Battery being drained

[Sambar56]

I have a 1970 gmc c2500 and I've replaced the battery twice and the starter and something is still draining it. Idk what to do, any ideas?

[mr48chev]

The starter won't drain the battery.

Have you checked the output of the alternator or generator to make sure that it is charging what it should? You may have a charging problem rather than a drain problem.
If you have a big sound system or high powered halogen lights the alternator may not be putting out enough amps to run those and charge the battery too.

A battery drain is usually caused by one of the following:
Something left on like an interior light or glove box light

Blown diode in the alternator that is letting power drain off.

A dirty battery top on top post batteries. Take your volt meter and check across the battery on different spots on the top of the battery to make sure that the dirt and moisture isn't letting current flow across the top of the battery. I've seen a battery go dead in three hours due to a 9 volt drain across the battery before

Someone in the family who goes out to the truck and plays the radio or CD player when you are gone. One of my students used to leave his lowrider at home while he worked at the local grocery store and his battery was dead too many times in the mornings. After jumping it and checking it in the school shop more than once his mom says "oh yes, your brother and his friends play your car stereo while you are at work real often".

Is there anything that you added to the truck that might draw power when the truck is parked? An amp that is wired so it has power to it all the time? A security device?

[Sambar56]

Thanks! I'm gonna remove the alternator tomorrow and take it to auto zone and see how it goes.

[SCHRUMGMC]

Don't take it there !! They will **** you for everything you have
Take it to O'Reillys, NAPA

[RichardJ]

Before you take it anywhere, just remove the Bat wire & plug connector from the Alt and see if the battery still drains down in the same time period. A bad diode may not stop it from charging close to normal and most flaps alternator testers can't detect the difference. If battery stays charged, the Alt is probably bad.


OR:
Remove battery cable from battery and place an Amp gauge between battery post and battery cable. You should be seeing a discharge reading, unless it's a dirty battery top as MR48 suggested. Remove Alt cables and if the Alt is the culprit, the discharge reading will go away. If the discharge reading stays, then start removing fuses and accessory power, one at a time.

OR:
Remove battery cable from battery and see if it sparks when you touch cable back to battery post. Remove Alt wires, then see if battery cable still sparks. Take your own chances with sparks around a battery.

[67 cst swb]

Originally Posted by RichardJ

Before you take it anywhere, just remove the Bat wire & plug connector from the Alt and see if the battery still drains down in the same time period. A bad diode may not stop it from charging close to normal and most flaps alternator testers can't detect the difference. If battery stays charged, the Alt is probably bad.


OR:
Remove battery cable from battery and place an Amp gauge between battery post and battery cable. You should be seeing a discharge reading, unless it's a dirty battery top as MR48 suggested. Remove Alt cables and if the Alt is the culprit, the discharge reading will go away. If the discharge reading stays, then start removing fuses and accessory power, one at a time.

OR:
Remove battery cable from battery and see if it sparks when you touch cable back to battery post. Remove Alt wires, then see if battery cable still sparks. Take your own chances with sparks around a battery.

Great advice right there...

[hamjet]

OR:
(Remove battery cable from battery and see if it sparks when you touch cable back to battery post. Remove Alt wires, then see if battery cable still sparks. Take your own chances with sparks around a battery.) Alot of good advice, but I would remove the ground from the opposite end of the battery, like from the engine. it doesn't happen often but sparks near a battery can be bad. also wear safety glasses at minimum when fooling with bats...google exploding auto batteries, it'll scare you... it did me...

[68K30]

Without a test ammeter you can CAREFULLY disconnect either cable and place a test light, or a spare socket with a known good bulb between the disconnected battery cable and the battery post. If the test light lights you have a draw. Remove fuses and disconnect components (alternator, stereo equipment) one at a time until the test light goes out. The doors have to be shut if you have door switches and the key has to be off.

Hope this helps
Bill

[68_Shortbox]

If your truck has the original external voltage regulator, the contact points inside can get stuck closed and drain the battery when the engine is not running.

[Sambar56]

I've checked the alternator, I know for sure the alternator is charging the battery as well. So I know that something is draining the battery now. I've come to realize that if I have the ignition switch in the off position the fan still turns on and so do all the lights even the headlights. Is that normal?

[Blazerowner]

I have a draw issue on my blazer. I just bought a trickle charger for when I go for a period of time without driving it. Fires up every time. Worked so good I bought some for my Chevelle and quads. Prob not a permanent fix but it sucks to come out to a dead battery and have to charge it.

[RichardJ]

An alternator is an AC generator. There are three windings in the Stator and they produce phased AC voltages like that shown in the first image. There are two diodes per winding with a total of six. The diodes chop off the negative portions of the AC signal and you end up only the positive 12 volts DC, with the ripple as you see in the second image.

If you lose one of those AC signals, the other two pulses can still produce a DC voltage that is above battery voltage. In other words, enough to charge the battery. If you lost that one AC signal because of a shorted diode, that diode will act as a short to ground and drain your battery when the engine is off.

