18 hp opposed twin discharging the battery a couple of hours after shutdown

Hello all, just bought an old lowes mtd with an 18 hp engine . When charged up the engine starts fine. But looses power on the battery within a couple of hours after shutting down. the amp gauge even shows a slight discharge to left of center after shutting off. When running, the amp gauge looks like it's charging showing about 1/8 of an inch to right of center. I got the mower for 30.00 dollars . had a new battery in it, so I figured the battery was worth the 30 bucks. I put a new carb kit in it and new plugs , replaced the dry rotted gas lines and cleaned out the gas tank and the mower started right up. smooth running machine when running . I have 2 new spindle shafts ordered for the deck. Any ideas where to start first on checking the battery discharge? Lowes number is 138-758-062 / 95198. Engine # is 422707-1251-01 code 940221. would appreciate any input that I could get. thanks

Welcome,
The most common cause of the battery running down when turned off is a duff diode in the stator.
When that happens the stator trys to become an electric motor which it can not do so it turns all of your electricity into heat.

Find the 2 pin plug from the stator and unplug it when you have finished mowing.
IF the problem goes away then the diode can be considered duff.
You test it by poking a pin through the red wire about an inch away from the plug.
Then test for continuity between the pin you put in and the pin on the plug.
You should get continuity in one direction but not in the other because a diode is a one way valve for electricity.
They burn out over time.
Be very careful when you replace it as you can cook the new one with too much heat.
I generally put the replacement a few inches further back as it is easier to solder it there