manny26

Hey i got a AC Delco AGM battery months ago and since last weekend its been draining overnight. I have HIDS installed and LED eye brow Lights , and LED Depo Tailights, but i have had them since before i even changed the battery to a AGM battery. Anyone know what can be causing a drain overnight? or what it can be ? or what to check for , any help would be great, 2010 Mercedes c250

Josh.A.Hussey

Did you do anything to correct it so it doens't overcharge and reduce the life of the battery?
If the previous battery wasn't draining, this one shouldn't either unless the battery itself is shot and won't hold a charge. This could happen if the charging system is not meant to charge AGM batteries. You can't brute force charge them the same way you can with flooded deep cycles.

I would charge it, then disconnect the terminals and leave it over night to see if it holds charge.

Of course a quicker method of doing this would be to charge it and use a load tester... Just a thought.


AGM requires careful charging methods. Although I have seen people just replace flooded batteries with them and apparently have no issues.... Not sure how though. considering they are not meant to be charged the same way.

Google AGM batteries and their differences/similarities to flooded batteries for more info.

If you find the battery is intact, then you've got something else which is taking charge when it shouldn't. That could be a pain to find...

c300white

I would first get the battery tested at parts store to make sure it can hold a charge. If battery is good, you have drain to ground somewhere - best way to isolate that is to get adapter you can put in the fuse socket , connect it to your multimeter and take out one fuse
At a time to measure current (with key out). Some circuits could
Be as high as 1 amp until the car goes into sleep mode - but if you list back here all fuses that measure more than 1/2 amp - we might be able to point you to the issue ...

Most autoparts stores carry those adapters ....

c300white

Btw - another possibility is bad alternator diodes - is alternator warm to the touch after leaving it overnight ?

manny26

I'm going to go see if I can get that fuse socket adapter now , and The alternator is not warm , just checked it . Car has been sitting for 10 hours . I still have my old Mercedes battery which I can install and get the AGM one load tested still has warranty but they said they can't replace it unless
It shows a fail on there load tester

manny26

manny26

Tested the battery and then cranking test , those were the results . Don't look to good ..

Josh.A.Hussey

Nope. Did you charge it first though?

c300white

I went through two batteries I thought were bad ... But actually bad alternator diodes were the issue. Was battery tested out of the car ? What voltage does alternator charge battery at when idling ? Even with bad diode voltage can still be ok - parts store may be able to test alternator (just make sure they are also testing diodes - not just charging voltage) easiest way to spot diode problem was that alternator body was warm when left overnight ...


Hope you just have a bad battery

Josh.A.Hussey

You need to follow procedure when testing these things.
I would suggest following a procedure outline by someone here.
Taking a few voltage readings unfortunately will not give you 100% confidence in your diagnosis.

manny26

Can I get a link to a testing procedure and here are the results for the charging
Voltage at high idle , doesn't hit 14V stays around 13.8 ish drops to 13.4

manny26

manny26

And also the car sat over night again the alternator was not warm but tested the battery it only showed that it was at 131 cold cracking amps not even close to 800...

manny26

Found out the AC Delco AGM battery went bad, replaced it with my old mercedes battery and has been starting fine for the past few days, thanks alot for your guys help.

Josh.A.Hussey

Thank you for the update. Sorry I did not reply to your inquiry, I missed that post!

I have a feeling the AGM went bad based on how it is being charged from the alternator. You may need some sort of charging module to regulate input to the battery. It is quite different from a regular flooded battery brute force approach.