Re: sell by?
"Why is it that the big name brands put a "Sell by" date of ten years into the future? I don't know the internal resistance of these batteries but that does seem a bit optimistic. Any battery boffins out there?"
The "shelf life", is part of a cells' (or battery's) specification that defines the length of time that will need to pass, at the nominated temperature, that the cell (or battery) will lose 10% of its capacity.
Typically, with good name brands, you're currently looking at 2 years for carbons, 8 years for alkalines, and about 15 years with lithiums. From memory, going back in time, Carbons were 6 months, and alkalines were about 5 years - so their battery chemistry fine tuning is certainly getting better.
Of course, for this "Shelf Life figure" to be useful, you need the date of manufacture - and this is where some FUD comes in. Sometimes I've seen cells with the month and year of manufacture stamped on the side, sometimes there is only a "Best By" date into the future. This means very little, partly because we don't know if it actually is the shelf target date, and party because if it's a fudged date, it's meaningless. Personally, in long term applications (blackout lighting, remotes etc) I use a labeler to mark the purchase date of said battery, and gamble it it wasn't very long between manufacture and sale. Usually, it's reasonably short.
The "sell by" date is a little more complex, and ironcially, much more arbitrary. This is a date set by a committee group, that juggles how long they can hold onto a product, and it still be in "salable quality", as defined by either some law or (in the case of batteries) how long you can get away with without the customer finding out. In real life, a battery sell-by date of 10 years will never *actually* be met, because it's simply not the type of product that would hang around that long.