Engine 721D Grasshopper won't restart after starting

I have a mid 1990's 721D with over 2,000 hours on it. Still mowing, but the last year with a fully charged battery (battery is good... I've had it checked), it won't start unless I use the jumper cables and my truck then if you turn it off, you have to have the jumper cables to restart the mower.

Any ideas on what I need to look into for a fix.

Thanks in advance!

Engine 721D Grasshopper won't restart after starting