For around $5 you would think the battery in an Oral B Pulsar toothbrush would last a little longer. Here is how I overcame that problem.
Step 1: The tools that I used
These are the tools I used. A pair each of vice grips, adjustable pliers, and small needle nose pliers (the smaller the better)
Step 2: The brush
This is what the toothbrush looks like.
Step 3: The seperation
Here you can see the dividing line near the center of the brush where the battery compartment comes apart.
Step 4: The grip
Clamp the vice grips on the upper portion then using the adjustables on the lower portion twist counter-clockwise. Some brushes are more difficult than others to break the seal. Screw off the cover.
Step 5: The guts
This is the separated battery compartment.
Step 6: The removal
Remove the battery by prying the metal contact out of the way and pulling upward on the battery.
Step 7: The replacement
Insert the new battery.
Step 8: Making contact
Bend the contact as close to it's original position as possible without actually contacting the battery.
Step 9: The modification
Many posts on the web on this subject suggest playing around with the contact to try and get the switch to work properly. Personally I didn't have the patience. The following works just as well for on/off operation in my opinion and is a lot less hassle.
Bend the tab on the contact backwards and squeeze it down as close as you can get it to the contact. By doing this you will effectively remove the on off switch from being usable.
Step 10: The "ON position
Screw the cover back on. When fully tightened the brush should start up.
Step 11: The "Off" position
To shut off brush simply unscrew cap a quarter or half turn.