For most people, a battery is a mysterious little pack of magic, containing the juice that makes our gadgets flash and bleep (and then run out just when we really need them). For Energizer, it seems, batteries are an opportunity to perform some mild deception: it turns out that inside the chunky, D-sized casing of an Energizer rechargeable is a spacer-clad sub-C cell AA battery offering far less power than you'd expect.
Update: Commenter huskyboy1978 points out that's not an AA battery, but a production-sized sub-C cell.
In fact the Energizer 'D' battery has less than a third of the power, at 2,500mAh, of some rival rechargeables, despite being priced at around 400-percent of the cost of the AA they're based on. What you're paying for, seemingly, is the plastic shell the company wraps the AA in.
Now you can go into conspiracy mode and accuse Energizer of offering poorly performing rechargeables so that they can continue to sell more powerful (but obviously only single use) disposable batteries, or you can say "it's just the same as food packaging" which so often is far bigger than the contents in the hope that buyers will pay more attention to physical bulk than the actual weight of the goods contained. What makes it slightly more dubious is the fact that few mainstream consumers understand mAh, and merely assume that a bigger battery means a more powerful one; yes, Energizer clearly mark the battery with its capacity, but it's hard enough explaining fat percentages on cookies, never mind what "mAh" are.