What are the Best rechargable C cells for a 9w LED torch?

Hi Guys,
Having purchased a 9w Tektite LS5 torch that uses 3x C cells, I decided to buy a set of rechargeables for it to save some money long term as it ate its supplied set in two dives and the performance was falling off real fast after only 10 mins.

So, I chose the "Lloytron Ultra" that offers 3000mah.

Those however, whilst better than alkaline, still fall off after about 20 mins so i find that ideally, I need to take a "set per dive" with me, and even then, I only get full performance for 1/2 a dive. Rubbish.

I am not looking at some 5000MAH Powerex ones and some 6000mah Extreme ones.

What are you guys using in yours? I really need some advice before I blow anymore money on these batteries!

9W/4.5v = 2A. That's a chunk of current.
If 3 A-h gives you 0.3 hour and you want 0.7 hour you roughly need 3 x .7/.3 = 7 A-h. I guess weight is not a factor in this application.

Bone up on this for the concept of apparent battery capacity being dependent the rate of charge withdrawal.
http://batteryuniversity.com/learn/article/calculating_the_battery_runtime
This battery sizing has to work on paper before it will work in the real world.

It turns out to be not so easy


at a 20 hour rate a 2 A-h battery gives 0.1 A
and at 2A it gives 0.3 hour
what is k?
If b = t/H and a = C/IH then k = log b/log a
0.3 =t in hours
20 =H in hours
2 =C in A-h
2 =I, discharge current in amps

-1.823908741 =log b
-1.301029996 =log a
1.401895995 =k

How long will the battery last at
3 amps?
t = H{(C/IH)^k}
0.033333333 =C/IH
0.008496296 =(C/IH)^k
0.169925918 =t in hours
so the apparent capacity of this 2 A-h at 3A is about 0.17 x 3 = 0.5 A-h.

The trick is get the manuf. to tell you how long you can pull some number of amps from his battery. Then you figure k using the rated values and then you can predict apparent capacity at any current drain.