My John Deer 125 lawn tractor won't start unless I hook it up to the jumper cables. Even after I have it running for long periods, it will occasionally not start up immediately after shutting it down. Last summer it did the same thing, but very rarely. The jumper cables ALWAYS work to start it up under this condition, and so far I have already had to use the jumper cables twice. The battery has a good charge and appears to hold it. Before I bring it to my John Deere dealer and get charged for something I don't need, I was hoping to get an idea of what the problem might be. I do all the services myself and have no problem attempting to fix this problem on my own. Any ideas?

Is this a closed cell battery? If not closed are there some cells that are low or close to dry.
A battery could seem fine except under a load ( a weak cell problem)

Run through this check list. On a side note A battery can be bought for that lawn mower at an auto parts store. They can also test it under load.

1.Check battery for a loose connection. Remove the corrosion on the terminals, if any.

2. Check the ignition switch. Make sure there are no loose connections and check with a test light if the switch is any good.

3. Check battery for charge. It must have at least 12 volts. If battery is dead and you charge it for and hour or two and it still does not have 12 volts, the battery is bad and must be replaced or it has a weak cell.

4. Check the starter. If the battery is good, check the voltage to the starter by taking a volt meter and grounding the black probe to a good ground and touching the red probe to the starter terminal. Turn the key. If all 12 volts are going to the starter and the engine still will not crank, then starter must be repaired. If less than 12 volts are going to the starter, check for exposed areas in the wiring which could cause a partial ground or corrosion. Remove the corrosion, if any, and tape up the wire with electrical tape.

5. Check the starter solenoid. After all the above checks have been done, finally check the starter solenoid. To check the solenoid, remove the wire from spade terminal. With a screw driver, touch the spade terminal and the hot side of the solenoid. If the starter turns, the starter solenoid is good and there is a problem in the safety switches. The solenoid is usually located on the back side of the accessories panel, in the engine compartment, on Riders and Tractors. On Rear Engine Riders, the solenoid is located under the seat or in a separate compartment. Follow the Positive (Red) battery cable to find the solenoid.

6. No Ground. Check the ground cable at the battery and where it connects to the engine or chassis of the power equipment. Replace if necessary cable if necessary.