of the battery.
Another POSSIBILITY for the loss is a high resistance ground in one of the stator windings insulator strips. This loss can be measured by performing a MEG OHM test, using a special test device, best performed by a reputable shop. Most shops would not do this test because of overall system voltage is low (14.4 volts), saying its not needed. But you are chasing a very low loss factor....micro amps....and this test may help in finding the loss. Others may say: ...don't worry about this small loss..., but any loss over time will drain the battery to a no start condition. The greater the loss, the faster the battery will be discharged. Just my opinion of course.
BOB RENTON

When I took the alternator to the shop a few weeks ago when this first happened, they said it tested good and I asked if that includes the diodes and they said yes. But I really wonder if they would check for such a small drain. Do you think if I leave the battery disconnected between driving, it could still lose voltage? I will try it. Also, I noticed, after driving 11/8, the battery was at 12.77 on 11/9, at 12.7 on 11/11 and dropped to 12.0 on 11/15 (4 days later). That's a big drop in only 4 days. I need to monitor it some more. Thanks for your help!

 

[Don Frelier]

Ed, I think you are reading the meter correctly.
0.0000022 amps is close to nothing.
@Ranger16 might be correct, maybe as simple as a bad battery.
A .7 Volt drop over 4 days from that low of leakage current doesn't jive.