1 Minute per Second Battery Start Rule

[MrAl]

Hello there,

I am posting this to see what other members think of this sort of 'rule'.

I call it the "1 minute per second battery start rule" because it looks like if it takes 1 second to start the car it takes 1 minute of engine idling to recharge the battery. Since it may take 3 seconds to start an engine under normal conditions, that means at least 3 minutes of engine idle time to recharge it. Of course if it takes 5 seconds to start the engine then 5 minutes of engine idle time to recharge it. If you have trouble starting and it takes 10 seconds then that means 10 minutes of engine idle time.

I get this from the nominal 400 amps it may take to run the starter although i did not measure that, and a nominal 15 amps charge from the alternator while running the engine. It could be less or a little more.
If we draw 400 amps from the battery and calculate the ampere seconds for that, then calculate the ampere seconds from an alternator putting out 15 amps, it would take about 53 seconds to recharge the battery for every second that 400 amps was flowing when starting the engine. That takes into account the lead acid charge acceptance which is usually fairly low so i set that at 50 percent.

I havent tested this thoroughly yet but it seems close as far as i did test.

If you have any comments about this please take a minute to post your concerns or suggestions or any ideas about this. Thanks.

[NOTE ADDED LATER]
These estimates are based on nothing else running like the heater or AC or headlights while the engine is running and charging the battery. I will do some more measurements to try to take that into account as many of us will run some other things too while the engine is running.

[scot850]

Al, if you can figure this one out, I would really appreciate it. AS you know Abscate's daughter was having issues with her car, which boiled down to short commutes and not sufficient time for the battery to recover. You may want to consider worst case scenario where it is cold winter day and dark so cabin fan on full, headlights on, rear windshield heater and door mirror heaters on all drawing the 'refill' charge down further, so longer recovery time.

Thanks,

Neil.

[volvolugnut]

A thought:
How about a voltmeter with memory that tracks battery voltage at key position II before each start and indicates the trend. If trends down over a couple days, you know you need to make longer drives or at least idle longer.
volvolugnut

[850oldschool]

The USB charger I use provides a voltage readout, making it easy to monitor charging. I supply the memory!

[volvolugnut]

Self memory works, but not for everyone.
volvolugnut

[scot850]

Now what were we talking about............?

Neil.

[June]

Starting for only a minute or even five could cause lawnmower syndrome? At any rate; if you run the engine long enough to reach normal operating temperature, at least five minutes will have passed? I'm curious what the practical reason for the original question? June

[MrAl]

Al, if you can figure this one out, I would really appreciate it. AS you know Abscate's daughter was having issues with her car, which boiled down to short commutes and not sufficient time for the battery to recover. You may want to consider worst case scenario where it is cold winter day and dark so cabin fan on full, headlights on, rear windshield heater and door mirror heaters on all drawing the 'refill' charge down further, so longer recovery time.

Thanks,

Neil.

Hi Neil,

I am happy you brought this to my attention i had forgotten to place a note in the original post about having other things running during the time the engine is running and recharging the battery. I did add a note to the original post, thank you.

My suggestion would be to measure the battery charge current while all of the other things are running such as the heater, AC, headlights, seat warmer, etc. Whatever is the norm here that should be the conditions under which the charge current is measured. I measured my charge current at 15 amps with nothing else running. Since something else running MAY take some of that current away from the actual battery current during charge, we could measure it in that condition and use that as the charge current instead. In that case the charge current could be less.

The idea then would be to take the 15 amps and divide by the current actually measured. That would give us an adjustment factor that could be applied to estimate the required minimum run time. I will try to do this with my car soon within the next few days i hope.
As an example, say i measure 10 amps with everything i normally run turned on, such as the heater. The factor would then be:
K=15/10=1.5
and that's quite simple to calculate.
Now the adjustment time would be 1.5 times the original time of 1 minute per second of starter run time during the engine start period. That would mean if it took 3 seconds to start the engine the original time would be 3 minutes, but because of the factor K it would have to be 1.5 times that amount, or 4.5 minutes which of course is 4 minutes and 30 seconds. So i should run the car for at least 4.5 minutes then.

