I bought the BC40 and have used it with some of the cheaper batteries with varied success. I bought good cells from Callie Kustoms, AW and Redilast and these have worked perfectly and are the best. I have decided to not use these batteries in the BC40 as I have other lights that I like better and I'm using these more expensive quality batteries in these lights that I use more often. Of the less expensive batteries that I have used, the Trustfire 2400 batteries with the black and red label with the flames on the side have worked the best. I have recently bought some Tenergy 2600 batteries that are proving to be good cells but they are flat top batteries and will not work in a two cell light. Otherwise I would be using these batteries in this light. I also have two Jetbeam 2300mAh batteries that have been satisfactory but I don't find them any better than the Trustfire batteries that are less expensive. I will not buy any Ultrafire batteries again, especially the 3000mAh ones which are the worst I have tried.
BTW. I just ordered 4 of the Trustfire 2400 flame batteries for about $17 delivered total for backups. I also bought 4 protected Tenergy 2600 and 4 Tenergy 2600 unprotected batteries and I'm really liking these. Like I said, the problem is the flat top but they work great in my one cell lights. These have been the best deal I have come across as far as cost to value. A 4 pack is only $14.99 delivered. Can't beat that price for the quality IMHO.