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Quick backstory (skip to the last paragraph if you just want the question haha)...about 3 months ago my alternator went out so I had to replace it. Yesterday I was out shopping and went to start my car - nothing. Lights and everything was at full brightness but the car wouldn't turn over. Tried to jumpstart my car and it still wouldn't turn over. So I called a tow to request a jumpstart and if that didn't work ultimately a tow home. While waiting I couldn't help but think I got a bad alternator that wouldn't recharge my battery.
The tow arrived and he was able to start the vehicle. While it was running we disconnected the battery and the car continued to run so I'm thinking the alternator is good. I figured I'd run to Advance Auto to get the battery tested. During the drive, about 20 miles, the volt gauge fluctuated between about 12-14 back and forth.
Once I got to Advance, I shut the vehicle off and they hooked up the tester. The Voltage came in at 13.7 and when I tried to crank it the amps came in at 775 (and the car again wouldn't turn over). He said that those numbers indicate something wrong with the cells within the battery. My question is - if both the volts and amps are higher than rated, why couldn't it start the car? It seems counter-intuitive that you have more power but can't turn over the vehicle. Is there some kind of regulatory between the battery and whatever draws off it?
I have had a similar problem in the past and was told the amperage delivered under load was the insufficient and that a 'dead' cell was the problem. A new battery solved the problem.
Location: God's Gift to Mankind for flying anything
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Before you buy a new battery ...
Disconnect the battery.
Clean the contacts of the battery really good, and clean the connectors.
Reconnect the battery and try to start.
If it starts, you are done.
If it does not start, then disconnect the old battery again.
Now start the car with another battery.
If it starts, then the starter is OK, but if it does not, your starter may be bad ?
Put your battery back on.
If it does not start, forget trying to figure out why.
Just buy a new battery, but I am almost sure, you have lousy contact between the connectors and the battery posts !
Your battery is not what it looks like. We call them a 12 volt battery but really the car battery is a 2.2 volt battery with 6 cells linked together. What can happen inside the the battery is that the strap/link between 2 cells can crack. Some times it makes contact and sometimes it doesn't. This is called an open cell fault. When you start the car is the biggest demand on the cells, this is when the fault will show up.
While your alternator can be charging it can still be defective. Inside most current alternators there is a regulator that tells the alternator to put the charge to the battery or that the battery is fully charged and run any excess voltage/amperage to ground. The battery is used only to start the car. The alternator runs the car after the engine starts. But, the regulator can malfunction when the engine is off. It can drain the battery slowly by running electricity to ground. This is a failure of what is commonly called the final diode. It acts like a gate. When it's charging the battery the gate is open to let the flow go to the battery. When the engine stops, the gate is supposed to close and stop electricity going back thru the alternator and to ground. When this fails, the alternator will charge but you can still have a dead battery after the car sits for a few days. If you bought a rebuilt unit, no rebuilder checks the final diode. All they check is that the unit charges. Taking it back to the parts house and they will check it for charge only yet the unit is defective. When it comes to alternators, always buy new to avoid the problem.
Quick backstory (skip to the last paragraph if you just want the question haha)...about 3 months ago my alternator went out so I had to replace it. Yesterday I was out shopping and went to start my car - nothing. Lights and everything was at full brightness but the car wouldn't turn over. Tried to jumpstart my car and it still wouldn't turn over. So I called a tow to request a jumpstart and if that didn't work ultimately a tow home. While waiting I couldn't help but think I got a bad alternator that wouldn't recharge my battery.
The tow arrived and he was able to start the vehicle. While it was running we disconnected the battery and the car continued to run so I'm thinking the alternator is good. I figured I'd run to Advance Auto to get the battery tested. During the drive, about 20 miles, the volt gauge fluctuated between about 12-14 back and forth.
Once I got to Advance, I shut the vehicle off and they hooked up the tester. The Voltage came in at 13.7 and when I tried to crank it the amps came in at 775 (and the car again wouldn't turn over). He said that those numbers indicate something wrong with the cells within the battery. My question is - if both the volts and amps are higher than rated, why couldn't it start the car? It seems counter-intuitive that you have more power but can't turn over the vehicle. Is there some kind of regulatory between the battery and whatever draws off it?
Thanks!
This highlighted above is a legitimate technique with a *generator* but not an *alternator*. You may have damaged the alternator by doing this. In other words, it was good up till about the mid-1950's.
I have seen an internal short in a battery cause a no-start, and couldn't jump start it, but had to replace the battery to be able to start.
Once I got to Advance, I shut the vehicle off and they hooked up the tester. The Voltage came in at 13.7 and when I tried to crank it the amps came in at 775 (and the car again wouldn't turn over). He said that those numbers indicate something wrong with the cells within the battery. My question is - if both the volts and amps are higher than rated, why couldn't it start the car? It seems counter-intuitive that you have more power but can't turn over the vehicle. Is there some kind of regulatory between the battery and whatever draws off it?
Thanks!
I'm thinking there was a little miscommunication here between you and the guy who tested the battery. Those numbers sound perfectly normal, but my guess is that those were the initial readings which really don't tell you much. It probably failed the load test that he should have run after taking the initial reading.
Location: San Ramon, Seattle, Anchorage, Reykjavik
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Quote:
Originally Posted by BoSox 15
Quick backstory (skip to the last paragraph if you just want the question haha)...about 3 months ago my alternator went out so I had to replace it. Yesterday I was out shopping and went to start my car - nothing. Lights and everything was at full brightness but the car wouldn't turn over. Tried to jumpstart my car and it still wouldn't turn over. So I called a tow to request a jumpstart and if that didn't work ultimately a tow home. While waiting I couldn't help but think I got a bad alternator that wouldn't recharge my battery.
The tow arrived and he was able to start the vehicle. While it was running we disconnected the battery and the car continued to run so I'm thinking the alternator is good. I figured I'd run to Advance Auto to get the battery tested. During the drive, about 20 miles, the volt gauge fluctuated between about 12-14 back and forth.
Once I got to Advance, I shut the vehicle off and they hooked up the tester. The Voltage came in at 13.7 and when I tried to crank it the amps came in at 775 (and the car again wouldn't turn over). He said that those numbers indicate something wrong with the cells within the battery. My question is - if both the volts and amps are higher than rated, why couldn't it start the car? It seems counter-intuitive that you have more power but can't turn over the vehicle. Is there some kind of regulatory between the battery and whatever draws off it?
Thanks!
I had the exact same thing happen on my Outback yesterday. 2 year old OEM battery. New battery fixed it. Batteries fail for lots of reasons even when, by all measures, they should be fine. I never got a battery warning or ever sensed an issue till it died.
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