Dead batteries are slowly injuring your car’s alternator

We all know that pit-in-your-stomach feeling that comes from turning a car key and only hearing a click, but what’s the best course of action when it comes to a dead car battery? Sure, it’s usually possible to just jump it with juice from another power source and be on your way, but the newest video from Jason Fenske of Engineering Explained takes a deep dive into the effects of a dead battery on your car’s alternator.

This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/03/07/dead-batteries-killing-you-alternator

This is a useless discussion because batteries that are dead enough to need a jump are not coming back. If you need a jump either your battery needs replacing or your charging system is already dead.
The exception to this is a car that has been sitting for a long time. Leaving a battery discharged may kill it.

Obviously keeping a good battery is key to a healthy electrical system. But things used to be a bit different. Everybody except me, of course, has left their lights on or done something to cause their battery to go dead. If that was why you needed a jump then we used to do it and go on about our business with no ensuing issues. You could even remove the battery from the car and drive it with no problems. Don’t try that with today’s computer controlled cars. In the 90s, manufacturers were trying to reduce weight and drag on the engine for better mileage, alternators got smaller and of course more expensive. But there was a new issue; alternators are now made to maintain a charge but not to actually charge a dead battery. If you jump started a GM with a completely dead battery you would probably blow the diodes in the alternator so it was recommended to charge it first. I’m sure other manufacturers’ alternators would also fail for this reason too.

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Right, because nobody ever had a good battery drained by a malfunctioning glove box or dome lamp switch, turning the ign. on while testing something and forgetting to turn it off overnight…

If that happens the battery recharges once and you’re done .
The article talks about the battery needing repeated charging. Usually if a battery goes dead spontaneously it won’t take a charge again.

Not so, as by definition, an intermittent problem is reoccurring. Trust me, I’m very familiar with the science of maintaining lead acid batteries, including training from C&D corp when dealing with a fleet of electric forklifts in private industry. We always have to consider and determine if a, “battery goes dead spontaneously” is actually an outside cause & effect, since batteries don’t have a sign pop up telling you why they were 13.4 yesterday and maybe 11.8 a day or two later. I have a car that delighted in making me grab the booster box once a month because the car was cranking over slowly. I seldom drive it after dark, and several tries with an ammeter in series from the battery would show no draw beyond the electric clock. Then one rainy day when I was in the garage with the door closed and a single overhead bulb on instead of the banks of fluorescents, I spied a gleam of light coming from the top crack of the “closed” glove box door. Sure enough, barely breathing on the door was enough to shut off the light. Older cars have many mechanical contacts that can stick as the get pitted, both on loads and mechanical regulators. All I’m saying is one can’t assume anything about a low battery until they can disprove all other scenarios with the Socratic Method, i.e. good, basic troubleshooting.


Granted any number of wierd things can happen.
But generally speaking car batteries tend to die without warning. The car starts fine, and then the next time it doesn’t. In this case usually the battery won’t even take a surface charge for one restart.

Not necessarily true. If one leaves an interior light on, etc., the battery can go flat. Then, after jump-starting (or manual charging), it will continue to function. Of course, a lead-acid starting battery will not take kindly to frequent full discharges, which will likely shorten its life.

That certainly has not been my frequent experience, in driving cars (many older) for over 45 years. Occasionally, sure; but, I have found that batteries usually give warning signs of trouble, as opposed to just suddenly dying.

Funny thing that alternators in general are much, much more powerful now (in terms of amperage output) than 40-50 years ago, yet they often cannot easily handle recharging a fully-depleted battery. Related, I had a 1992 Plymouth V6 Duster that blew a fusible link, while trying to recharge a depleted battery.

The 45 years is the difference. I can remember when a battery would get weak but still take a charge. I think battery construction has changed so every dead battery I have encountered since my '01 and several others that I helped have done the sudden death sinerio.

From what I’ve been told of the newer cars, they are more current based, where unless the battery is exceptionally low, they increase or decrease their output based on what the current draw is of the vehicle. Cars like my sixties babies are voltage based, where they will simply crank out all they can whenever the system voltage is lower then whatever the voltage regulator is set to.

I believe the newer cars that are computer controlled, and fuel injected need a minimum of 11.4 volts to start. If the voltage is lower it will turn over, but not start.
Older cars - manual trans can be started with a simple push, and in either direction, forward or backward if need-be. and will start even if the battery only has enough power to click the starter solenoid. Just remember if pushing backwards - the transmission needs to be in reverse.
When replacing the alternator on my 1980 TR8 with one rated for more amperage- I discovered both a bad ground by the battery and connection to the engine, and a bad terminal on the connector to the alternator. I believe once these were corrected the original 45 amp alternator would have been enough. But even the lights work now , even in the rain with the wipers and defroster on.