That's why I have a couple of ZTS computerized battery testers, both in my bailout bag, and just to have around the house, because they take the guesswork and frustration out of so many battery-related problems.
The smaller ones (about $40) test many types of electronic, rechargeable, and plain ol' consumer batteries, plus lots of oddball ones. The bigger testers test more types, but run up to about $150. The biggest ones also have NSN numbers, which means they are listed in the government's online parts catalog, but you may not have enough "pull" to get one, especially in a war zone.
The alkaline batteries in my cordless mouse stop working when the battery charge drops to only 80%. But those same 80% batteries will run a LED flashlight for quite a while, so all is not lost.
In a barter situation, you might like to swap for batteries, but how do you know they'll work for a while, or are they on death's doorstep, regardless of the "good until" date?
With a tester, you'll have a much better idea of battery health, whether for your own peace of mind, or in a trade.
People have already mentioned booze, ammo, TP, tobacco, etc. as being a form of currency. Most of the other items can be checked visually, or with a quick demo, to give an idea of how useful they will be. Batteries, as desirable as they may be, are a bit trickier without one of these.
And consider having a couple of these testers as spare barter goods. If someone "found" a truckload of batteries, what better way to clinch the sale than testing something at random one one of these?
Batteries will be "luxury goods", but people will be more willing to do business if they know they're getting good merchandise. How much enjoyment can you get from your MP3 player if you know the batteries are good for several weeks?
The smallest model is about $40 list, but I'm sure a bit of online shopping can do better. Just remember that the testers use AA batteries themselves, and make sure you have known fresh ones available. For something about the size of a cell phone, here's an item I haven't seen mentioned elsewhere, but it won't weigh down your wallet or your pack, and might just make you the smartest and coolest kid on the block.