How do you fix a battery problem with my lawn mower because it wont start but it is charged

Have a close look at the blade. It may have jammed up with a stone or stick or something and melted the inside mounting. The blade may be spinning on the shaft and not actually be turning by the motor now.

Probably bent the shaft. Disconnect the spark plug to prevent accidental starting and lay the lawn mower on its side. Slowly turn the blade with your hand and watch the center of the shaft. If it wobbles at all it is bent. $150 to $200 job. Probably time for a new mower.

Check cable that is not loose when you adjust power or check that there's nothing rapped around the blades could decrease the power it's supposed to give when blade is spinning

The batteries are defective in this unit. It should be returned to the dealer that you purchased it from and get exchanged, or your money back. Another way to test this mower is to charge the batteries fully, then test them. If they are truly fully charged, run the mower until it slows down, and test the batteries again. If they are still charged, (unlikely) then the problem is in the motor itself and still should be returned.

does the blade spin freely if not bearings going out check make sure blade brake is fully releasing the blade to spin you will probably have to take it in it could be in the activation swicth but you need special tools to check it

You haven't given enough information of the problem. The blades themselves wont turn when you try to spin them by hand or they wont turn when you engage them with the lever or the switch?

i have seen this problem before. your going to want to replace the battery with a bigger one. when i baught mine my battery was to small

check your crank case for fuel,
bowls stick open an fills it up
if this is the case , you can buy a plastic fuel cut valve from lawn mower shop
todays fuel isn't very good
also try sta-bil fuel coditioner

hav you tried charging the battery

It sounds to me that you need a new mower deck drive belt, or your blades need sharpening. If the blade edges are rounded, they won't cut butter... Also, when mower is off, check mower deck drive pulleys to make sure you don't have any that are locked up or are dragging. You can check this by pulling the belt away from pulley and spinning the pulley by hand. MOWER MUST BE OFF before attempting any of these checks. You would be surprised how many people would try this with the mower running.
Hope this helps