Car battery problem

[strantor]

My brother in law said his car is having problems starting. I checked the battery voltage, 10.6V. I had him start it up - it was a weak start, but it did start, and after it was running I read 14.6V. So I told him that his alternator is good, but his battery is shot, probably a shorted cell, and that he should replace it ASAP, as the alternator could fail as a result of trying to charge the bad battery.

I was pretty confident in that answer but the more I think about it, the more I have doubt that there might be another problem. Automotive electrical is my Achilles' heel. Just looking for a sanity check here.

 

[ronv]

Makes sense to me.

 

[cachehiker]

2V per cell, 6 cells, open circuit fully charged should be over 12V.

that was the diagnosis on my old 92 toyota pickem up truck.

The way I understand it, that's the way they fail in the summer.

In the winter, and winter means well below zero here, they lose their cold cranking amps.

I'd fix it soon but waiting until the next free evening is no biggie.

He's just increasing the probability of being unable to start it.

When the voltage drops to 8.5 Volts, and it will soon enough in that Texas heat, you'll be getting one of those calls.

 

[rfranzk]

Other Checks

Hey Strantor

You are basically on the right track. 10.6 volts is too low BUT this could be caused by a key off drain dropping voltage and not the result of a bad battery. The best test is to apply a high current load for 15 seconds and view battery voltage drop. The load is typically 300-500 amps, and minimum voltage for a good battery under load is 9.6 volts after 15 seconds load. Most people don't have access to this type of tester. Also a hydrometer test is recommended if the cells can be accessed.

The best way to check for key off drain is to remove negative cable and place a digital ammeter in series with the cable. Newer computerized vehicles can have a couple of amps drain until the computers go to sleep.(This can take up to 45 minutes) The final resting key off drain should be less than 500 miliamps on most vehicles. (Unless your meter is autoranging don't open doors or turn on accessories that might let the smoke out of your meter or fuse)

Typical automotive battery life is about 5 years depending on the quality of the battery and the way it was maintained.

You are correct about alternator problems if the battery is faulty.

Thanks. rfranzk.

 

[strantor]

The best way to check for key off drain is to remove negative cable and place a digital ammeter in series with the cable. Newer computerized vehicles can have a couple of amps drain until the computers go to sleep.(This can take up to 45 minutes) The final resting key off drain should be less than 500 miliamps on most vehicles. (Unless your meter is autoranging don't open doors or turn on accessories that might let the smoke out of your meter or fuse)

thanks rfranzk
500ma seems a bit high to me. I've read more than once that the typical automotive battery is not a high capacity battery, 60AH typical. So using that number, and assuming that the battery should be around 13.8V is fully charged, I do the following math:
13.8V (good battery) - 10.6V (measured battery) = 3.2V
3.2V / 13.8V = 23% discharge
60AH (capacity) / .5A ("normal discharge") * 23% = 27.6 hours

If you are correct about the 500mA typical discharge and if my math and capacity assumptions are correct, then just having sat in the driveway for a little over 1 day could explain the drop to 10.6V. That doesn't seem normal to me. I use my truck maybe once every 2 weeks to once per month, and I never have a problem with the battery being dead. Also he doesn't use the car every day, and never had this problem in the past. I'll check the the amp draw at rest and report back. It's a early 2000's acura.

 

[dr pepper]

Makes sense to me too.

Alty's self regulate to an extent so you'd be unlucky to burn it out.

 

[MrAl]

Hi,

Batteries are tricky sometimes. But 10.5v is a bit low for sure. What you'd have to do is charge it and see how it works over time.
A good battery charge will work well too. Put it on charge for a couple hours.

The real test is in the winter time if you have cold weather. In the morning when is is very cold it is hard to start because the battery has to heat up a little bit first and if it is on it's way out it has trouble starting the car. That's the time when batteries fail around this area of the country. If the battery is past it's life time (2 years, 3 years, etc.) then it may be the time to start thinking about a new one. They can last longer than their life time too though.

Harbor Freight for one sells a couple of battery testers for auto batteries. They are decently priced. I bought a 100 amp model which means it puts a 100 amp load on the battery. The idea is to load it heavily and test the voltage while it is under load. The best bet is to test it when the battery is new and then compare readings after a year, two years, etc., and this will give you some idea how the battery is holding up over time. Im sure other places sell these testers too. They are usually called something like "battery load testers'.

 

[rfranzk]

Hey Strantor,

Yes 500 miliamps is high and you will notice I stated it should be LESS than .5 amps. On the 2000's Acura I would expect around 100 miliamps. Newer computerized vehicles have 10 plus onboard computers that stay awake to some extent and can draw up to .5 amp. Fords are notorious battery crunchers. I didn't recheck your math but it sounds close!!

I just thought a couple of tests prior to battery replacement might be prudent.

Lots a luck. rfranzk.

 

[audioguru]

My car battery was 5 years old when it was tested by a high current tester. The test results were printed by the tester. It said my battery was below the performance of a new battery (DUH!) and should be replaced.

That was more than 2 years ago and the battery still works fine.
They simply like to collect money.

 

[debe]

I prefer the old way of testing with a load tester & Hydrometer. Usualy get 6-8Yrs out of batterys in my vehicles. When buyng a new one i use a Hydrometer to check the cells first. Have quite often found one cell will be down in SG, so i put battery on a charger untill all cells read the same then fit to vehicle. This makes sure the battery has a good start in life.

 

[cachehiker]

I sucked an additional year or so out of mine but it wasn't suffering from a shorted cell.

It had most likely lost capacity due to sulfation during the wicked cold winters here.

But as long as I gave it a good topping off every month or so, it kept getting the job done.

I finally got torqued off enough to replace it when I lost the radio station presets about three times in ten days.

It still started the car in spite of that though, all three (or was it four?) times.

 

[audioguru]

I had an old car battery suddenly stop working while driving on a hot summer day. Either a cell or two became shorted or the water in it evaporated (but it was maintenance-free and sort-of sealed).