I Need A Battery...
Yes, indeed you do - unless you are going to take all your pictures with a Leica M2 or a Hasselblad 500C/M you are going to need some sort of battery - either for a light meter or a motor drive or a digital camera. Nit-pickers may be able to point out that there has been some sort of photo apparatus that ran from a hand-generator or burning olive oil, but they are welcome to the sort of images it might have produced - along with the nits...
There are two very boring pack-shots attached to this post - similar packets, similar colours. Observe, Watson, and think about what you see.
The first picture shows a number of different sizes and shapes of battery - from flat button cells to large torch cells. Each one of these has a place in some photo gear - from flash triggers to cameras to speed lights. The cylindrical lithium cells are needed for many of the quite-decent late model film cameras from major manufacturers. They are real powerhouses in this application - keeping their capacity even if they are not drawn upon for months.
If you are going away on a trip with your vintage gear, why not grab an extra one - you won't be able to find it on a Greek island or up the coast of Alaska, so be prepared.
They generally cost less than the batteries marked with the camera manufacturer's name - I do not believe that they are inferior products. Indeed, the editorial Fuji X-10 is powered alternately by Fuji-marked and aftermarket brand batteries and the pictures look the same - the run life of the aftermarket battery is just as good.
That might seem to be a funny thing to say from a shop that sells the branded battery at a higher price and derives a profit from that, but it is true. Please note that the Promaster batteries come in resealable plastic boxes so they can rattle around the inside of a camera bag safely.
I think that any photographer who is serious about their field work needs to take at least one charged spare with them, and travellers would be wise to take two spares as well as the one in the camera. Wedding shooters carry three spares plus three cards of lithium AA cells.
The business of chargers is another topic. You get one in every camera kit, and if you are a traveller you are expected to lose it in a hotel room somewhere on your journey, so perhaps it would be wise to take a spare universal charger as well.