What exactly is going on with my car battery

highlander2099

Okay first of all, it is a 4.5 years old battery that has 60K miles on it so it is due for a replacement, however what happened recently is just what I couldn't understand.
I went on a 3 weeks vacation. After I came back, the car is still able to start. I could drive it for shopping at different stores the next day. Everything seems to be fine.
Then 3 days later the car failed to start. I checked it with a battery checker and it says SOH 5%, SOC 0%. So I drive on the highway for 40 minutes and able to charge it to SOH/DOC 90%.
The next day I am still able to drive it to multiple stores. But in the night, the battery has dropped to 60% and the next day it further dropped to SOH 7% SOC 0%.
This is what I know: alternator is good since it is able to change it to 90%. There is no parasitic drain. The drain is only 10mA.
The question: if the battery is bad, why was I able to start it after 25 days of not using it. If the battery is good, then how to explain that it is no longer able to hold charge?

thecardoc3

An alternator is not designed to charge a battery. It can replace what is taken from the battery after starting the engine but when a battery has been completely discharged it should be charged with a regular battery charger. Something is wrong with your test results. Even with a shop's battery charger it takes all night long to restore a battery that was discharged to the point that it couldn't start the engine. At forty minutes if you actually got it up to 20% that would be a really good result. I suspect the test result was done under a condition called surface charge, and the tester failed to identify that was the case because the voltage wasn't high enough because of starting with a totally dead battery.

How did you measure the parasitic drain?

As far as why was it able to start the engine after sitting for nearly a month and then it displayed and issue a few days later and wondering why, at this point there is no answer for that, but sometimes parts just fail without a warning.

highlander2099

Hi, Thank you for the post. What you said does make sense. The battery tester does have the option of removing surface charge by asking the user to turn on the headlight for about 10 seconds but it never gave me the such a warning (it is supposed to detect the surface charge and ask the user to remove it first).

To check for parasitic drain, I simply disconnect the battery from the cable on one side and put the multimeter between them (on DC A mode). I disconnected the positive side since the negative is on the far side to reach but I don't think it matters.

Cheers.

thecardoc3

Excellent on using the DC amps setting and putting the meter in series with the battery. There are some occasions where a car can play a dirty little trick and result in this routine not confirming that there is a drain on the battery. Sometimes a parasitic drain occurs and lasts until the battery is depleted then it goes away. Picture a module that has a problem and doesn't go to sleep and just keeps pulling on the battery. Then once the battery is sufficiently dead, that forces the module to finally turn off. So, then the battery is charged, or replaced and when the car is tested no drain is observed. This can even happen pretty quick during the time to take the battery cable off and attach the meter or if someone just pulls the fuse and they see the drain go away only for it to not come back when the fuse is put back in. When we suspect that might be what is happening, one testing routine is to use a low amps current probe which can just be clamped around the battery lead and measure the current flowing without opening the circuit. Another trick is to make sure the car is parked in shade, or in a garage. Attach the charger and just let the car sit turned off for three to four hours. Then use an infrared camera to look for heat. If something has stayed turned on it will be hotter than the rest of the car. I'll attach a photo here for an example. This is a Dodge Ram truck that would kill a battery over the weekend. The instrument cluster has failed and doesn't turn off.