I have learned something about the behavior of lithium polymer batteries, since spending a few years with them now. They take a few charge cycles before optimum performance is reached. You’ll notice that the charge will last longer after a few charge cycles.
Pair this against reality: the rated maximum efficiency of any portable media player is entertaining at best. It’s like mileage comparisons- your mileage may vary, of course. All manufacturers benchmark their devices using a basic audio file, then choose a moderate bit rate, and then let them play along without disturbing them. That’s all fine and dandy, great for advertising and bragging rights, but we all use our devices a little bit differently.
From what I’ve seen, marginal performance is not common. Either your Sansa will fly through the battery in a few hours, meaning that there’s a genuine battery issue, or you’ll get a reasonable service time. Video playback, with its high bit rate and constant-on display, will consume those electrons at a precipitous rate, four or five hours. Audio can run the gamut of consumption, with high bit rate FLAC being the biggest consumer, and audiobooks at 32kbps the lowest.
Any time you wake up the display with a command, the backlight LEDs are active, and they are a big draw, so set your display to a reasonable 15 seconds or less.
Give your battery a chance to “break in”, and I’m sure you’ll see an improvement!