Bad Battery or Alternator?

[Phishin]

Just put a new water pump and radiator in this thing last weekend, now it wouldn't start this morning for my wife. Dang it.

There is juice in the car (interior lights come on, radio works, etc). Just when you turn the key....nothing.

Put a volt meter on the battery...only about 10 volts. So, I put a battery charger on it this morning. I'll check it after work when I get home.

The battery looks like a fairly cheap model. It's 3 years old according to the born on sticker. The guy I bought it from must have put it in right before he sold me the car.

How do I tell if it's a bad battery or a bad alternator? With my last repair bill, I don't have the money to simply replace both battery and alternator. How do I know if the battery won't hold a charge or if the alternator isn't charging it?

IF I can get the car started, can I simply disconnect the battery while running and see if the car shuts off...and if it does, the alternator is therefore bad? Someone told me this might work.

I just want to replace what needs to be replaced and not go further into debt over repair bills on unnecessary parts.

Thanks in advance.

 

[nwjones18]

Put a volt meter on the battery while the car is running. If it is showing around 13.5 volts with the a/c and everything running the alternator is charging. If its showing less than 12 it isn't charging. You can also drive it to a parts store and they will test it for you. Or buy a battery/charging system tester from harbor freight.

 

[HerrStig]

I wouldn't disconnect the battery with the engine running, that might have been OK with carbs and the old Kettering ignition, but the electronics in modern cars can stand only so much overvolting before the expensive electric smoke gets let out. A "bad" battery is like a leaky bucket, you can pour current into it but it "leaks" right out. Voltage looks OK with the engine running, but drops quickly when the alternator isn't turning. Measure the voltage with the engine on, it should be around 13.5, if not suspect the alternator, then disconect the battery and measure it after an hour or so. If it's less than 12 and dropping, the battery is failing. Is the battery "weeping" a bit around the terminals, if so it's probably on the way out, although some batteries will do that for a long time.
Interior lights and a standard radio don't put enough load on a battery to be a good indication of anything. Turn on all the electrical stuff including headlights with the engine off and see how fast the battery gets to below 11 volts or so. If it happens almost at once the battery is failing. With the engine on, the alternator should be able to run everything in the car and keep the voltage at 12.5 or so. That voltage isn't charging the battery but it isn't discharging it either. Harbor Freight (among others) has very inexpensive digital voltmeters which are quite useful.

 

[SteveSRT8]

The battery/alternator/starter triumvirate is one of the most misdiagnosed areas on cars.

Get thee to an auto parts place with a machine. Then you can quit guessing...

 

[440Magnum]

Its really nice to have (beg, borrow, buy) a clamp-on ammeter that can read DC current (yes, they're much pricier than AC-only clamp-on ammeter). Then you can isolate the wire from the alternator, clamp onto it with the meter, turn on every accessory the car has, and see what the actual output is at idle, 1000 RPM (fast idle), and 2000 RPM or so. That gives you a real good idea of how strong the alternator really is.

This looks like a decent one for a reasonable price:
http://www.amazon.com/Sinometer-MS2138R-...7494&sr=1-2

 

[tig1]

Originally Posted By: nwjones18
Put a volt meter on the battery while the car is running. If it is showing around 13.5 volts with the a/c and everything running the alternator is charging. If its showing less than 12 it isn't charging. You can also drive it to a parts store and they will test it for you. Or buy a battery/charging system tester from harbor freight.


This is correct. If you have a volt meter this method will tell the tale.

 

[mechtech2]

Charge the battery.
Note the voltage.
Start her up, and note the voltage at idle and 2,000 RPM.
It should go from about 12.5 to 14 -14.5
Too low, like 13, or too high, like 16, are both no good.
That indicates a bad alternator [with enclosed regulator].

But first, clean the battery cable ends. BOTH ends of EACH cable, AND where they seat - nice and shiny.
This fixes all sorts of car problems, and is often overlooked. At least, it is a very good start and base to go from.