There is at least one catch i can think of right off though, and that is the battery cant be really old and the battery must be a type that is recommended for that exact model (and year) car. If it is not, the battery may have a much harder time charging.
The second is that the charging system must be working properly or at least good enough. If it is not charging at 15 amps with nothing else turned on, then you have to calculate a second factor K2 in the same way and apply that factor also. If the battery normally only charges at 10 amps, then you have to apply that factor of 1.5 before any other factor (with stuff turned on). That could make the charge time much longer, even two or three times longer.

What i long suspected about the match between the car and battery i think i have seen proven now. That is that the battery has to be matched to the car or else it may never charge properly. For my car if i use a more modern battery it probably wont charge enough ever, and i think that would happen with the AGM batteries. The one i have now and the one before that were both non AGM batteries and so they seem to charge good enough at 13.8 volts. Many cars charge at a higher voltage like 14.5 volts or so, and they would probably have to have an AGM battery not a non AGM battery. This is important because the different battery types do well with the correct normal charge voltage for each type, which seems to be different.

Another point may be the starter. If the starter draws more than 400 amps then another factor has to be applied. If it draws say 500 amps (cold weather maybe) then another factor would be K3=500/400=5/4=1.25 and that brings the original 3 second time to recharge time up to 4 minutes, and that is before applying the other two factors if needed.
It may be more difficult to measure the starter current while starting but i should be possible if you care to try that. You need a good clamp on meter. One that can catch the surge is better too i think you can do it with just one person then instead of one to start the car and one to try to read the highest reading on the meter as it starts, which could be difficult.

There is another way though too. If you start the car and run it for 5 minutes several times and it under the same run and environment conditions it starts to sound like it is starting slower (engine turning over slower that is) then you have to increase the recharge run time, of course. This takes a little time to figure out but this is probably the ultimate way to tell if it is recharging enough. If it starts to sound slower, you have to increase the charge time, even if you have to sit in the parking lot for 5 more minutes.
If you are lucky enough to be near a power outlet you can always charge with a plug in charger and that would help a lot.

Help this helps and if any other ideas or comments that might help please don't hesitate to add them here, thanks. I think i covered a lot of scenarios but you never know if something else will pop up. I think the most important of all though is to measure the charge current of the battery during a time when you are running everything you would normally run. I dont run anything else because i just sit there in the driveway letting it charge up again.

[MrAl]

A thought:
How about a voltmeter with memory that tracks battery voltage at key position II before each start and indicates the trend. If trends down over a couple days, you know you need to make longer drives or at least idle longer.
volvolugnut

Hi there,

The best way i think is to have a fast responding volt meter connected right to the battery and note the lowest voltage it goes down to while the car engine is turning over during the time you are trying to start the car.
When a battery gets old or does not charge long enough the load voltage goes down more and more over time, and this can be seen during the highest load times and that is when starting the engine. It doesnt take too much memory to remember the lowest voltage every week or so and if you like just right it down or enter it into one of your phone apps.

The main point here though is that you have to have a meter that responds quickly most digital meters probably wont work fast enough. I used an analog panel meter for my measurements in the past as they respond fast. If you really want to get into it, use an oscilloscope. Even a cheap $35 scope will work here, as long as it is connected to the battery not the cigar lighter. You'll get a detailed profile that way and will be able to predict an upcoming dead battery with almost no trouble at all. Might have to be able to store the waveforms though.

Oh is key position II the start position? If so then yes that's the key

[MrAl]

Starting for only a minute or even five could cause lawnmower syndrome? At any rate; if you run the engine long enough to reach normal operating temperature, at least five minutes will have passed? I'm curious what the practical reason for the original question? June

Hi June,

Yes 5 minutes may be enough. I run mine for about 5 minutes when possible and it seems to work fine.
I dont turn anything else on though while idling like heater, ac, lights, etc.
I am going to cut that down to 3 minutes though and see how that goes.