Battery not charging, alternator is working? - PeachParts Mercedes-Benz Forum

Loren · #1 03-27-2007, 04:06 PM

Hello all,
I am new to this group. I have a '91 500SL with 60K miles on it that I am taking out of winter storage. Over the winter I replaced the spark plug wires (wow, now I see why MB wanted $500 in labor to do the job) because there was a hot idle/missing problem that came up in October. Now it runs like a top (timing was good) but discharges the battery as it runs. I don't think this is related to the spark plug wire replacement but good background.
The battery will be discharged and unable to start the car within 30 minutes when the engine is running. The battery appears to be good, starts the car fine, holds 12.6 volts overnight while connected to the car. The alternator/charging system appears to be working as the battery voltage jumps to over 14 Volts when the car is ideling but still the battery drains (this is what is confusing to me

). The lights and functions of the car are all normal while running but once shut off the dome light goes dim (alternator working?).
While it was in storage I had it on a battery tender trickle charger (without disconnecting the battery). The car also had a ground disconnect on the battery post that I found was intermittently rattling itself to the off position while I was driving. I discovered this when the car refused to start after a pit stop on a 1000 mile trip (who knows how long it was running with the battery disconnected). It has since been removed but may have caused the latent damage that I am trying to resolve now. I went through the manual and a workshop CD and they all say DO NOT CHARGE THE BATTERY WHILE CONNECTED TO THE CAR. Well I did and know better now but none of them say what I toasted and now need to fix / replace. I can't afford to take my cars to the dealer every time something goes wrong so I am asking for your help in troubleshooting this (probably common) issue. Thanks in advance.

Loren

GSS0816 · #2 03-27-2007, 04:31 PM

I would have the battery tested, it may have a dead cell. If the battery has one, it will start the car when charged or jumped, but when it sits overnight, or even several hours it won't start.

fpharryc · #3 03-27-2007, 04:45 PM

I would suggest test the battery first also. Discnonect it, using a charger to check. If battery is not holding the voltage, then you have the culprit. If the batter is fine, then I would suspect the alternator itself. I used to have a alternator had internal short, that drained the battery fast and also damage the battery at the same time.

Loren · #4 03-27-2007, 05:27 PM

I thought it might be the battery at first too. The battery seems to take and hold a charge just fine, connected to the car, disconnected, after a few days. Starts the car multiple times without apparent weakening. Voltage drops to about 9 for a couple seconds before the engine fires and the voltage goes up to 14 once started (seems normal). A weak cell in the battery would exibit the same condition if I charged it using a charger or it was in the car running... right???

The problem is when the car is run/ideled for 20 minutes or so. The car drains the battery as it runs rather than charging it. While the car is running all electrical functions work properly, the lights are very bright and do not fluctuate by RPM. Voltage at the battery remain around 14 while running but if the car is shut off there is not sufficent power in the battery to restart and all the lights go dim. Doesn't make sence! I am no Electrical Engineer (Mechanical) but this seems to defy the first law. Power is supplied yet the battery is still drained? Is there a way using my trusty voltmeter to measure the amperage going into the battery? Say the alternator is supplyig the required voltage (14) but insufficent amperage to compensate for the ignition and other running gear it would make up the difference by draining the battery... right? I have not run the car any more than 30 minutes or so but if I did, once the battery is dead the car should stall? Ok, now it may be the alternator. I am not familiar with the death signs of an alternator. Do they die catestrophically or just slowly dwindle away? Is there a verification I can do to pinpoint the problem exactly.

Loren

david s poole · #5 03-27-2007, 05:40 PM

hook up your voltmeter and start the car.put on emergency brake and put in drive.now watch the voltage at the battery terminals while idling in gear and see what the voltage does over time.if it runs for twenty mins or so and maintains 14+volts and then you shut it off and very low volts you will need to change the battery.[by the way how old is that battery?]

mpolli · #6 03-27-2007, 06:26 PM

A 12V battery cannot discharge with 14V applied to it. The only way this could occur is if it is discharging internally. What does this mean? It means BAD BATTERY. More likely it is not taking a charge. What does this mean? BAD BATTERY. Now I am taking your word for it that 14V is on it all the time. How old is the battery? It is probably sulphated.

Mike

Loren · #7 03-27-2007, 08:10 PM

The battery is less than 3 years old! The first battery lasted over 12 years. I know that doesn't really mean anything but I was quite surprised to get that much lifeout of it. Guess it helps being in the trunk away from the engine heat and vibration.
If it were discharging internally it would do that whether it is hooked up to the car or not... right? Why would running the car make it discharge faster if it were the battery? Connected or not the battery maintains 12.6 volts and will start the car many times no problem. It will take a charge from the AC charger and maintain the charge for days once disconnected and again start the car many times no problem. Start it up and it drains quickly (sounds like alternator / regulator problem). Surprised none of the leads get hot from the discharge of a battery that big in such short order (sounds like internal battery problem). The battery should have more of a charge on it after the car is run for a while than before.
Why would the battery discharge while the car is running and not while the car is off if it is an internal battery issue?

