How many miles does it take to charge a battery?
Number_Cruncher
Active Member
Taking perhaps pessimistic values, you might draw 400 Amps for 15 seconds while starting the car.
Charge = Current * Time
= 400 *15
=1600 Coulombs
Then, during running, if the alternator can supply 10 Amps more than is being used to power the vehicle's systems
Time = Charge / Current
= 1600 / 10
= 160 seconds
So, being pessimistic again, you should be somewhere near restoring the charge after 10 minutes or so. Of course, the numbers may vary for your car, but, I don't think they are many miles out!
Charge = Current * Time
= 400 *15
=1600 Coulombs
Then, during running, if the alternator can supply 10 Amps more than is being used to power the vehicle's systems
Time = Charge / Current
= 1600 / 10
= 160 seconds
So, being pessimistic again, you should be somewhere near restoring the charge after 10 minutes or so. Of course, the numbers may vary for your car, but, I don't think they are many miles out!
I think someone worked it out to be around 30 mins of town driving or 15 mins on motorway... thats average of course... so many variables
Dieselman
Banned
- Joined
- Jul 13, 2003
- Messages
- 34,199
- Car
- Peugeot 403 Convertible
Using your example where does the 10 minutes come from, 160 seconds is ~2.5 minutes.
Nice working out, I can't remember the last time anyone refered to coulombs.
You can do the same calculation in Amp/hrs.
400A for 15 secs is 400 x 15/3600 = 1.666 Ah.
1.666 Ah divided by 10A charge current gives 0.1666 hrs ... 10 minutes.
400A for 15 secs is 400 x 15/3600 = 1.666 Ah.
1.666 Ah divided by 10A charge current gives 0.1666 hrs ... 10 minutes.
Number_Cruncher
Active Member
Yes, I was thinking along the lines that the charging probably wouldn't be constant at 10 Amps, and would probably have something like a first order lag, with more charging straight after the engine starts, and a long tail with the charge going in more slowly - so, I was pessimistic, and rounded up by a factor of 4 or so.
All the numbers in the calc are just estimates, and I think the only thing we can say is that it takes a small number of minutes - not seconds, and not hours. A rough order of magnitude estimate would perhaps be a better desciption of my post.
All the numbers in the calc are just estimates, and I think the only thing we can say is that it takes a small number of minutes - not seconds, and not hours. A rough order of magnitude estimate would perhaps be a better desciption of my post.
shadowninja
Active Member
- Joined
- Jan 27, 2007
- Messages
- 240
I don't think it's anything to do with distance. Once the engine is turning, it's charging the battery therefore 30 minutes stationary or flying at 150mph will do it. I am happy to be corrected by an auto-electrician.
MBManInKen
MB Enthusiast
Wrong track, I'm afraid. What matters most is not the maximum current that the alternator can provide (current is drawn from the circuit), but the voltage difference between the output of the alternator and that of the battery. It's a bit like two connected buckets and you need to pour electricity from one bucket into the other. If both are at the same voltage level, no current flows - like two connected buckets with water at the same level.
When the voltage difference is larger, the current drawn from the circuit will be larger too (Ohm's law gives the basic principle, although as a circuit it's more complicated than just a power supply and a resistor). So, in its initial charging stage, the battery will draw a lot more current than at the later stage were it has taken on more charge. Again, to use the water analogy, given that the pipe connecting the buckets remains of the same size, the way to put more water from one bucket into another is to increase the water pressure, pushing more of it through the pipe in a given time segment. But you can't increase the pressure beyond what the source can supply, so the maximum current that the battery draws will be limited by the output the alternator is able to provide.
The maximum current the alternator can provide depends obviously on its capacity, but also on its age and its rpm. It will need a certain level of rpms to deliver its maximum current.
If very flat, a battery would draw a lot more current than the alternator could provide, so the charge rate is limited by what the alternator can provide, but as the charge is topped up, that drops significantly and while the voltage difference exists (i.e. water levels between the buckets are different), the battery will take on charge.
Battery age, state of the electrolyte, ambient temperature are all additional factors in the calculation.
Number_Cruncher
Active Member
>>It's a bit like two connected buckets and you need to pour electricity from one bucket into the other.
Yes, that's the first order lag I mention in post #6, the effect is included, although I didn't talk about it - my posts are usually confusing enough!
I said 10 Amps again as a pessimistic average - alternators on MBs are rated much higher than this.
As I have said, the calc gives us an estimate, we can say that the charging process isn't completed in seconds, and it doesn't drag on for hours.
Yes, that's the first order lag I mention in post #6, the effect is included, although I didn't talk about it - my posts are usually confusing enough!
I said 10 Amps again as a pessimistic average - alternators on MBs are rated much higher than this.
