Yes thank you that helps a lot! But of course it raises more questions as well. How significant a difference is there between the Panasonic batteries and the UltraFire I've been using? I already have a few of them and ordered a couple more last week. Also I saw a P60 style 6V Xenon replacement dropin advertised as an upgrade. Would it work with these batteries? Would it have less life per battery since it's not an LED and uses 6V, if it does work? Also, is the mode control in the dropin? For some reason I had always thought that was in the tail. I took apart an UltraFire 501 and 502 trying to get a better idea about what you explained. I've got one of each no longer being used because they started giving problems. The 502 got so it wouldn't shut off and the 501 got so it never behaved consistently, sometimes dim, sometimes bright, sometimes strobing, changing at any time...
I would suggest the following:-
1. If you do get additional UF-502b hosts, find the ones with better quality, however, that in itself is a challenge as there are dozens of different types of clones with varying degrees of build quality.
2. Your most cost efficient, next step up from your ultrafire 501/502 would be the Solarforce L2 (with or without a p60 light drop in). If you are hesitant to purchase from their main website in hong kong, I believe there are many CONUS dealers that you can go to. One example off the top of my head would be customlites.com (since I just looked there yesterday). You could also go with a Surefire 6P and bore it out yourself to accept 18650 batteries, or buy one already bored.
3. Panasonic/Sanyo/Samsung 18650 batteries are much better than Ultrafire brand ones, more importantly, they are also
SAFE with protection circuit (if you get those) to prevent overcharging (BOOM!) or overdischarging. Based on my own limited testing, on Ultrafire/...Fire batteries that my friends purchased either at the night market sellers, or through online, or obtained them free with Chinese lights...Their stated capacity is a
LIE. I have tested more than 20 of these and all of them have capacities in the range of 600-1000 while on the label they state it as 3000mah/4000mah etc.
SUMMARY Avoid Ultrafire/Trustfire/...fire batteries (except SureFire) if you can help it, the additional cost spent on quality 18650 batteries go a long way, in terms of REAL capacity and SAFETY.
4. 4. If you want runtime, always go for LED. Xenon or other Incandescent lamp will drain batteries faster (at any input voltages), not saying they are not good, but if you want efficiency, go for LED. There are a good number of manufacturers with very good drop ins with very affordable prices, some examples would be Lumens Factory, Solarforce and for more options you can also look up mountain electronics, customlites.com or malkoff devices.
Hope this helps!