How many aa batteries would it take to get decent runtime out of a 5w bulb?

I'd like to see Fenix sell a 5w that uses aa batteries, and I'm curious how many you'd need to get both decent output and a flat 2 hour runtime.

About 4 or 5 Lithium AAs. Slightly less with NiMH.

I think.

Let's make the calculation for 4 NiMH's. 2500mAh*4.8V=12Wh. Let's presume the driver circuit has an average efficacy of 80%, than the output to the LED will be 9.6Wh. That will give a runtime of 1 hour and 55 minutes. That's almost 2 hours

I would guess 6, but that would depend on the bin rating.

chiphead

Well, I'd want the option to use alkies.

alkies would be useless they cant put out the current.

Rifter said:

alkies would be useless they cant put out the current.

Click to expand...

Not even in large numbers?

Raven said:

I'd like to see Fenix sell a 5w that uses aa batteries, and I'm curious how many you'd need to get both decent output and a flat 2 hour runtime.

Click to expand...

According to Sanyo, you can get pretty close to the rated capacity at up to "C" rate.

Their 2.5Ah cell is rated at "2.3Ah minimum" capacity, so we'll calculate around 2.2Ah.

You will need six cells and a buck/boost converter, because the forward voltage of Luxeon V is around 6.5v(average) and six cells in series start off around 7.8v and ends at 5.5-6.0v

Assume 75% efficiency.

5watt/0.75 = 6.67watt

nominal voltage = 7.5v
6.67w/7.5v = 0.89A (increases to 1.21A near the end as battery drops in voltage)

2.2Ah/0.89A =2.47 ~ 2:30 hours. The regulator reduce the LED drive current to 25% or less once battery voltage drops to 1v/cell and shut down at 0.9v/cell to avoid polarity reversal damage to batteries.

8-10 alkaline AAs should work as well, but at the rate of five alkaline batteries per hour, its gonna get kinda expensive...

So, assuming you were using alkies, and if the configuration was 6aa, then you'd need to boost them in order to get anything approaching a decent runtime.

Ok, got it.

I was just checking out an old ad for the Brute flashlight, and wondering whether it could be modded.

Thanks

Raven said:

So, assuming you were using alkies, and if the configuration was 6aa, then you'd need to boost them in order to get anything approaching a decent runtime.

Ok, got it.

I was just checking out an old ad for the Brute flashlight, and wondering whether it could be modded.

Thanks

Click to expand...

No.. you need 8-10 cells in series to keep the current under 500mA if you're gonna use alkalines to get a decent runtime.

Well the Brute uses 7aa batteries, so I guess modding one with a 5w bulb is out of the question.