As I have said, the calc gives us an estimate, we can say that the charging process isn't completed in seconds, and it doesn't drag on for hours.
popuptoaster
Active Member
From experience i'd say a minimum of 20 mins driving (not sitting in traffic) if you dont use any of the electrics, more if you have to have stuff on.
bigbobs cl
Active Member
I would say none of the above!! You can`t completley charge a battery by driving, give it a good trickle charge overnight is the best way.
Bob
Bob
Why can you not completely charge a battery whilst driving? Surely this would imply the need to trickle charge all batteries which I never do on cars that are used regularly - it is only the ones used little that need to be charged.
Number_Cruncher
Active Member
>>I would say none of the above!!
I would disagree with that. For my own cars, I have never needed to charge a battery - even on a car that was parked up for weeks at a time.
The alternator is designed with the intention of charging the battery.
I would disagree with that. For my own cars, I have never needed to charge a battery - even on a car that was parked up for weeks at a time.
The alternator is designed with the intention of charging the battery.
MBManInKen
MB Enthusiast
Not quite, actually. The alternator and the charging system are designed to maintain the battery charge, not to charge a fairly empty battery. That might sound like nit picking, but it is an important difference.
Charging a battery is tricky business. You don't want to overcharge it, so the system regulates the voltage and current on the safe side. The battery should be kept close to, but not quite at its maximum charge.
bigbobs cl
Active Member
Sorry i was assuming we were talking about a flat battery! If you keep driving and using the car then yes the battery will remain topped up but from flat a trickle charge is the only way to get a battery completley charged.
Bob
popuptoaster
Active Member
rubbish, i run old bangers all the time and have cars in storage for weeks at a time, i have often had to jump start a car that wont even light the dash lights cos the battery is so dead, but as long as the battery hasnt died from being flattened and the charging system is good then normal driving will fully charge a battery, i usually go for a 20 min drive to be able to restart the car, or a good hour of driving around to know the car wil then be useable for short journeys as well as longer ones.
Its all the crap using up power in a modern car that require them to be trickle charged while parked up for a few weeks, given a chance the alternator will run the whole car AND charge the battery untill it reaches peak voltage at the same time.
Its all the crap using up power in a modern car that require them to be trickle charged while parked up for a few weeks, given a chance the alternator will run the whole car AND charge the battery untill it reaches peak voltage at the same time.
Number_Cruncher
Active Member
>>not to charge a fairly empty battery.
The OP wasn't asking about flat batteries, he was asking what happens after the car is started.
The OP wasn't asking about flat batteries, he was asking what happens after the car is started.
MBManInKen
MB Enthusiast
Quite a few errors in that statement...
You won't take my word for it, I'm sure, but perhaps someone else's might do?
A fairly modern circuit should at least be designed to prevent a massive current peak when you fire up the engine with a dead battery connected, but it's certainly not great for its longevity to keep doing that kind of thing.
More to the point, sulfation will start to build up in a discharged battery and that will seriously restrict its life and could very well make it suddenly fail completely and without warning. If depleted, trickle charging is the better way to recharge in order to avoid sulfation.
Old bangers are generally less fussy about jump starting etc, but on a modern car you could easily damage the electronics.
popuptoaster
Active Member
Who's Mike Pardee whan he's at home and what are his qualifications? i have no official qualifications but i have been mucking about with cars for 30 years and whatever the science is, i have on plenty of occasions put flat batteries on cars with known good alterators and then jump started them as it happens to be the most conveniant way to charge a battery at the time.
I know you shouldn't jump many modern cars and it may well shorten the life of a battery if you mistreat it, but it was just an example to show that an alternator will charge a battery right up, whater ever mr wassisname on the "unofficial honda acura" site says, the only time i have ever killed an alternator by accident is when i left one conncected when i struck an arc with a MIG, i have run all sorts of split charging systems with multiple batteries and mulitiple alternators on various cars due to my habit of running large ICE installs or dirty great lorry batteries and hydraulic pumps in the boot to power the suspension system in my lowriders both of whcih are perfectly capable of flattening batteries in hours (or even minutes if you get carried away bouncing the car up and down)
I know you shouldn't jump many modern cars and it may well shorten the life of a battery if you mistreat it, but it was just an example to show that an alternator will charge a battery right up, whater ever mr wassisname on the "unofficial honda acura" site says, the only time i have ever killed an alternator by accident is when i left one conncected when i struck an arc with a MIG, i have run all sorts of split charging systems with multiple batteries and mulitiple alternators on various cars due to my habit of running large ICE installs or dirty great lorry batteries and hydraulic pumps in the boot to power the suspension system in my lowriders both of whcih are perfectly capable of flattening batteries in hours (or even minutes if you get carried away bouncing the car up and down)
MBManInKen
MB Enthusiast
Too right! Sod the laws of physics too, what has that Newton bloke ever done for cars, anyway! 
