Dead battery while driving but alternator appears to be charging? (1 Viewer)

[smwmustang]

Just got the LX back on Sunday and it drove great and started fine. My son took it to school yesterday and called me when he was leaving and said it wouldn't start. He has jumper cables so he called and said it jumped off fine and was headed to work.

About 10 to 15 minutes later he called and said he was sitting in the turning lane and when he went to take off it just died.

I rushed out to help since it was in one of the most populated intersections in the Memphis area and tried to jump it off but everything would come on but the battery was completely shot. Called the local police to help us with traffic so we could push it to the side of the road where I had already had my other so go buy us a new battery. We managed to get the car home with no other issues.

This morning I got out the volt meter and the battery read 12.8 before cranking the car. I assume it has been sitting on the shelf at AutoZone and we had to have it immediately (my son works at AutoZone coincidently) so I had no idea what my other son bought. It happens when you need it right away.

Anyway, once cranking the car it read 14.3 and then after turning on the lights, A/C and radio it still read 14.3 and never fluctuated. After running the truck for about 15 min and shutting it off the batter now read 13.1 as if it actually charged.

I guess my question is, since this has never happened to me, can the battery be so bad that a good alternator cannot charge it. All other times the alternator has been bad. We will take today to test the alternator and I am ok with getting a new one just in case.

Thoughts? Sorry for the long ramble....facts matter.

 

[SteveH]

The regular voltage of a wet-cell lead-acid battery (all the ones I test that are a year or two old) is around 12.62, so a battery off the shelf that read 12.8 is just fine.

Yes - old wet-cell batteries can be so bad that the car won't run, but they usually don't fail that way. Batteries act as a filter on the alternator's output, and that feature disappears when the battery is shot. If (at this point) you are seeing solid voltage at idle with all accessories turned on (fan on high, headlights on high, rear defroster, etc.) then you're good.

I just replaced an alternator in an 80 series (395K miles) that showed 14.3 at idle until you turned on all the accessories, at which point it tanked to 12.75v or so - and upon replacement with an OEM reman., it shows good voltage at idle now with all the junk turned on.

 

[smwmustang]

The regular voltage of a wet-cell lead-acid battery (all the ones I test that are a year or two old) is around 12.62, so a battery off the shelf that read 12.8 is just fine.

Yes - old wet-cell batteries can be so bad that the car won't run, but they usually don't fail that way. Batteries act as a filter on the alternator's output, and that feature disappears when the battery is shot. If (at this point) you are seeing solid voltage at idle with all accessories turned on (fan on high, headlights on high, rear defroster, etc.) then you're good.

I just replaced an alternator in an 80 series (395K miles) that showed 14.3 at idle until you turned on all the accessories, at which point it tanked to 12.75v or so - and upon replacement with an OEM reman., it shows good voltage at idle now with all the junk turned on.

Thanks, I am going to test it some more tonight and drive it around the neighborhood so in case I get stranded it wont be too far from home. In my experience it has always been my alternators that went bad when this happened so this would be a 1st.

 

[donco]

That just happened to me Sunday. Drove fine all morning, parked it. Came out later in the day and "dead". Turned out the battery clamp on the positive post was loose. Tightened it up and all is good, battery is fine.

 

[smwmustang]

Ran it again yesterday morning and drove it about 15 min in the neighborhood. Battery was at 12.8 before cranked, 14.29 at lowest with all accessories on and then 13.3 after driving around neighborhood. I was out of town yesterday so taking to get alternator tested today just to make sure we are good to go.

 

[plainjanefjc]

The connections leaving the battery can be slightly loose- not enough to even notice and it’ll do what you’ve described. I replaced the battery and alternator and it was a loose connection…

 

[smwmustang]

The connections leaving the battery can be slightly loose- not enough to even notice and it’ll do what you’ve described. I replaced the battery and alternator and it was a loose connection…

Thanks I will double check all the connections as well. The battery was toast so now having a new battery we can eliminate other things as well.

 

[Mike NXP]

Ran it again yesterday morning and drove it about 15 min in the neighborhood. Battery was at 12.8 before cranked, 14.29 at lowest with all accessories on and then 13.3 after driving around neighborhood. I was out of town yesterday so taking to get alternator tested today just to make sure we are good to go.

This is all normal for sure, same vbltage readings from my system,,,,. Sounds like your original problem was caused by that battery being so dead it wouldn't run off the alternator alone, hopefully the new battery was the only fix needed

 

[smwmustang]

Last update...I think. Had starter, new battery and alternator tested and everything looks good. My son drove to school today so I guess I will find out later if any other issues arise. Thanks for all the tips and comments.

 

[XRayFish]

Hi Guys,

I have a 1998 Lexus LX 470. I replaced my original 80 amp alternator with which, I think, is a 130 amp alternator some time ago.
Last year around September my battery died for no apparent reason. It wasn't old. I claimed a new battery on warranty thinking that it was the problem,.
A couple of weeks ago the same thing happened again. Battery died...... Absolutely dead.
So I bought a new fully charged battery, (reading 13.2V}, upgraded to the "Super Charge" Gold Plus with 40 months warranty.
I drove the wagon approximately 5 minutes from where I broke down, back home and that new battery was utterly dead flat. I have checked and been through everything and it keeps coming back to the alternator. When I leave the wagon on idle, it charges normally. When I rev the engine, massive discharge. When I turn the engine off, the battery charges for a few moments. Weird huh!
After a ton of research it seems that if you replace an alternator with the new one having too many amps, this is the effect you get. I never looked at the amperage of the replacement alternator because on checking if it suited my vehicle I got the thumbs up! I believe it is either 100 or 130 amps output. The original one had an 80 amp output. I guess I get to check it when I get it out!!! Lucky me - YEHAAAA

 

[smwmustang]

Hi Guys,

I have a 1998 Lexus LX 470. I replaced my original 80 amp alternator with which, I think, is a 130 amp alternator some time ago.
Last year around September my battery died for no apparent reason. It wasn't old. I claimed a new battery on warranty thinking that it was the problem,.
A couple of weeks ago the same thing happened again. Battery died...... Absolutely dead.
So I bought a new fully charged battery, (reading 13.2V}, upgraded to the "Super Charge" Gold Plus with 40 months warranty.
I drove the wagon approximately 5 minutes from where I broke down, back home and that new battery was utterly dead flat. I have checked and been through everything and it keeps coming back to the alternator. When I leave the wagon on idle, it charges normally. When I rev the engine, massive discharge. When I turn the engine off, the battery charges for a few moments. Weird huh!
After a ton of research it seems that if you replace an alternator with the new one having too many amps, this is the effect you get. I never looked at the amperage of the replacement alternator because on checking if it suited my vehicle I got the thumbs up! I believe it is either 100 or 130 amps output. The original one had an 80 amp output. I guess I get to check it when I get it out!!! Lucky me - YEHAAAA

Interesting as the alternator was replaced right before we bought it. I guess I need to check that out. I tested again this weekend after he had been driving and the numbers seemed a little low. This morning the battery was at 12.8 and then when cranked at 14.3. This was however at idle and I have not tested it under load....good idea.