 

[Dave Sherman]

Originally Posted By: Phishin

IF I can get the car started, can I simply disconnect the battery while running and see if the car shuts off...and if it does, the alternator is therefore bad? Someone told me this might work.


Don't ever disconnect the battery with the engine running. Alternators need something to "absorb" some of the output to keep the voltage stable. With no battery there, the alternator may not be able to stabilize the voltage, and it could go to something much higher than what light bulbs, ignition systems, and engine computers can tolerate.

Even my old MG has a big warning label under the hood advising against that. A friend tried that on his motorcycle, burned out his neutral indicator light, made the mistake of stepping on his brake and burned out his brake lights.

 

[ajchien]

Agree with all of the above ...

Except, 10V is really bad for a battery. That's more than I would expect for a complete discharge. I would have thought it was a bad cell in the battery.

 

[Vikas]

The clamp-on DC ammeter looks very tempting but I am hoping to find a cheaper version.

 

[Warstud]

My guess is a bad battery. If the alternator was bad it probably would have quit working the last time she used the car and left her stranded.

 

[tig1]

Originally Posted By: Warstud
My guess is a bad battery. If the alternator was bad it probably would have quit working the last time she used the car and left her stranded.


And a volt meter will tell you this.

 

[kb27]

10 volt is usually indicative of a bad cell in the battery.

 

[eljefino]

Just get a battery from walmart and save the receipt. Stick it in for diagnosis. Make up some cock and bull story when you take it back about you're not satisfied.

I'm leaning pretty hard on a battery fault. Agree with the cell theory. Had a car like that. Voltage would dip dramatically at idle but perk right up at 1500 RPM when the alt was making enough juice.

Your sig shows two cars, temporarily switch the batteries out even if you have to run with it jury rigged up on a fender or whatever.

 

[Phishin]

Mystery solved.

When I had taken the car in to my mechanic for new radiator and water pump, he had cleaned up my battery posts and sprayed some sealant on them. The sealant wasn't the problem.

But the bolts clamping the cable to the battery terminal were not real tight. When I pulled on the negative cable, the cable pulled right out from the terminal. There just wasn't sufficient contact for all the amps the starter was pulling, but there was enough for the radio, lights, etc.

CHEAP FIX. My kind of solution!!

 

[tig1]

Originally Posted By: eljefino
Just get a battery from walmart and save the receipt. Stick it in for diagnosis. Make up some cock and bull story when you take it back about you're not satisfied.

I'm leaning pretty hard on a battery fault. Agree with the cell theory. Had a car like that. Voltage would dip dramatically at idle but perk right up at 1500 RPM when the alt was making enough juice.

Your sig shows two cars, temporarily switch the batteries out even if you have to run with it jury rigged up on a fender or whatever.



Actually, I don't think that serves us well to do that. Besides I don't think WM or anyone else will take it back unless the new battery was defective.

 

[Phishin]

Oh, and obviously, my cheapy volt meter is a little off.

When I got off work, the battery was about 11 Volts according to my meter. Once I fixed the problem (bad ground connection), it fired right up.

Battery still read 11 Volts.

Turned on head lights and AC Max, and battery still read 11 Volts while running at slow idle.

So, obviously my alternator is taking care of business and my battery must be okay. My volt meter is just off.

Jeez. Thanks everyone for the input. Much appreciated.

And yeah, that [censored] "switch out" at Walmart is bad advice. I don't want a new battery from WM to have been previously installed!! I'm not cool with that either.

Maybe the same people return air filters in the new box, but with their old filter in it....and return a case of M1 oil with their old nasty drained fluid in it too.

 

[mechtech2]

10 volts could also be a discharged but perfectly fine battery.

Run my tests.

 

[Vikas]

simple; just clip your "broken" voltmeter to the battery and sit in the driver sit. Watch how much voltage you lose during cranking and then observe how much voltage you gain after the car has started. The former tells you about the health of the battery (over 2V drop mean bad battery or corroded connection) and the later (less than 1V over resting battery voltage means broken alternator) tells you the health of your alternator.

Obviously, you should throw the "broken" voltmeter and buy one which actually works but until then just use the method suggested.