How long to charge a battery?

[wdfifteen]

I tired to google this but couldn't formulate the question well enough to get an answer.

Assume you have a modern car, V-8 engine in good shape, starts easily, charging system working, battery in good condition - every thing optimum. How many miles/minutes would you have to operate the car to replace the energy removed from the battery by starting the engine?

[Mrmerlin]

It would be best to charge the battery with a charger capable of 6 amps charge the battery for 2 hours every other month if the battery has been left disconnected.

To gauge how much time it will take the alternator to replenish the charge will be harder to measure as you will need a specific gravity to measure acid density.
Though a volt meter may also be used as a guide.
Generally the older the battery to lower its recovery performance will be.


If your having issues with a battery it will usually let you know its on the way out.

The reason to use a 6 amp charger is so you can fully charge the battery,
the car charging system may only replenish a partial amount of charge

[wdfifteen]

Originally Posted by Mrmerlin

Generally the older the battery to lower its recovery performance will be.

If your having issues with a battery it will usually let you know its on the way out.

That's basically the question. How would it let you know? By not fully charging in 5 miles, 10 miles, 20 miles? How many miles will it take to bring a good battery in a properly operating vehicle to replace the charge depleted by starting the engine? Knowing that I can determine if the battery is on the way out.

[Scott Douglas]

This should be a simple math question (but it's a lot higher math than I can deal with this early in the morning) but there are a lot of variables in play, like how many amps is the alternator/generator able to put out, how many amps does it take to run the car etc etc. I suspect there are a lot easier ways to determine if a battery is on its way out, like getting a load test done at the local PEP Boys.
But if I were guessing, I'd say a normal car should be able to recover the power in about 10 miles at highway speeds. Remember it's really an amps x time equation.

[aschen]

back of napkin:

starting a car 150A for 5 seconds: is around 0.2 W hours (not much)

Lets say the average charging rate for the cars system is around 10 amps: recovery time is around 75 seconds

These numbers are illustrative, but I think the point is that a normal start hardly depletes the battery at all.

[rusnak]

I was going to say 2-3 minutes at max. But there are other questions of course such as how many tries to get it to start, amperage of starter, and whether the battery even holds charge.

[wdfifteen]

Originally Posted by aschen

back of napkin:

starting a car 150A for 5 seconds: is around 0.2 W hours (not much)

Lets say the average charging rate for the cars system is around 10 amps: recovery time is around 75 seconds

These numbers are illustrative, but I think the point is that a normal start hardly depletes the battery at all.

This is what I was looking for. Thinking back as far as I can remember:

Start, drive 1 mile - shut off
Start, drive 4 miles - shut off
let sit overnight
Start, drive 3 miles - shut off
Start, drive 5 miles - shut off
Start, drive 4 miles - shut off
Start, drive 100 ft - shut off
Wouldn't start anymore.
I was wondering if I was asking too much from the battery or if it should still have enough charge to crank the engine. Looks like the answer is the battery is on its way out.

[rick-l]

Originally Posted by aschen

back of napkin:

starting a car 150A for 5 seconds: is around 0.2 W hours (not much)

Lets say the average charging rate for the cars system is around 10 amps: recovery time is around 75 seconds

These numbers are illustrative, but I think the point is that a normal start hardly depletes the battery at all.

Batteries are rated in Amps * Hours (typically about 90).
Using the numbers above 150 Amps * 5 Sec / 3600 Sec per hour = 0.208 AH
A typical alternator puts out 60 to 100 Amps. 60 Amps * Hours = 0.208 AH so Hours = .0035 or 12.5 seconds.

Something does not seem right?????? The first second it will draw much more current when the starter is not turning very fast but even if it doubles the time that is not much.

[aschen]

batteries are rated in amp hours and voltage. The actual capacity of the battery in energy units is in Watt hours.........90AH X 12V for your example.

I used 10 amps to be conservative. An 80 amp alternator is typically not charging the batt at 80 unless its heavily loaded and the engine is at high rpm.

[T77911S]

Originally Posted by wdfifteen

That's basically the question. How would it let you know? By not fully charging in 5 miles, 10 miles, 20 miles? How many miles will it take to bring a good battery in a properly operating vehicle to replace the charge depleted by starting the engine? Knowing that I can determine if the battery is on the way out.

just take it to one these parts stores that load tests them for FREEEEE. (adam sandlers voice).

you jsut cant tell with batteries no matter what you do.
i have a 40 battery UPS at work that puts out 480v. i do tests on all the betteries every 3 months. i check internal cell resistance, voltage and load voltage.
the best check is to put the UPS on load and watch for any of thebattery voltage to drop more than the others. i have seen batteries with good cell resistance fail under load. even though i have a fancy $10k tester, the load test tells me more, but in the end, thay can fail at any time.
i have had 1 battery take the entire string down.



car batteries are rated in hundreds of cranking amps. like 700+
you dont know what the starter+solenoid requires to start so you dont know the drain on the battery. if your battery went dead after the starting sequence above, i would suspect a bad battery. but first i would start with connections and charging, also depends on the age of the battery.

as a former auto electric guy, one of the biggest over looked problems on cars is the grounding circuit. people will clean the positve and ususally neg posts on the battery but the rest of the connections are over looked.
the negative connection to the body needs to be cleaned, the body to engine connections need to be cleaned, even connections on the alt and alt to ground. then you have the positive side. clean the connection at the starter.
all these things should be done before testing or replacing anything charging/starting related.

just as a note. i know a lot here like the optima, they are now made in mexico and are crap.
i just put cheap parts store batteries in my car. these days the batteries just are not what they use to be.

