The MJ-872 is a great light but IMO surpassed now, I’d be looking at alternatives such as:
Neninja’s Dual Inton or smaller glow worm units
Currently out of stock but the XML T6 [last model back] or MJ-880 units are available on the bay and else where. I have friends with Neninja’s and MJ-872, and the Inton would be what I’d buy, but only as a bar unit.
Torchy’s Fluxient Triple XML Unknown quality at the moment with potentially huge outputs, but IIRC the MJ-856 was first sold under the fluxient branding. The worry with this is that there won’t be enough heat dissipation, as the dual XML Inton’s get around this by being a huge/heavy heatsink. We await real world reports, but again I’d suggest it would be only useful as a bar light.
Some useful info & beam shots on Torchy’s website
Though for me my next unit will actually probably be a small dual XML unit for helmet unit. Neninja’s is on the link above, Smudge will have one out soon or you can take the risk on a chinese one here.
As for batteries, I’d just run what ever you get with the unit until it dies, the worry about another (or talk to Smudge)