The third image doesn't give a DC reference voltage indication, but the scope pattern in (B) can still be above 13 volts and charge the battery as if nothing were wrong.

[Sambar56]

I got a negative 13 reading. Is that what you mean?

[Sambar56]

I got a -13 in the voltmeter between the negative cable and the negative post on the battery while the truck was running. Is that good?

[68gmsee]

Originally Posted by Sambar56

I got a -13 in the voltmeter between the negative cable and the negative post on the battery while the truck was running. Is that good?

The best way to read the charging voltage is to put the meter neg. lead to battery neg. post and meter + lead to battery + post with cables still connected to the battery and engine running. Voltage should read app. positive 13.5 VDC or better depending on alternator. My 68 reads plus 14.5 VDC while running. It will vary some but it should be around there.

You didn't say if you disconnected the alternator completely from the battery or not but that would be the easiest way to check to see if the battery is still being drained. (forgot to add: Posts above are saying that even if it's still charging, you could have a leaking diode.)

[Sambar56]

Originally Posted by 68gmsee

The best way to read the charging voltage is to put the meter neg. lead to battery neg. post and meter + lead to battery + post with cables still connected to the battery and engine running. Voltage should read app. positive 13.5 VDC or better depending on alternator. My 68 reads plus 14.5 VDC while running. It will vary some but it should be around there.

You didn't say if you disconnected the alternator completely from the battery or not but that would be the easiest way to check to see if the battery is still being drained. (forgot to add: Posts above are saying that even if it's still charging, you could have a leaking diode.)

Alright so I did as you suggested except with the truck off and I got a negative 10.5. When I tried it with the truck on the multimeter went crazy

[luvbowties]

Originally Posted by Sambar56

Alright so I did as you suggested except with the truck off and I got a negative 10.5. When I tried it with the truck on the multimeter went crazy

One item: you said, "and I got a negative 10.5". That indicates you have polarity reversed--swap your pos and neg leads* to read positive 10.5 v. Reading 10.5 volts indicates a partly-discharged battery: a charged battery should read 12 volts or above. **If you definitely have leads connected correctly, then some multimeters have a switch you can flip to reverse polarity.
sam

[davepl]

Are you sure they just use diode packs to "chop off" the inverted part of the waveform? I'm a software guy, but if I were tasked with inventing this 50 years ago, I'd have used a four-diode rectifier to just flip the negative back to positive.

[davepl]

The absolute first thing to check would be without the car running insert your standard old digital ammeter inline with the negative post and cable (cable off battery). Should be 0.000 A flowing. If you see any current (and everything including dome light and so on is off, as well as aftermarket stereo that might have a clock, etc) then you've got a leak somewhere.

In other matters, I've found that a shorted diode in the alternator will cause the alternator to run very hot, and when you disconnect the field wire you'll see the idle speed pick up because its not trying to generate infinite power across the short.

[Sambar56]

Luvbowties, yup totally right lol thank you for that. Alright, I still can't pinpoint what is draining the battery though... I did buy that trickle charger though.

[68gmsee]

Originally Posted by Sambar56

Luvbowties, yup totally right lol thank you for that. Alright, I still can't pinpoint what is draining the battery though... I did buy that trickle charger though.

Okay. Polarity is correct. As mentioned above a fully charged battery should read 12 vdc or better.

With cables connected to the battery and the engine running at idle, place the meter leads as above + to + and - to -. You should get a reading of at least 13.5vdc-15vdc (be sure you have the meter setting at 20vdc or higher).

Now if these readings are correct, it just lets you know the battery is getting charged. The draining could still be caused by the alternator or other accessory draining it (read my similar problem below). It could even be a defective ignition switch.

One of my Rangers trucks has a similar problem. If I don't drive it for 2, 3 days battery goes totally dead. I can jumper it, drive around for 30 miles and it starts right back up.

Everything checked out but still wasn't sure if it was a weak battery so had it checked and it was okay. The first thing I disconnected was the radio and that fixed it. Somehow, it works fine but discharges the battery. I still haven't fixed the radio but I take the fuse out when I'm parking it for more than couple of days.

[RichardJ]

05-10-2014
>>I got a -13 in the voltmeter between the negative cable and the negative post on the battery while the truck was running. Is that good?<<

05-12-2014
>>Alright so I did as you suggested except with the truck off and I got a negative 10.5. When I tried it with the truck on the multimeter went crazy <<


I am doing my best to understand what you have been doing. The best I can make of your posts is that you are removing the battery cable from the battery and placing the voltmeter between the battery cable and the battery positive post.

No one here ever told you to do that. No one.

It was suggested you do that with an Ammeter, WITH THE ENGINE OFF. Never with a voltmeter.

NEVER EVER remove the battery cable from the battery when the engine is running on an engine with an alternator charging system. No one told you to do that.

[Sambar56]

We'll, I obviously don't know what I'm doing and I really do appreciate all the help you guys have given but I guess I'm just taking it to a shop. I just keep messing stuff up. Once again, thanks guys!