You don't need to sacrifice a light. You can take the screws out, have a look inside, then put it back together, and continue using it. There is another solar headlight, not shown, which appears to be a sealed unit. The light is not as bright as this model.

The headlight shown here uses an 18650 battery. This is the largest battery in any of the solar headlights I have seen. Other solar headlights, not shown, use a battery similar to a small phone battery.

The tail light shown here uses two button batteries, approximately 10 mm in diameter.