I have a Husqvarna 21 HP Lawn Tractor and I went out to start it and the battery was totally dead, so I jump started it. It started but died out a couple times. So I got it going again and drove it up the hill and came back down and went to cut the grass, pulled the knob out to engage the blades and the Tractor died out. My question is doe's the battery have to be fully charged to engage the blades, and since the battery is so low, is that why it is shutting down when I engage the blades.

Check your charging system as purplewg recommends. Remove the battery, bring it to full charge and take it to an automotive store or battery shop and have them load test the battery to see if it will hold the full charge. Most places will do this free of charge....if your battery is more than two years old it probably needs to be replaced anyway....Autozone carries a really good tractor type battery for about 35 to 40 bucks that has a lot more amp hours power than most batteries you can buy...if you have to replace the battery get one of these....you'll be glad you did !!! I bought one for my personal mower which sees a LOT of use, and it's been in my mower for three and a half years and going strong...Hope this helps...;-D

The electric clutch needs full power in the battery to stay engaged. I would recommend you check your charging system before replacing the battery. Put a digital volt meter across the + and - terminals of the battery and while running at about 1/2 throttle you should be over 13 volts DC. Probably around 13.5 or so. If not you have a charging problem. Lawn mower batteries I have seen seem to last only 2-3 years. I would also recommend a trickle charger for extended periods of non use. Don't forget the Stabil also.

Hope this helps.