hahah, the Ultrafire has never ever 4000mAh
= Use google, but I mean, this has in reallity ony 1600mAh (look at the rating at the end of the side
" my other 3000mAh 18650 battery last for almost twice as long, than these."
The biggest existing Cell is from Panasonic with 3600mAh build into Tessla Electric car.
If I would be you, I would use LiFePo4 Batterie, because it's much easier to charge and absolutly safe!! like this
**broken link removed**
or smaller from A123, but nearly the same price...
This you can charge with 3.6Volts, but nothing happend, if you charge it to 4V, if you go higher there is only smoke at about 4.2Volt
You can it charge with 3.4V too to 100%!!! Charge Level