How to figure out battery life- Mtbr.com

# Thread: How to figure out battery life

1. ## How to figure out battery life

What is the formula for figuring out expected battery life?
For instance, if I run my double XML light at 3000mA how long will a 14.8V (4S1P) 2600mAh pack give me on high compared to a 3S2P?  Reply With Quote

2. I convert everything to Watts for this type work. It will get you close but the only true way is to do a run down test.

If your light is wired in series then ~6.4Vf x 3A = 19.2W. I'll round to 20 to sort of account for efficiency.
Your 4S1P battery is 14.8V x 2.6Ah = 38.48Wh. I'll round down to 38.

38 / 20 = 1.9 hour run

A 3S2P battery of the same 2600mA cells would be

11.1V x 5.6Ah = 57.72. Round down to 57

57 / 20 = 2.85 hour run  Reply With Quote

3. Thanks  Reply With Quote

4. I used this calculator when figuring out my battery needs. Again, its a ballpark estimate.
LED Runtime Calculator  Reply With Quote

5. A quick and easy way is to take the current rating of the battery divided by the current draw. in this case it would be 2600mAh/3000ma=0.87 hour. Another way to think of this is a 2600mAh battery can have a current draw of 2600mA over 1 hour.  Reply With Quote

6. Originally Posted by n9tkf A quick and easy way is to take the current rating of the battery divided by the current draw. in this case it would be 2600mAh/3000ma=0.87 hour.
This is totally wrong. Your formula ignores the voltage difference between the battery and LEDs. The OPs battery is 14.4V but the Vf of his LEDs is a bit below 7V. Because of that difference, the draw from the battery will be about half of the LED drive current.  Reply With Quote

7. Ok. Now I know why my batteries last about twice as long as I calculated them to last. Thanks.  Reply With Quote

8. Originally Posted by Vancbiker I convert everything to Watts for this type work. It will get you close but the only true way is to do a run down test.
It's been a while since I've posted here. I agree, on the battery side of the driver, the current is not the same as the led side of the driver, and since the driver is a constant power device, it's better to calculate Watt Hours. Originally Posted by Vancbiker If your light is wired in series then ~6.4Vf x 3A = 19.2W. I'll round to 20 to sort of account for efficiency.
Your 4S1P battery is 14.8V x 2.6Ah = 38.48Wh. I'll round down to 38.

38 / 20 = 1.9 hour run
It's easy to add the efficiency to the Power calculation, and it gives a more conservative result...

~6.4Vf x 3A /0.85 = 22.6W (assuming 85% efficiency, maybe a bit harsh)
Durration=1.7h ..

What runtime did you end up with, n9tkf? Twice than 0.87h ?  Reply With Quote

9. for those who don't want to do the math themselves, the runtime calculator linked in an earlier post does the same calculations - and you can dial in the efficiency where you want it as well.  Reply With Quote

10. Originally Posted by bwack What runtime did you end up with, n9tkf? Twice than 0.87h ?
About twice .87 hrs. I totally misread the the question though thinking that 3000mA would be presented to the battery,not so.

The LED calculator is a good place to start.  Reply With Quote

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