1 Minute per Second Battery Start Rule

[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

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.

I wonder if the age of the battery is a factor? Like a newer battery recharging faster than a older one? June

[abscate]

Yes it is, June

Starters pull about 100 amps on cranking, they are nominally rated at about 1 Horsepower, or 750 watts. At cranking amperage the voltage will drop to about 10 Volts.

I’m guessing the rule might be closer to 1 minute RUN for each second of start.

Don’t forget , on a 1999 on , if you coast your alternator is not outputting, due to overrun clutch

[June]

Lucky for me the T6 version does not use the clutch pulley. June

[850oldschool]

My thinking is that the resting voltage (when the battery has sat idle overnight) in combination with the charging voltage tells you everything you need to know. If the charging voltage is 14 or more, but the resting voltage is 11.8 or less, the battery is getting near the end.

[rguzz]

How do we factor in the potential negatives, idling a car several minutes, especially in cold weather, with nice thick engine oil? Another question altogether I guess. Or did I misunderstand the original question...

[volvolugnut]

I think the better approach (than idle a cold engine just to charge battery) is to try for longer trips when you do start the engine. Sitting without starting for a couple weeks should be OK for a normal battery. As I understand charging systems, at idle with a weak battery, the alternator may not do much charging at idle. That may not be true.
volvolugnut

[scot850]

Alternators do charge at idle, but dynamos/generators did not. Did not know about the 'coasting' feature on alternators though.

Neil.

[850oldschool]

Permanently installed battery tender? Plug in when not driving?

[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.

I wonder if the age of the battery is a factor? Like a newer battery recharging faster than a older one? June

Hi June,

Yes i mentioned that in my longer post just after your first post here.
As the battery ages the charging characteristics change and this is reflected in the charge current. If you measure the charge current with a clamp on ammeter, you can see the current go down as the battery ages because the internal resistance goes up. That's why i like to stress measuring the charge current. You can then get some idea how your battery is doing by comparing it to the measurement when it was brand new, or just keep tabs on it from then on and see how bad it gets.

[MrAl]

Yes it is, June

Starters pull about 100 amps on cranking, they are nominally rated at about 1 Horsepower, or 750 watts. At cranking amperage the voltage will drop to about 10 Volts.

I’m guessing the rule might be closer to 1 minute RUN for each second of start.

Don’t forget , on a 1999 on , if you coast your alternator is not outputting, due to overrun clutch

Hi Steve,

Oh that's all they draw is 100 amps? I guess that is in warm weather?
I was under the impression they drew more than that but i never measured that on this car i only measured the charge current.
Maybe we can lower the 1 minute per second starting rule then to 15 seconds run time for every 1 second starting. I'll have to look into this more. I also did not know there was a clutch on some of these alternators either.

Do you have any direct measurement of the starting current?