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?

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

3. Thanks

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

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.

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.

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

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 ?

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.

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.

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