# Thread: My run-time sucks.

1. ## My run-time sucks.

I have a pair of 7.2v 4200mAh NiMH batteries I got from batteryspace. I have them running a single 6v 15w halogen. (one battery is a just a spare) From what I've read here, I calculated that my run-time per battery should be about 2.1 hours, give or take. Instead, I'm only getting about 1.1 hours. Which is totally weak. I've gone through the procedures that came with the battery, and run it through it's cycle 4 times now with no change.

Any ideas?

2. From my understanding of Ohms law by overvolting you are effectively running the bulb at 21.6 watts and drawing 3 amps. At best you will have 1.4 hours until it is completely flat, so 1.1 hours is about right before it drops below a usable level.

Perhaps post how you calculated the 2.1 hour run time.

Tim

3. I'd read that Time = Ah * V / W

4. But your W from Watt is probably wrong because you didn't calculate the effect of the over-volting. 3A that goes to light means a current draw of 1,5A out of each cell of your batterypack. When you draw so much current from a AA cell the capacity will decrease. And that makes the calculated 1.4 hours will be less in the real life.

5. Originally Posted by Finski
I'd read that Time = Ah * V / W
Yes, but as I point out you are effectively running a 21.6 watt bulb because of the overvolting. So the equation is 4.2*7.2/21.6. This equals 1.4 hours.

Tim

6. Well then, explain why I'm getting 2 hours of run time, even better as the weather gets cooler, out of mine. TurboCat 6v, 15W Halogen bulb run off the 7.4v 4800mA pack I got from Batteryspace.

7. Originally Posted by mofoki
Well then, explain...
Are they factoring in the over voltage into the bulb wattage? The math works out to a little over two hours if you use 14W, 7.4V, and 4.8Ah. If i were a company, that's what i'd do...

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