DO NOT TOSS your Dremel Model 770 Type 1 tool just because the nicad battery pack has failed! You can replace the nicads with new ones! Even 'tho the pack is no longer available from Dremel and battery technology makes you want a tool with lithium batteries do not toss this tool. The light weight and easy-to-handle features of this Dremel tool have not been replaced by newer tool models.
When I queried the Dremel tech support team they advised to not attempt a self-repair.
But, a little help from a related Instructables allowed me to open the battery pack. Replacement of the batteries then became obvious.
Resist the temptation to vary from nicad cells since the Dremel Charger No. 758 that came with the tool is designed for nicads only.
Step 1: Open the Battery Pack, 7.2v No. 757
Detach the pack from the tool. There are 4 small tabs which are part of the black case and hold the red battery pack cover. This pic shows 2 of the tabs. All 4 tabs are shown in a later pic. Using a small thin screwdriver or knife blade, gently insert the blade near the tab while pressing on the red finger grip and applying upward pressure. Work around the pack to release all 4 tabs while pressing upward on the red finger grips on the two sides of the pack. Be gentle! If you break a tab, you're in trouble.
Step 2: The Battery Pack Is Now Open
These pics show the open battery pack with the red pack cover removed. Note the 4 tabs on the black cover. Also note the inside of the red pack cover and where those tabs seat when reassembled.
Step 3: More Pics of the Open Pack
Study the orientation of the two 3-cell packs, red pack cover and AA cells. The two 3-cell packs can only be placed in the black case in one direction - so it will be difficult for you to load the packs wrong. Also note the black rubber O-ring inside the red cover. This serves to hold the batteries against the bottom of the pack when assembled. Do not remove the black O-ring. I took pics of the pack at each step of disassembly to help me later.
Step 4: Remove & Replace Two, 3-AA Cell Packs
Study carefully the polarity of the two three-cell packs before you remove them from the black case. Note that the case bottom is marked with polarity. As the two three-cell packs are withdrawn from the black case, note their orientation. A side view of the packs shows how the cells are assembled. Each 3-cell pack provides 3.6v. The switch on the tool delivers power to the tool from one or both packs, thus the 2-speed tool.
When looking at the pack bottom note the silver "buttons" which are the contacts for the tool. These "buttons" may need to be added to the AA batteries by you or your cell supplier. The battery size and pack construction are critical and must be EXACT replacements for the bad ones.
My local Batteries Plus Bulbs store had the AA cells, tools and connector plates to make the replacement packs for me. I left one pack with them to serve as a model. Their price for the packs was very reasonable. Turnaround was one day.
I am now a happier DIYer with one of my most favorite tools performing as good as new!