Just bought a 01 scrambler 500 4x4 rode fine when I bought it but after like an hr lights dimmed and didn't have enough to electric start. Put on charger battery charged up fine
When running only getting 9v to battery.
What should I check first.

 

I would guess the rectifier is bad. it's like the regulator on a car controlling the voltage from the charging system. it might just be a bad or corroded connection. it should be up under the front panel by the radiator. look for a plug in wire harness that goes into a box.

 

Like stendori mentioned usually it's a reg/rectifier either bad or in some cases wires have pulled loose from vibration. If you just bought it I'd load test the battery also to see what shape it's in. No telling how long this has been going on with the previous owner and sucking a battery down constantly can shorten it's life.A battery can show 12 volts,but may not be able to hold this charge even when you replace the regulator if it's needed.