I am so confused!

I can take the battery out and have it tested at the auto parts store but I don't think they will be able to test for internal shorts. They can test it under load which is essentially what I did by monioring voltage drop under load (starting). Even if it is weak with respect to its rating it starts the car fine when charged and left for days so why would it deteriorate when the car is running? (something in the charge system is not working correctly?) The accesories and lights work great while the car is running so the alternator seems good. The power is just not getting to the battery to recharge it. But the voltage on the battery while the car is running is measured around 14 Volts. Turn the car off and the lights go really dim (they were bright before the car was started and while the car was running).

What would be damaged from me charging the battery while it is connected to the car?

What would be damaged by the ground being disconnected from the car while driving (faulty ground disconnect switch)?

These are all things that I unknowingly did to the car that the owners manual say will cause damage to the electrical system. Naturally their advice it to take it to the dealer (and drop $500 for them to tell you what needs to be fixed and what an idiot I was then another $500 to fix it).
I am sure most people on this forum drive their cars enough to keep the battery charged but surely there are a few in the northern climates that store them for the winter. Do you disconnect the battery from the car as the owners manual insists? I have been charging the battery with it connected to the car for 8 years and I guess it is about time it caught up with me. That may be the reason the previous owner had the ground disconnect on the car.

Thanks for all the suggestions!

Loren

mpolli · #8 03-27-2007, 09:13 PM

There is no harm from charging a battery conected to the car that I know of.

If you have 12.6 resting voltage then you do not have a shorted cell.

You need to verify your assumptions. You seem sure that the battery has 14V on it all the time while it is "discharging". This is technically impossible. How do you "know" that it has 14V on it? Did you put a volt meter on the battery posts? I am not trying to argue with you. I am just saying that you are making some assumptions, one of which is not possible. Put a volt meter on the actual battery posts, not the clamps or anywhere else. If there is 14V there then the battery cannot discharge. Perhaps the discharging is occurring after you turn the car off?

Mike

joselu43 · #9 03-27-2007, 10:02 PM

I am with mpolli. With the given info the most logical reason is a bad cell. You need to take some measurements. Again, I am with mpolli, disconnect the battery (to eliminate the drain factor), charge it for as long as you want, let it rest overnight and take a voltage reading in the morning, if you have less than 12 volts, you have a bad cell. Alternatively, after charging for a while, you should see bubbles coming out of each battery cell. If a cell is not bubbling, it is bad and you will not get 12 volts after the battery has been at rest for a while. Mind you, you may still be able to start the car with less than 12 volts, but I would no depend on it.

Jl

Loren · #10 03-27-2007, 10:17 PM

I agree it doesn't make any sence! Charged the battery holds 12.6V for days. When running the voltmeter reads +/- 14V on the battery yet still drains as it runs. I will take the readings again. Otherwise I may have discovered a black hole (not just a potential money pit).

Loren

fpharryc · #11 03-27-2007, 11:46 PM

Your battery sounds OK with me. It is chargeable and will hold the charge. Next step is, figure out what is draining it while your motor is running. My money is on your alternator.

david s poole · #12 03-28-2007, 09:47 AM

perhaps just to get rid of the assumptions you should run two lengths of wire from battery into car and hook up voltmeter and watch while you drive.is it poss that alternator belt slips while driving?

GSS0816 · #13 03-29-2007, 10:12 AM

Just a thought, check your starter. I had one stick with the bendix in the out position, and it will drawl serious current. Car will run but voltage will drop.

Loren · #14 03-29-2007, 01:21 PM

Ok, last night I started with the same battery (fully charged, 12.6). Started it up and ran it around the block. At 2500RPM the battery read 14.12Volts. At idle it read 13.8 (about the same as before) Ran it for 30minutes shut it off and the battery reads 12.4 Had no problems restarting. I am not sure what happened. The only thing different is the ground interupt switch was removed. Maybe the vibration from running disconnected it enough to not be able to pass enough current to start? I know I checked that before! That still does not explain the drain on the battery while running (I know it was there as the voltage droped and the charger took more than 4 hours to recharge it). If the starter was sticking wouldn't that make a hell of a chatter? Is there something I can do to prevent this? Grease?
Glad it appears to be working correctly again. Confidence in my troubleshooting ability has certainly been shaken. I am going to be packin' the jumpers in this car from now on. Please stop if you see me holding cables in some parking lot. Thanks for all the helpful hints, I am sure I will have more questions for you all in the future. Who knows maybe I will be able to answer a few of yours.

Thanks,
Loren

bwheitman · #15 04-22-2007, 11:22 AM

Sounds to me like a regulator problem. When you start a car, the alternator kicks about 14V back to the battery for a short period, this replenishes the juice from the start up, then the regulator kicks it down to required voltage to run the car and accessories, I think this shift is when you are losing power. If it were my car, I would remove all after-market add on's, trickle charger, shut off, etc.. buy a new alternator/regulator, and a new battery. I don't like to get stranded! Good luck!
Brian