[rick-l]

Originally Posted by aschen

The actual capacity of the battery in energy units is in Watt hours.........90AH X 12V for your example.

So if I go into Autozone and ask for a 1 kilo Watt Hour battery they will know what I need? How come the published figure is Amp Hours?

[T77911S]

car batteries are in cranking amps.

your deep cycle batteries like for UPS's are in amp hours.

they are designed to put out power for a long time as where a car battery puts out a lot of power for a short time. thats why for one, the red top optima, is not a good car battery. it is a deep cycle battery. its great for something like an electric fishing motor.

[Rusty Heap]

you let the smoke out of the wires, didn't you...

[porwolf]

Originally Posted by wdfifteen

That's basically the question. How would it let you know? By not fully charging in 5 miles, 10 miles, 20 miles? How many miles will it take to bring a good battery in a properly operating vehicle to replace the charge depleted by starting the engine? Knowing that I can determine if the battery is on the way out.

For a long time now I have been working with the rough formula of, to replace battery energy used for starting a cold engine, not deep winter cold, is about 1/2 hour of driving time. Seems to work.

[aschen]

Originally Posted by rick-l

So if I go into Autozone and ask for a 1 kilo Watt Hour battery they will know what I need? How come the published figure is Amp Hours?

Because people at auto-zone arnt great at high school physics? More likely is that all batteries in question are at 12v nominal so the charge capacity is directly proportional to the energy storage.

If you want to do an energy balance back of napkin calculation it is more rigorous to do it in energy than amp hours as energy is conserved but current is not.

Practically speaking none of this matters as everything in a car is at 12v

[notmytarga]

I was looking for a nice net article that described the importance of "state of charge".
In order for a "12 Volt" battery to be 100% charged it needs to be at 12.7 volts. At 12 volts it is only 25% charged. The alternator output, voltage regulator and battery all have to be working and adjusted to make the system effective and reliable. I have ended up replacing 2 out of the three before really fixing the problem. But batteries are the most likely problem and are temporary service items at best. I have swapped out a DuraLast Gold from AutoZone for free several times on my 2007 Sienna because up in Redding it bakes under a hood in 110F heat all summer.


State of Charge/Specific Gravity/Voltage/12V/6V
100% 1.265 12.7 6.3
75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0

[rick-l]

Originally Posted by notmytarga

I was looking for a nice net article that described the importance of "state of charge".
In order for a "12 Volt" battery to be 100% charged it needs to be at 12.7 volts. At 12 volts it is only 25% charged. The alternator output, voltage regulator and battery all have to be working and adjusted to make the system effective and reliable. I have ended up replacing 2 out of the three before really fixing the problem. But batteries are the most likely problem and are temporary service items at best. I have swapped out a DuraLast Gold from AutoZone for free several times on my 2007 Sienna because up in Redding it bakes under a hood in 110F heat all summer.


State of Charge/Specific Gravity/Voltage/12V/6V
100% 1.265 12.7 6.3
75% 1.225 12.4 6.2
50% 1.190 12.2 6.1
25% 1.155 12.0 6.0
Discharged 1.120 11.9 6.0

Lead acid batteries also have an effect called surface charge where the chemical reaction is concentrated on the surface of the plates and hasn't diffused into the lead. This makes everyone say my battery fully charged measures 13.5 volts.

I could be wrong and have overlooked something but in my experience all the car batteries I have looked at have two ratings:

1. Amp hour capacity
2. Cold Cranking Amps

[rick-l]

Originally Posted by aschen

If you want to do an energy balance back of napkin calculation it is more rigorous to do it in energy than amp hours as energy is conserved but current is not.

If you assume 12 volts you can easily go between Watts and Amp hours.

I recall (I don't remember where) car batteries were rated in amp hours because it was more like the battery would be used. I believe starter batteries are rated over 10 hours i.e. to be rated a 90 Amp Hour battery it would have to maintain a discharge rate of 9 amps for ten hours.

[nota]

starters are surge type draw

why no capacitor like a fan or a/c uses

[rick-l]

Originally Posted by aschen

back of napkin:
starting a car 150A for 5 seconds: is around 0.2 W hours (not much)
Lets say the average charging rate for the cars system is around 10 amps: recovery time is around 75 seconds
These numbers are illustrative, but I think the point is that a normal start hardly depletes the battery at all.

I was wondering why our numbers didn’t agree.
Discharge ( 150 Amps * 12 Volts)Watts * ( 5 Seconds/3600 seconds per hour)hours = 2.5 Watt Hours.
Charge ( 60 Amps * 12 Volts) = 720 Watts 2.5 Watt Hours / 720 Watts * 3600 seconds per hour = 12.5 seconds