Formula to calculate Mah/Whr at different voltages?

[Carnage9270]

Hello all,

Is there a magic formula to calculate run time on a battery at different voltages? For instance, I have a 4S lipo rated at 20,000 mah. I would assume that the run time for that battery is calculated using the battery's voltage (16.8v fully charged). But is there a way to calculate the run time I would get if I was running that pack at 12v instead of 16.8v?

 

[KenD]

The mAh is calculated for the nominal voltage, so that would most likely be 14.8v for that pack. You can convert that to Whr (296 whr), but not sure if the mAh can be converted based on a different voltage at all.

Sent from my M7_PLUS using Tapatalk

 

[Mooch]

Hello all,

Is there a magic formula to calculate run time on a battery at different voltages? For instance, I have a 4S lipo rated at 20,000 mah. I would assume that the run time for that battery is calculated using the battery's voltage (16.8v fully charged). But is there a way to calculate the run time I would get if I was running that pack at 12v instead of 16.8v?

No need to use voltage at all. The capacity number already is the run time.
20Ah = 20A for one hour.

But this assumes zero internal resistance, i.e., a 100% efficient battery, and that doesn't exist. For example, that 20Ah capacity rating was measured at a discharge rate of less than 0.5A. As the current goes up the actual capacity of the battery goes down.

But for comparing one battery to another, it works ok just to use the capacity rating to estimate differences n run time. This assumes roughly comparable batteries in terms of C ratings and internal resistances.

 

[Carnage9270]

No need to use voltage at all. The capacity number already is the run time.
20Ah = 20A for one hour.

But this assumes zero internal resistance, i.e., a 100% efficient battery, and that doesn't exist. For example, that 20Ah capacity rating was measured at a discharge rate of less than 0.5A. As the current goes up the actual capacity of the battery goes down.

But for comparing one battery to another, it works ok just to use the capacity rating to estimate differences n run time. This assumes roughly comparable batteries in terms of C ratings and internal resistances.

Thanks Mooch! Another question though, when they run the tests don't they run it at .5A at nominal voltage? And wouldn't it be different if you ran it at a lower voltage at .5A?

 

[Mooch]

Thanks Mooch! Another question though, when they run the tests don't they run it at .5A at nominal voltage? And wouldn't it be different if you ran it at a lower voltage at .5A?

The battery takes whatever voltage it wants to for the discharge current level being used. You can't select the battery voltage.