# Time taken for battery to drain completely

Discussion in 'Power Electronics' started by rajat, Aug 27, 2012.

1. ### rajat

16
0
Jun 3, 2012
Dear All,

Battery : 24 volt 8000 mAh

Motor : 24 volts 350 watt

Can anyone show me formula to calculate the drain out time of this battery ?

I am naive so apologies if it is very simple question or if I am missing some information.

Many thanks

1,114
159
Aug 13, 2011
350W / 24V = 14.6A

8Ah / 14.6A = 0.5h

3. ### CocaCola

3,635
5
Apr 7, 2012
Edit bad math in original post of mine...

And KJ6EAD has beat me to the calculations, mind you that is in a perfect world, and we don't live in a perfect world...

I would speculate run times are more like 15-20 minutes in the real world, possibly less...

Last edited: Aug 27, 2012

1,114
159
Aug 13, 2011
Factoring in the high (almost 2C) rate, some unknown discharge curve for the battery, the unknown load and the probability of "optimistic" battery rating, I'd guess even less, maybe 5-10 minutes, but I didn't want to confuse the issue with too many facts so I presented only the basic ideal formulas.

Last edited: Aug 27, 2012
5. ### CocaCola

3,635
5
Apr 7, 2012
True, it really depends on the battery chemistry and battery quality... I was actually going to post 10-15 minutes originally (and mention the high drain) but gave the benefit of the doubt in the final post...

16
0
Jun 3, 2012

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
It depends on how you use the device

I have a 24V 10Ah battery and a 200W (nominal) motor.

If I use the motor continually, and in a way where it is under moderate to high load, it will flatten the battery in about 45 minutes of use.

The problem with electric motors is that their rating does not tell you the minimum, maximum, or even average power consumed.

I know, for instance, if the motor is stalled, it will dissipate more than 200W (and the same if operating under a very heavy load). This is also not particularly good for it.

However, if running freely, it dissipates far less than 200W.

If the use is a motor on a bicycle, you typically see high power demand at low speed, reducing as speed increases. At higher speeds (unless geared in some way) the motor is likely to have far lower torque and consume far less than its rated power.

I will add that on my bike, the motor is connected via the gears, and I am able to keep it in a low-RPM, high torque, high power region. This is not true of hub motors. As has been stated, a lot has to do with your battery pack. I have a Lithium (LiFe I think) pack and it gives very good capacity at high discharge rates.