Car trouble: battery or alternator?

[charliex]

Thank you for " Alternators are 3-phase AC generators" I did not know about the 3 phase part. Good info.

[LCHIEN]

auto battery testers don't test capacity. To do that you have to charge it completely and then run it down and see how many hours it can deliver a fixed amount of amps (which is not the usual use for a auto battery anyway), the product of which is amp hours capacity.

Auto battery testers check voltage vs. load to give an idea of the internal impedance ( fancy catchall word for resistance). Car batteries have ratings called cold cranking amps( CCA). usually something like 400, 600, or 800. the higher the CCA, the lower the internal resistance. When you r battery ages the CCA falls and the impedance increases. The battery testers usually load the battery with like 100A for a few seconds (simulating car starter loads) and they measure the voltage at that point. The lower the voltage, the higher the internal resistance. So they have a scale which is actually driven by the voltage but is marked in equivalent CCA. If you have a 800 CCA battery but it only registers 200 CCA, then your battery is in bad shape and should be replaced. CCA generally goes down gradually with the life of the battery but at some point it starts falling precipitously and that's when your car won't start.

[billwmeyer]

The way we used to test alternators on the farm was with the car running, disconnect the negative terminal on the battery. If the car dies, the alternator is bad. Be careful of course. I recommend shutting the car off before reconnecting the battery.

Bill

[cabinetman]

The way we used to test alternators on the farm was with the car running, disconnect the negative terminal on the battery. If the car dies, the alternator is bad. Be careful of course. I recommend shutting the car off before reconnecting the battery.

Bill

I remember that one. With today's electronic sensors, I would guess if the car is running off just the alternator, or the battery, there will likely be all kinds of "check engine" lights going off.

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[LinuxRandal]

http://www.youtube.com/watch?v=JLUBt8sgKHA

Start at 2:29

[JimD]

While an alternator is a AC current device (as opposed to the generators really old cars had) the car doesn't really know about that. I think the diodes are usually in the alternator these days but sometimes they were a separate device. But the diode blocks the negative part of the wave form and I think they use a rectifier to clip the voltage too. But the main point I wanted to make is the car doesn't really get AC power. The AC from the alternator is converted to DC before the rest of the car sees the output.

Another small clarification point I want to make is about "sealed batteries". I have successfully removed the caps several times on sealed batteries. It is nothing like the old caps but usually there is still a cap that can be removed but you have to put a knife or chisel in the part line to get it started. I mention this because I agree this sounds like Sears needs to honor their warranty and admit it's the battery. Pulling a cap and using a hygrometer might convince them. It would at least allow you to know for sure what the issue is. If all the cells test fine and consistent, then it isn't the battery. I still have a hygrometer or two in old tool boxes but even if you had to buy one, they're cheap.

Jim

[gerti]

http://www.youtube.com/watch?v=JLUBt8sgKHA

Start at 2:29

Cool! I'll add a compass to my electric toolbox!

Thanks for that one.

Gerd

[gerti]

Just to follow up: after some talking Sears gave me a new battery, and the problem went away.

So somehow that less than a half year old battery was discharging itself, who knows why.

Thanks everybody for all the tips and suggestions!

Gerd

[cabinetman]

Just to follow up: after some talking Sears gave me a new battery, and the problem went away.

So somehow that less than a half year old battery was discharging itself, who knows why.

Thanks everybody for all the tips and suggestions!

Gerd

Sometimes it's the most obvious or simplest fix. I always just start with the battery (no pun intended).

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[dusty]

All seem like pretty good comments. I will add two I did not notice. Lots of short trips, especially in cold weather, can drain a battery. If this is the vehicles normal use, you might want to invest in a trickle charger.

Harbor Freight sells a tester that will load test the battery and also tell you if the alternator is putting out correct voltage for about $20. Seemed like a good investment to me so I bought one. It is very easy to use. You do not need to disconnect the battery.

Jim

I should probably own one of those. Especially now that the battery indication on my dash is so questionable. I used to be able to tell when the battery was charging by watching the voltage gauge. Now that does not happen. The voltage comes up to what a call 12 volts and hangs right there.

Voltage measured at the battery, with the truck running, gets up to about 12.5 volts. It seems to me that used to be 14-15 volts.

[durango dude]

So somehow that less than a half year old battery was discharging itself, who knows why.

Because it's in December in Minnetonka, MN (Minnesota) ---- where even penguins would think it's too cold.

Why on earth would a sensible person from Minnesota accept car advice from someone that lives in Florida????

[LCHIEN]

Gerti:

excessive internal battery (self-discharging current) leakage is a fault mode of batteries. You can't detect it with a current meter since it's internal and bypasses any external loads.

Glad you were able to get the battery replaced.

Dusty:

either your voltmeter is reading wrong or your alternator is not charging the battery. If it hasn't gone dead for a long time then you probably have one of two situations:
1. Voltmeter is wrong. You can confirm it with an external meter or the HF battery checker etc. Also, rev the motor some to see if that raises the voltage to the normal 13.5-14V charging voltage.
2. at 12.5V output Alternator is partially broken, you are charging the battery just enough to keep it working but maybe not topping it completely off. Have an auto parts store check your alternator.

[cabinetman]

Why on earth would a sensible person from Minnesota accept car advice from someone that lives in Florida????

All weather batteries.

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