Can A Dead Battery Ruin Your Car's Alternator?

Will Jumping Your Car’s Dead Battery Damage Your Alternator?

What should you do if your car battery dies? Is it okay to jump the car and drive off, or is it better to use a battery charger, rather than relying on your alternator to charge the battery? This video will test various parameters of a car’s electric system to determine what effects will result from running with a dead battery, versus running with a charged battery. We’ll examine battery voltage (engine on and off), alternator voltage, alternator amps, and alternator temperature. Does a dead battery cause your alternator to overheat? We’ll find out in the video!

Note: If you’re stranded because of a dead battery, you should get your car jumped so you can get where you need to be. Obviously in emergency situations, do what’s safest for you, rather than what’s ideal for your car.

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[videogalore]

"If you have a project car that you haven't touched for a few months, don't know who that would apply Detailed experiments, combined with your subtle humour is what keeps bringing me back! :D

[AdamWebb]

FULL BRIDGE RECTIFIER (been watching too much electroboom)

[Artoconnell]

I have been an Auto Mechanic for 35 years, and I LOVE how you explain things....you would be the worlds BEST service Adviser..been watching your work since we were both much younger. thank you sincerely for what you do here.

[tullgutten]

9:42 that is about the complete oposite of how it really is.

The alternator has a set max voltage.
A bad battery that doesn't take charge anymore draws significantly LESS current to rise the voltage to the max voltage and the alternator won't do much work.
A good battery with low voltage however will draw significant ammount of amps.

An alternator only outputs as much amps as your car and battery can receive.

If a battery is bad it usually take maximum 20 to 30 amps charge if it is low voltage, a good one will easily charge over 70 amps or more.

Work as a full time mechanic and is the head of electrical diagnostics at work.

It's not like you said "a bad battery will take more current to charge"

Still a good video tho

[Madehack]

I imagine that alternators are sized according to all potential use cases, including charging a dead battery. I mean, if you are weekly charging your dead battery, you might kill the alternator prematurely. But on the rare occasion, it should cope fine.

[MrSlashr]

The alternator has the "potential" to charge a dead battery? Good one Jason.

[alexogle]

At the end of your first run, the alternator was supplying ~13A, which tells me that it takes ~13A to run the engine and the car electronics.
At the end of your second run, the alternator was supplying 40A which tells me it was supplying the 13A for the car and still putting 27A into the battery to charge it. Therefore, the battery was still far from charged.
Measuring the open circuit voltage of a battery is a very bad way of assessing it's state of charge.

[JohnAudioTech]

You were charging a drained good battery. A weak old battery would have higher internal resistance and may not draw as much charging current.

[Sandy-oylr]

I keep a battery tender on my sports car's battery while it's sitting through much of the winter. I understood it helps both preserve the battery's maximum life as well as make things a lot easier on the alternator. I've read somewhere a lead acid flooded battery can loose up to 25% of it's life if it's deep discharged even once. And that keeping the battery well charged helps to reduce sulfide buildup on the battery plates. Which can also shorten it's life. Might be interesting to dig more into this....

[louisswanepoel]

The live of electromechanical components (such as alternator) can be directly influenced by its operating temperature.
Generally it would perform exactly the same and for most people it wouldn't be a problem, especially if you sell your car after 5 years or so. But if you want to extract the maximum life out of your alternator you must have a healthy battery.

[sreduos]

Nice to have some fact to back up all these old “assumptions”. Great video!

[AdrianoCasemiro]

Bump start a car on a dead battery is one more reason I love manual transmission. I've been driving for decades and lost count of how many times I had to resort to this trick. Having said that, I never had any alternator problems after driving and charging a dead battery on the alternator alone. Good to see your test gave me some confirmation, though. On an emergency, find a way to start the car, drive it for a while and charge your dead battery. Loved the video, as always.

[FGuilt]

Great video! I guess I'd say the takeaway is, if your alternator can handle the amperage (say up to 100amps) and the OAT isn't super hot, you should be fine to charge a dead battery. temps can messup bearings and electronics so, having to jump the car over and over can eventually cause increased wear and thus, a dead battery that is REALLY dead CAN damage your alternator. A dead battery that isn't REALLY dead is GTG. This was very interesting. Fin.

