Car Starter Draw relative to Battery Recharge

[UNC201]

Hurt my leg recently so I've taken some time off...
I have the wife swap cars now and then just to get some time/heat/moisture out of it.

Just thinking outloud..
Let's say the starter draws about 100-150 amps to crank.
If the car is not driven, just idles for about 15 minutes.
How much is ~put back~ in the battery.

My car had a nice long drive about 4 days ago.
Since then, probably had 4-5 Short Trips local just for odds/ends.
No issues starting. I did not even take the MM out to check voltage to be frank.
Just wanted to top the battery off.
Was sorta surprised on how low it took from whatever SOC to float
I used my ~chargers~ often enough to have a benchmark idea of how long it would generally take given the same conditions. Today, it took about twice that long

 

[cjcride]

A slow recharge from a battery charger is better for long life than a high amp recharge.
A weak battery from long sitting is best charged by your battery charger as opposed to the alternator.

 

[BigCahuna]

If you have access to power, you can hook up a battery tender to it. It will keep the battery fully charged while it just sits there. So you won't have to play games with a charger, or try to let the alternator bring the battery back up to charge.,,,

 

[The_Jeff]

It's a curve. When your battery is at 85% state-of-charge it absorbs current faster because it has lower internal resistance. So a UPS truck that turns off every 2 minutes probably sits around 85% all the time.

Sure, it's good for a battery to desulfate it by bringing it up to 100% every once in a while, but the difference could be lasting 7 years instead of 6... and people change batteries every interval anyway as "cheap insurance".

 

[UNC201]

I have quick leads in my car. If it going to sit, I'll have the tender on it. Mainly just due to hungry electronics in modern cars.

The OP was more a test/observation of how theoretically, on a fully SOC battery, about 8-10 starts, with maybe 8-10 minutes of max driving on 3-4 sessions, the charging time it took with my charger took almost 2X as long as what it theoretically would took on a similar SOC.
Again, this is not a accurate statement as I did not take any readings....
Just more a seat of the pants observation..

 

[901Memphis]

Sometimes short trips drain more than they put back In, especially with headlights, rear defroster, heaters, wipers and all the electronics on.

It's good to bring them to full often

 

[HerrStig]

You have to put in about twice as much as you draw out. Rough figure.

 

[SHOZ]

For me it is usually 15 minutes of driving at high way speeds. Load makes no difference.

 

[Wheel]

If I remember correctly, an old Popular Mechanics article indicated 14 miles of driving minimum to put back what starting took out of the battery.

That was back in the early 80's - it is likely different now.

 

[SHOZ]

Smaller engines, more efficient starters and easier starting engines are a plus too. I don't think either of my two cars starters draw much more than 40 amps.

 

[Vikas]

It is not the amperes, you have to talk about ampere-hours. If your vehicle starts in 0.5 seconds and the starter takes 150Amps, then you have used 75 Ampere-seconds from the battery. If the alternator is feeding back 10Amps to recharge the battery, then it will need approximately 7.5 seconds to put back those spent electrons

 

[SHOZ]

Originally Posted By: Vikas
It is not the amperes, you have to talk about ampere-hours. If your vehicle starts in 0.5 seconds and the starter takes 150Amps, then you have used 75 Ampere-seconds from the battery. If the alternator is feeding back 10Amps to recharge the battery, then it will need approximately 7.5 seconds to put back those spent electrons
That's true. But the amps are the base. If you get a voltmeter for the car you will see when the voltage backs down after the start.

 

[The_Jeff]

Originally Posted By: Vikas
It is not the amperes, you have to talk about ampere-hours. If your vehicle starts in 0.5 seconds and the starter takes 150Amps, then you have used 75 Ampere-seconds from the battery.


The first "chug" of the starter draws the most amps and it gets easier from there.

On the flipside there are cars that slowly turn on the alternator after the engine catches to "help stabilize the idle."

 

[wrcsixeight]

I have a digital Ammeter that reads and counts amp hours into or out of my AGM battery.

When my Northstar AGM battery is truly 100% charged it requires very little current to be held at nearly any voltage from 13.6 to 15v. Somewhere below 0.05amps.

So when I crank my engine, my ammeter will see -126 amps, but the engine catches very quickly, and I expect amps drawn are likely higher.

When the amperage into the battery falls to 0.0x amps at 14.9v with the engine running, I consider the battery 100% fully charged.

It takes under 30 seconds for the amps to taper from ~30amps to less than 0.05amps at 14.9v, starting with a truly 100% charged AGM battery.


Now if I were to draw this battery to 50% charged, it will get to 80% charged in about an hour of driving, but that last 20% will always take about 4 hours, and that is if the voltage regulator allows mid 14's to be held for those 4 hours.

Fully charging a depleted battery takes time. The alternator is not some instant magical battery charger that the general public believes of it.

 

[TFB1]

Great explanation!!

Again assuming a fully charged battery, only takes a few seconds to maybe a a couple minutes to bring it back to full charge... Also assumes the engine starts quickly within five or six seconds...

BTW today's permanent magnet starters require approx 2/3 the current vs ones of old that used field coils to create a magnetic field... Still I doubt there are many out there that can do the job if not pulling at least 100-125A... Initial surge for maybe ¼ second is likely over 200A...

 

[Vikas]

Quote:
the engine starts quickly within five or six seconds...
I should have made my point more clear. Five or six seconds is eternity when it comes to starting any late model vehicle. Next time, do measure it.

 

[TFB1]

Originally Posted By: Vikas
Quote:
the engine starts quickly within five or six seconds...
I should have made my point more clear. Five or six seconds is eternity when it comes to starting any late model vehicle. Next time, do measure it.

No doubt, most start within two seconds or less...

My point as long as there isn't excessive crank time, there won't be enough charge pulled out of the battery that it takes several minutes to recharge...

I have a '98 Grand Marquis that takes around five seconds to start when temp is approx 45* or less(starts almost instantly in warmer weather)... Been been that way since I bought it in '10 @ 33K mi... Battery replaced, has new plugs, runs perfect, just turned 71K mi, not going to worry about it...

BTW I've tried cycling the ign switch three or four times to build fuel pressure in case it was leaking off, no difference..

 

[The_Jeff]

^ MGMs have a computer that won't allow the car to fire until four complete engine revolutions. IDK why, maybe to build oil pressure.

 

[TFB1]

Originally Posted By: eljefino
^ MGMs have a computer that won't allow the car to fire until four complete engine revolutions. IDK why, maybe to build oil pressure.

The car in question runs a '99 Crown Vic PI computer, not for performance but to get rid of PATs, has since Nov 10...

Oh and it just made a liar out of me, went on a errand a hour or so ago, started instantly at 32*... Must be a issue at between 37* & 45* LOL...