Please help with run-time calculation

[Paul Therrien]

Ok, here is what I need to know...

I have a battery pack that contains 4 nimh AA's running at 2600mah. So we have a 2.4 volt / 5400mah pack.

I have a micro puck 350ma driver running a single 1watt led. FV when driven at 350ma is 3.2.

I believe my run time is low which seems to be hitting about 7 hours before dimming out. I was trying to calculate run time at first by simply dividing the 350ma the driver will pull plus the voltage boosting etc. That seemed to come out to about 450ma draw. But that would not make sense since I am running out of power at 7 hours. 450 x 7 only equals 3150mah which is well below the battery packs ability. So I should get a few additional hours more then I am.

So, for one, am I calculating this wrong? I need to figure out the actual amount of draw off the battery in mah to know if the batteries I have are not doing the trick. Or am I just not understanding this right and regardless of all this driver stuff it is a one watt led drawing more power then I know.

Thanks for you help!

Paul

 

[Mr Happy]

If 2.4 V is being boosted to 3.2 V, the minimum current draw from the battery pack will be:

350 x 3.2/2.4 = 470 mA

Now we need to allow for inefficiencies, so conservatively we can apply another 0.8 factor:

470 / 0.8 = 590 mA

Your pack capacity is nominally 2 x 2600 = 5200 mAh; however you don't say what specific batteries they are. Since 2600 mAh cells do not always measure at 2600 mAh we should de-rate them a bit, say to 2400 mAh. That would give you a pack capacity of 4800 mAh.

Therefore we might estimate the run time at:

4800 / 590 = 8 hours

Apparently you are not even getting that much. Your next steps would be to measure the actual current draw between batteries and driver using a meter, and to measure the actual operating capacity of your AA cells using something like the Maha C9000 charger. That will start to give you a better picture of where the shortfall is coming from.

My guess is that your 2600 mAh cells are coming in somewhat under 2600 mAh in practice due to age, poor quality, self-discharge, incomplete charging or some other problem.

 

[LuxLuthor]

Paul, Mr. Happy has you covered in terms of doing the numbers...but one of the most overlooked things is the quality and brand advertising the 2600mAh capacity, even at low drain rates. I'm sure you thought of that, but some are notoriously bad and/or dishonest...for example the Energizer 2500mAh's.