I don't think anyone has told you this yet but... congratulations for the 4k videos. I really appreciate when youtubers think about the visual quality of their contents.

[NebukedNezzer]

I fixed a friends 85 buick. he burnt up the alternator and replaced it. they checked his battery and said it was ok after it burnt out a warranty replacement alternator he made the judgement that the alternators were no good. I said maybe but not likely. we let his car run idling with the air conditioner on, hood shut, hot day and after about 30 min opened the hood. the alternator was extremely hot and the battery read 11 volts. I took the caps off and said to him. notice all the cells but one are bubbling a lot but one is not bubbling at all. I told him his battery has a cell that shorted only when the battery got hot. he replaced the battery and all was fine. your video does not answer all questions. people still have to use their own brains. I am amazed the vehicle solid state electronics can survive the extreme environment they are put in.

[vBunny]

The Temperature of the diodes would be more interesting in my opinion. The dead(and or dying) alternators I came across (10-15) almost all had diode failures in the rectifier. The ones that end up dead have all failed with a short. Which makes short work of the rest of the diodes ones they get reversed biased by the other coils. I suspect the amount of cooling of the rectifier-bridge is sufficient for a half drained battery but might push them past their limits on "dead" or shorted ones. Early signs I noticed on "older" cars which had incandescent headlights was that once one phase had failed lights would flicker in tune with the alternator rpm later to be followed by complete loss of electrical power.

[nickhill]

Usually, the capacity of a failing lead-acid car battery falls to near zero - in terms of amp-hours, not voltage. It will behave like a much smaller car battery. It will not place a significant load on the alternator.

A failing car battery will rise to a fully charged voltage with little current, and take little charge.

I am surprised your battery took charge at such a fast rate. This could damage the car battery. May be interesting to re-run that test measuring the temperature of the battery.

Slowly charging a dead battery not only helps preserve the alternator, but equally importantly, preserves the battery. You will likely also get a higher capacity in your battery if you slow charge (less than 5 amps), as the charging chemicals can pack much more tightly on battery plates when slow charged. You also save some fuel, which would otherwise be burned to produce the power needed to charge the battery.

[GearheadSchool]

This topic brings back the memory of sitting in a 3-hour NAPA training class over 10 years ago that covered this exact topic. They were having a large number of failures on their rebuilt alternators. After a lot of research, the failures were attributed to overheating of the alternator mainly due to replacing alternators and letting the car leave with a battery in a low state of charge.
An alternator is designed to power all the OEM electrical equipment on the car and to recharge the battery after a normal start, with some extra capacity. Most alternators will overheat and die when they are pushed to 100% output to try and power the car's electrical circuits, plus a few more owner installed accessories and a dead battery.
The simple fix is to use a battery charger to charge the battery before the car is returned to the customer.

[RJ_Cormac]

Most modern alternators are rated 100-140 Amps, more than enough for large entertainment systems and charge a battery. The battery is also used as a smoothing capacitor while running, to reduce electrical spikes from the alternator and vehicle accessories changing electrical demand quickly.

[julianomse]

Here in Brazil we faced in the 90’s some inconveniences of miniaturisation. When cars where fed by carburettors, the alternators had the job basically to replenish the charge of the battery and feed the lamps and the coil, and the currents were under 40 amps. At that time, the rectifier and the voltage controller were outside the alternator. When our cars began to be fuel injected, the current to drive the system led to a sharp increase in the current supplied by the alternators. At the same time, as we live in a warm climate, we began to buy cars with air conditioning - which draws at least 30 more amps from he alternator to drive the fans - and the alternators became more compact, with its rectifiers now on the inside, making difficult to cool down the semiconductors. As a result, since we use the air conditioner all over the year, the stresses over those new alternators were to high to bear: our batteries began to fail with less than 2 years, and within 4 years we had to replace the rectifiers