The battery can be replaced, but this is for devices that have aged beyond the warranty period, 1 year in the US, and 2 years in the EU.
Replacing the battery does require disassembling the Clip or Clip+, and does involve carefully prying the face of the device free from the back. The internal battery is a lithium-polymer unit, 290mAh for the Clip+, and 330mAh for the Clip. I cannot stress strongly enough that this type of battery requires caution when working with it! LiPo batteries are extremely reactive and can catch fire if damaged or pierced, irritated, or taunted.
When prying the back free from the device, a short plastic blade as used for disassembling mobile phones is best. The wedge is moved gradually around the perimeter to free the back. One must be very careful that the tool is not inserted too far into the gap, as the battery “pouch” is very fragile.
There are three wires connected to the battery, two are power, and the third is for battery monitoring. MOST of the time, in cases where there is no power, one of the two power leads has broken free from the circuit board. This situation is most common with the original Clip versus the newer Clip+; the battery would break free from its little adhesive pad, allowing movement.
Replacement battery units are out there on the internet, of similar size (Radio Shack even has one for the original Fuze as a kit). If you have a damaged Clip, the battery can be swapped from the broken device. Again, it’s all about electronics repair skills and patience.
Before committing to opening up a Sansa Clip, be sure that the device is operational when plugged in to a source of power.
The Sansa will not charge if the processor is “stuck” or “frozen” as in a corrupted file. In this case, it will not charge, a condition that has a safety benefit. The processor of the Clip manages the battery. If it has encountered a problem, charging is interrupted. In this case, the Sansa appears dead. Do a soft reset to see if the device will come to life. With the Clip, slide the power switch to the spring loaded ON position, holding it there for 20-30 seconds, then try powering up as normal. For the Clip+, hold the power button depressed for 20-30 seconds to reset.