Charging issue

[BeratE]

Hi all,
I have a Toyota with a 12V diesel engine. I have an Odyssey AGM battery that has awesome cranking ability when its charged, but there is the problem. My battery is not charging correctly or staying charged for some reason. I have charged the battery inside my house to a steady 12.85V, and then after a drive and a period of the truck sitting for a day or two, the charge is at 12.3V or less if it sits longer. Then a drive around town and the next day its at 12.18V. At idle the voltage across the battery is 13.6V, so it seems that the alternator is doing its job. I have a nice modern ctek battery charger, and when charged its lights seem to say that my battery is healthy and fully capable of taking a charge. And using my multimeter inline with the negative battery cable and the battery (engine off) shows a current draw barely in the micro amps. Measuring for AC voltage from the alternator at idle comes up empty.

What am I missing? What could be wrong?

 

[george_tlc]

Rev the engine up and you should see around 14.4 or a bit higher across the battery. If so, the alternator/regulator appears to be doing a good job.

Disconnect battery cable and let the battery sit for a few days, measure the voltage across the unloaded battery every day and see if it self discharges (bad cell etc).

You are sure you multimeter works ok, yes? Many have a 200 or 400mA fuse protecting the mA/uA range and that fuse can easily blow and then it just appears you have very low parasitic current...

I prefer to start current measurements with a DC clamp meter and then go to the 10 or 20A range on a multimeter and then to the mA range. This protects the meter if you have a higher current flow than you expect.

cheers,
george.

 

[BeratE]

Hi George,
Thanks for the reply. I just took the alternator off and took it to an Advance Auto store and had them bench test it. Won't say that they are the gold standard at all, but their tests show the alternator working just fine. I have been doing exactly what you said to test the battery discharge for the last couple of days now. It started at 13.0V two days ago and is at 12.82V now. The battery seems great, and so does the alternator. I tried a friend's multimeter on the off chance that mine was not testing correctly and still no obvious parasitic current. I am totally stumped

1/16/15 - Battery at 12.78V

 

[george_tlc]

First step is to determine if it is a parasitic draw or the charging system isn't working.

I would fully charge the battery with your ctek charger. Then ONLY connect the battery when you drive it, disconnect when you stop the engine. Do this a couple of days and see what the battery voltage ends up at. If it drops over a few days, then the charging system isn't working correctly (since you disconnect the battery each time the vehicle stops, you can rule out parasitic issues).

This of course all assuming your battery is not a culprit, though your bench test of it holding a charge for several days seems to imply it is not the problem.

cheers,
george.

 

[Fast Eddy]

I've got this problem on my 6.5td Chevy tow rig. New batteries, new alternator, no appreciable rise in voltage when running. It's like an electric car. It will run for about 100 miles or so as long as you don't use the lights, then it needs to be charged.

Isn't the regulator built into the alternator?

Edit: Answering my own question: it is connected to the alternator, separately servicible.

Edit2: My truck ended up having a bad connection at the alternator.

 

[BeratE]

George, I will try what you suggest and see what happens

Eddy, have you measured what the voltage is at the battery when the engine is running? And at 2000 rpms? It is possible that you got a DOA alternator when you bought it. Also, yes the regulator is part of the alternator on my truck