[SOLVED] Please verify simple battery power calculation discharge

[Zak28]

To find how long battery will power a circuit a 12v 3000mAH battery discharging @ 300mW/s - battery power is 12*3000*3.6=129600 Joules
battery can discharge for 1129600 / 300 = 3765 seconds
or 62.75 minutes. Is this simple calculation correct?

 

[FvM]

No. I presume you mean discharging with 300 mW or 0.3W. 300 mW/s is no meaningful quantity.

12V*3Ah = 36 Wh
36 Wh/0.3 W = 120h

 

[Zak28]

...discharging with 300 mW/s is no meaningful quantity...

You divided watt hours by watt seconds. Im pretty sure the calculation I did is correct because its unlikely to last 120h the results being from your calculation.

 

[FvM]

You divided watt hours by watt seconds.

There are no "watt seconds" specified in your original post. Just meaningless mW/s. I concluded that you have 300 mW continuous load. If so, there's no time quantity in the load specification, just plain mW or W. Respectively the battery energy content has to divided by the load power to get the discharge time.

You should better specify the problem consistently, and may be read a bit about electrical units in the meantime.

 

[KlausST]

Hi,

Im pretty sure the calculation I did is correct

So why did you ask, then?

BTW: I fully agree with FvM.

Klaus

 

[albbg]

To find how long battery will power a circuit a 12v 3000mAH battery discharging @ 300mW/s - battery power is 12*3000*3.6=129600 Joules
battery can discharge for 1129600 / 300 = 3765 seconds
or 62.75 minutes. Is this simple calculation correct?

129600 Joules is the energy stored in the battery, not the power.

129600J/0.3W = 432000 s = 120 h the same result of FvM.

As already pointed out by FvM, 300 W/s is a really meaningless quantity.

In any case you did the passage from W*h to joules, then you converted back the result seconds to hours. These passages are useless (you multiplied and then divided by 3600).

 

[bobTPH]

In your original calculation:

12*3000*3.6=129600 Joules

Is correct even though the two terms 3000, and 3.6 are both incorrect but in opposite directions.

You then calculate:

1129600 / 300 = 3765 seconds

Note the extra 1 in front of the total energy calculated in the first line.
Note also that you are dividing by 300, not 0.3. 300 is mW which is 0.3W

That is why your calculation is incorrect.

Bob

 

[Zak28]

So why did you ask, then?

Wanted verification and it turns out I was wrong, I find it shocking the battery will last ~120 hours. I dodn't believe Im coming off here rude or anything similar at least I hope I didn't.

Note the extra 1 in front of...
Note also that you are dividing by 300, not 0.3. 300 is mW which is 0.3W...

Typo but I am really glad to be mistaken in this case. Thanks for the overview.