Does a Car Battery with a dead cell cause failures of alternators?



My wife’s 2002 VW TDI Jetta Wagon recently had the battery dashboard red battery light come on. My attempt to recharge the battery recognized that a battery failure was the cause. I had previous checked with my VW service people at my VW dealership to assure that any attempts to recharge wouldn’t endanger the computer electronics.

I also recognized that replacing this battery would mean a reset of the same VW computer system, because as you already know when you remove the battery on the newer VWs it causes the computer to require a reset which if not done correctly can cause serious problems. Therefore I decided to have my VW dealership do the job. While driving to my VW dealership the following dashboard lights appeared - Airbag, ABS, and the Battery light. When the RPMs reached above 2500 the Airbag and ABS light would go off, but the Battery light wouldn’t.

The VW dealership testing found that there was a dead cell in the battery. They also found that the alternator was only producing 50% of the power needed. Therefore we agreed to replace both the battery and alternator.

My question here is could the battery problem caused alternator problem???

This battery was the original batter placing it’s age just over 5 years. What made this situation a real mindtwist was that I had identified that this battery should be replaced one day prior to the Battery light appearance.

My thought here is that if such a battery failure could cause a related alternator failure, then periodic battery testing when the battery gets older should be a recognizable desired preventive maintenance issue - right?


Yes periodic check is good and avoids alt. damage. Subaru recommends this check.


Yes. A defective battery can damage a perfectly good alternator.

The alternators function is to recharge the battery once it’s been used to start the engine. And to keep the battery charged when various electrical systems are used in the vehicle. The alternator determines how much to charge the battery by measuring the reference voltage from the battery. If there’s a problem with the battery where the reference voltage is very low, the alternator will try to recharge the battery. The alternator isn’t designed to recharge a dead battery. It’s intended to maintain the charge on a good battery. When this does happen, the alternator begins to run hot, and this is what kills the alternator.



Furthermore, several of the auto parts stores will perform the tests of the battery and alternator in the car, for free. Just tell them you think you have a problem…perhaps, slow cranking.


The dead cell overworks the alternator, no matter how hard it tries, it can’t charge the battery…Alternators produce 3-phase alternating current. A “diode trio” rectifies (changes) this AC voltage into the DC needed to charge batteries. When ONE of the diodes fails (burns out) from overload, an “AC ripple” appears in the cars electrical system. Computers don’t like that and that’s what made all those dash lights come on. The failed diode also greatly limits the output of the alternator.

Back in the Old Days, mechanics would dissemble the alternator and replace the failed diode(s) and you would be on your way for about $40-$50. But those days are over, as you have just learned…


There is a lesson to be noted here which is to not try to milk the last month of life from a battery, especially after the warranty has expired. Auto batteries, on a yearly basis for the warranty period, are cheap. I will check the ages of ours for sure now.


The only thing I will add to the good advice you’ve been given is that periodic testing is a good idea and should be performed a certain way.

The battery should be placed on a charger for at least 30 minutes (preferably an hour) and a load applied to the battery with a carbon pile.
The load should be approx. equal to what the normal starter draw is on a warmed up engine. NOT the intial current surge, but the normal steady amperage draw.
(125 amp draw, so 375 amp load applied)
This load should be applied for 15 seconds and the battery should maintain a voltage reading of 10.2 at a minimum. If it drops below 10 you’ve got a weak battery and if it drops to 9 volts, then to the boneyard it goes.


Actually, it takes two things to combine to damage the alternator, a bad cell and revving the engine. At low RPM the alternator doesn’t put out enough current, even to a dead short, to cause it to overheat and burn out the diodes. As you rev the engine, you also rev the alternator, increasing the current out and increasing the heat the diodes are subjected to.

The reset procedure after replacing the battery isn’t that difficult, you shouldn’t let the dealer scare you away from doing it yourself. Once a 5 year old battery begins to give you trouble, replace it immediately. No testing needed, just do it, its cheaper and easier in the long run. If you had a red battery light on the dash before the battery went dead, then the diode was the first thing to go, it took the battery out, not visa versa.


You can replace a battery without “reset” worry by simply using a another 12VDC source to “jump” the battery connectors as the battery is replaced. A cheap source is a pair of 6V lantern batteries wired to provide 12 volts. A lighter plug can be used on the cord and plugged into the cars lighter socket, Keeping the system powered up while the battery is replaced…This eliminates anti-theft radio problems also…


This is no doubt a superior test method but typically, a home mechanic will not have this equipment nor the inclination to bring it to a battery store to pay for having it done. I use an informal (quick and dirty) test if there is doubt about the battery. Turn the headlights on and start the engine and immediately shut it down before the alternator recharges the battery. If you can do this 10 times in cold weather, your battery should be good for a while. Not very scientific and the alligators might snap their jaws about deep cycling a battery but it has worked for me.

Before multiple coil electronic ignition, I would ground the ignition coil output and let the starter crank the engine for 30 seconds if it would.


Even VWs, with their dealer only reset, can supposedly have their batteries disconnected for up to 90 minutes without making the car’s computer die. After that, you’re screwed. I got stung on a New Beetle a couple of years ago. It was dead when I bought it. A new battery allowed it to start and run for about two seconds. Reset = $140, after the tow. Some indy shops may have access to the system to reset VW computors by now. It’s been out for a while.

I like Caddyman’s idea too, just in case.