I have a black and decker 20v max string trimmer one year old and it will not remain running when I push the blue slide button. The battery shows fully charged.

SOURCE: CST1000 Black & Decker Cordless Trimmer/Edge won't start

http://www.toolpartsdirect.com/cgi-bin/schematic.cgi/blackdecker/CST2000_TYPE_1

Part number #243614-00.
It's a microswitch made by Honeywell that costs about 5 bucks plus shipping.
The attached file has the schematic and parts list.
The CST2000 is well worth spending a few bucks to keep it going. Finding a local B&D authorized service center near you will even save you the shipping as they can order it for you.
Every other component is pretty dependable- the motor, battery, wiring, charging system, spool system- all built to last.
Only down side is you really only get 20 to 25 minutes of hard weedwhacking before you need to recharge. Of course, it's time for a cold beer in the pool by then.

SOURCE: Black & Decker 8209 Weed Trimmer

I went to the B&D web site and downloaded the manual for this unit.
Here is the link

http://www.blackanddeckerservicenet.com/Products/ProductManual.aspx?q=8209

SOURCE: B & D auto feed string trimmer won't advance string

Is the string spool seated on the rachet correctly? If so and all is well, it is worth noting that these trimmers auto feed fuunction is problematic at best, requiring manually unwinding some line every few minutes as the unit gets older.

SOURCE: Black and Decker gh1000 type 2 string trimmer the

I had the same problem, there is another screw hidden under the black sticker. If you feel around, there is a soft spot. That's it.

SOURCE: batteries run down fast

if you are lucky you will get about 40 mins out of it but it will loose it charge when stored

Testimonial: "This was helpful. I was leaving the charged battery stored.and was only getting about 15 min. I will charge before I use now. charles"