Piston Slap: Jump-Starting, Charging, and Plumbing Analogies

Jay writes:

Sajeev,

A friend of mine left their car’s lights on, and once she noticed the problem her battery was too drained to start the engine. I had jumper cables with me and used them. After a minute the car was able to start. She asked about concepts pertaining to time and charging rates in this procedure. I didn’t know these answers. Do you?

Obviously, there are many variables, but typically, how fast can an alternator bring back a battery? Is there a noticeable difference in alternator output between idle and higher RPMs? Can an alternator power every car accessory and still put a charge on the battery? How big of a role does the age of the battery play?

Sajeev answers:

Before I try to answer these questions, the best way to explain the concept is by showing how electricity moves from one “space” to another. My go-to reference is plumbing, especially a faucet.

Imagine the dead battery in the recipient’s car is the glass in the sink, while the charging vehicle’s battery and jumper cables are the faucet. The specifications of the electrical system in the charging vehicle and its current load will influence how much “water” is poured out, or the rate of charge it can give the dead vehicle’s battery.

First, let’s maximize the amount of water that reaches the faucet. You should turn every accessory off in the running vehicle to ensure the other battery charges quickly. Turn off your stereo, HVAC system, headlights, and interior lights (close your door while jump-starting!), for the same reason you’d deactivate washers, showers, toilets, etc. to fill a bathtub as quickly as possible. After turning off as much as possible, you are now fully at the mercy of the charging vehicle’s voltage regulator and alternator.

The alternator is like the internal bore size of the faucet, while the voltage regulator is like the aerator at the end of the faucet. (Or maybe the regulator is the handle?) While we may sometimes find aerators annoying, they ensure the right amount of water comes out to fill glasses without spraying or spilling. Your voltage regulator is tasked with a similar job, except it’s trying to prevent the alternator from overcharging, which leads to destroyed batteries, blown fuses, and damaged modules.

With this knowledge, let’s try to answer your questions:

Q: How fast can an alternator bring back a battery?

A: I’m gonna weasel out of answering this question by saying it takes minutes, not seconds or hours. That assumes that the jumper cables, battery terminals/cables, and positive/negative connections are all of high quality. To the best of my recollection, I’ve never spent more than 10 minutes jump-starting a dead battery. Of course, it depends on how “dead” the battery is. If the dead battery in question still powers lights but cannot turn the engine over, it’s gonna recharge quicker than a battery that powers nothing.

Q: Is there a noticeable difference in alternator output between idle and higher RPMs?

A: Almost always yes, as alternators tend to stall out below 1000 rpm. A good rule of thumb is to keep the revs around 2000–3000 rpm to ensure the alternator produces current efficiently for the other vehicle. More engine revs equate to more charging over the same period of time, but that may not be relevant or necessary to charge the dead battery. (And running an engine close to redline certainly seems like a bad idea for the charging vehicle.)

Q: Can an alternator power every car accessory and still put a charge on the battery?

A: Usually yes, otherwise batteries would deplete their reserve capacity and cars would die on the interstate at a terrifying rate. When it comes to jump-starting, it depends on the age, capacity, and condition of the battery, alternator, and the wiring between them. Let’s say you are jump-starting a dead battery with a 2002 Toyota Corolla, which sports an 80 amp alternator and Group 35-sized battery. That Toyota probably will not provide the same amount of current as a 2002 Ford Crown Victoria Police Interceptor with a hearty 200 amp alternator and a larger Group 65 battery, so turning off all accessories will be needed to charge at a rate closer to that of the Crown Vic.

Or not, because this is a thought experiment, not a scientific test. Another variable to consider is the energy consumption rate of your accessories, as audio systems with Apple CarPlay and speakers/subwoofers galore will expend more juice than an AM/FM radio and four small speakers. If you have a modern SUV with tri-zone HVAC controls and a family needing every accessory running while you jump-start a stranded motorist, maybe you do need to rev the motor closer to redline to get the job done.

Q: How big of a role does the age of the battery play?

A: Lead acid batteries do degrade over time, and so do gel batteries. I’ve noticed that modern cars are harder on batteries, especially in extremely hot climates. In terms of jump-starting another battery, however, I doubt this degradation plays a significant role. You’d need that aforementioned SUV with every accessory running to deplete the battery and tax the alternator as it tries to recharge two batteries instead of just one. The bigger variable is the amperage of your alternator and the quality of the wiring between the alternator and the dead battery.

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Whew! That was a heckuva theoretical discussion about something most of us have done but generally neglect to ponder. While it would be fun to test some of these answers out with vehicles in a laboratory-like setting (I wouldn’t mind being around a Crown Vic, twist my arm!) that’s beyond the scope of this Q&A column. But I hope this installment got your thoughts flowing, because I am looking forward to the comments section!

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.com—give us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.