Ok now, before everyone goes running out to slash open battery packs I need to say this.
Some might have something like that in it. That's a cheap way out, a company did an over run on small batteries and did that to get rid of them since they couldn't sell them as is as stand alone, ie the batteries are getting close to their expiration date on the package, so throw them inside something that is NOT guaranteed for years and the customer can't see the date on them.
Most companies though will put D size batteries in there, 4 of them, and cardboard filler. A lantern battery that is made properly, will have 4 F size batteries in them, because the F's are longer and provide more power overall.... You can tell which ones have the full F batteries in them .vs. ones where you are getting ripped and they have D's in them with a cardboard filler by balancing the thing on the edge of a table... or the counter shelf in the store. A 6 volt that has the F's in it .. the better ones will balance about midway through, because the battery pretty much takes up the length of it, one that has D's padded with cardboard, the balance point is going to be off to the side of center quite a bit.
Just a little trick to see if that 'bargain' battery really is a bargain when compared to another one that might cost a buck more but provide twice the power.
aaron