Dead Battery - Where to start? - PeachParts Mercedes-Benz Forum

chetwesley · #1 10-22-2007, 02:19 PM

Ok, so this morning when I started my car, the engine cranked slow. I thought it was just cold oil being thick, but when I went to take my lunch break, first try it only went 2-3 cranks, then 1, then "click."

Luckily I keep a battery charger in my trunk, but I am wondering what to look for first in figuring this out. I have been driving the car for several months with no problem, but I rarely drive at night until recently when it has been getting dark earlier, so glowing and the radio is the usual extent normally of the use of electrical from the car. How long would it take to kill the battery if you had no charging happening from the alternator? Probably less than a few months, right?

The battery light normally has only come on in the brief moment between when I pull the key out of the car, and when the engine actually shuts off, which means to me that the alternator would seem to be working fine (as it seems to stop sending a charge in that brief time between when I pull the key out and when the engine stops), but I might be wrong about that.

Could it just be that it is a tired old battery? I don't know the history of the battery - came with the car when I bought it in April.

By the way, when I hooked up the charger, indeed, the meter showed a pretty low charge.

JimmyL · #2 10-22-2007, 02:29 PM

That pretty much describes every poor old tired battery I've ever had. Not sure what else a bad battery would act like.....

rrgrassi · #3 10-22-2007, 02:30 PM

Yes, batteries do get tired, and do not hold a good charge if the electrolye (water & sulfuric acid) falls below the lead plates. Check the water level, and check the output of the alt with the engine at idle.

chetwesley · #4 10-22-2007, 02:32 PM

My battery is "Maintenance Free." Doesn't that mean I can't check the water level?

vstech · #5 10-22-2007, 03:20 PM

no, it just means it's a little harder to check. pry up the plates and have a look inside. if it's low, put in some DISTILLED water and charge it up. let it sit overnight with no charge or load on it, and measure the voltage with a digital meter. if it's between 12.45 and 12.78 the battery should be ok. to be sure put it in your car, crank it for about 10 seconds without using the glowplugs. and then disconnect the battery and measure the charge again. if it's still in the range of 12.45 to 12.78 then the battery is pretty good, if it's below 11V, the battery is probably bad, if not, move on to checking the alternator. with the car idling the battery voltage should be above 13.5V. if it is at or below 13.5, your alternator is not working properly.

84300DT · #6 10-22-2007, 06:12 PM

as vstech has said , get out your multimeter and do the tests. if the battery is shot then get one made by johnson controls. autozone or walmart or sears diehard .. etc... they are all the same battery. we are currently running a die hard international for the past 6 years no problems.

http://www2.johnsoncontrols.com/bg/

http://www.autobatteries.com/

yellowbenz · #7 10-22-2007, 11:34 PM

I'm going through a similar process. New battery didn't solve the problem. I should have done the testing first. Multimeter showed 12 volts with the engine running on my 300D. My 240D is at nearly 15 volts idling. I've removed the voltage regulator and will replace this next. I thought about switching the VR on the 240D with the 300D, but I wasn't sure about the wisdom of removing an electrical part from a functioning car and trying it on my new to me 300D. I wish there was a way to test the VR but I haven't come across this yet. If this fails I'll pull the alternator and have it tested. A common theme from the archives seems to be that a bad battery can kill the alternator and vice versa. I would think these are rare circumstances but I'm kind of worried about getting into a vicious circle of parts replacement without solving the problem or perhaps creating new ones. Good luck.

Kevin
1979 240D manual
1978 300D

chetwesley · #8 10-22-2007, 11:50 PM

I sort of doubt it is the alternator, as it was just replaced a few months ago...

The battery does not seem to hold much of a charge for very long and I had just let the car sit without running over the weekend, except for last night when I only ran it briefly to test something out (probably didn't recharge the loss from starting a couple times when I ran it, which may have put it over the edge).

However, there are some weird little electrical gremlins in the car, so this may not be the end of it... we shall see.

tangofox007 · #9 10-23-2007, 09:34 AM

Quote:

Originally Posted by vstech

if it's between 12.45 and 12.78 the battery should be ok. .

It's common for a battery to show good voltage but not put out enough current to operate the dome light. I just had one the other day that charged up to 12.75 volts, but would only deliver three amps.

Many parts vendors will check your battery and alternator for free.

rfdillon · #10 10-23-2007, 09:42 AM

Some times batteries work fine when the ambient temperature is warm, but that first cold snap is enough to kill it. I would start by taking the battery to Autozone / Walmart and asking them to test it, and probably purchase a new battery in my process to isolate the issue.

tangofox007 · #11 10-23-2007, 09:59 AM

Quote:

Originally Posted by rfdillon

I would start by taking the battery to Autozone / Walmart and asking them to test it, and probably purchase a new battery in my process to isolate the issue.

I don't think you will find the correct battery at Wal-Mart.

123Guy · #12 10-23-2007, 10:07 AM

Whatever else you do, replace the battery - you're in Wisconsin - don't take a chance on it with winter setting in - particularily if you don't know how old it is. When you replace the battery, don't get some auto supply house, 650 cranking amps deal of the day - get the best battery you can afford. Die Hard makes a good battery, but they no longer make the biggest MB battery, they have a close substitute that will work, but it's 750 CCA's, the best for diesels is the Interstate Megatron Plus, 850 CCA's - monster with an 85 month warranty.

chetwesley · #13 10-23-2007, 10:10 AM

I replaced the battery, as I don't have time right now to diagnose it, and if this is a problem, it is only a slow problem and not likely to hit me suddenly (and I know what to watch for now).

However, I discovered a sticker on the side of the old battery that said July 05, which means the battery was only a little over 2 years old. I am a little concerned by that, but I will just keep an eye on my new battery now to see if the problem repeats. It was a diehard international battery, which supposedly is a good battery.

Can a parts vendor just check a battery and see right there if it is bad? What I mean is, I charged the old one enough that it started my car when I left work yesterday, and it was still charged enough to start the car again when I got home, so if I take it in, might they just say, "yea, it's fine."

About how many times could you start the car (on a car that starts with only 2-3 cranks) without charging before you really run the battery down to where it wouldn't function to start? I have no idea how much the alternator really should be charging the battery when you drive.

tangofox007 · #14 10-23-2007, 10:23 AM

Quote:

Originally Posted by chetwesley

so if I take it in, might they just say, "yea, it's fine."

It's a tough decision. You just have to weigh the risks against the benefits. Put some thought into it; it's a decision only you can make.

To sufficiently analyze the situation, you might start by listing the top five things that could go wrong if you were to subject your old battery to a free test.

chetwesley · #15 10-23-2007, 10:46 AM

Very funny, but that's not the point. What I was asking is if they can accurately analyze the state of the battery. I know it isn't totally dead because it just started my car before I took it out. Can they actually analyze if it is capable of holding a charge for any significant time?

Quote:

Originally Posted by tangofox007

It's a tough decision. You just have to weigh the risks against the benefits. Put some thought into it; it's a decision only you can make.

To sufficiently analyze the situation, you might start by listing the top five things that could go wrong if you were to subject you old battery to a free test.