18 hp Briggs and Stratton Starter?

I have remedied a condition that sounds similar to what you describe by making sure all the cables are in good shape and the connections are free of corrosion or anything that can act as an INSULATOR.

Try here and find a location near you and they'll ascertain the issue

Check start relay, start switch and associated wiring for proper operation or errors. Also make certain charging system is working properly if you have not already done so. A slow but constant drain on the battery will kill it in a short time as standard lead-acid batteries will not tolerate even one complete discharge without damage. One possibility is the ignition switch/start switch having moisture in it causing leakage at times.

Pull cable from spark plug, connect amp/meter to positive cable and monitor amp draw as you try to start, check specs for starter and battery. Disconnect or bypass ignition switch check for draw with multi-meter.

I'd check for a ground somewhere draining your battery to the point of ruining it. How old is it? Maybe worn brushes in starter grounding? Check all your connections, ohm to ground with the key off and see if you're reading anything. Should be quite a bit of resistance but don't know what to tell you it should read for sure. Up in the Megohms.

Well let's take a rounder look...if you set a battery on the ground it will discharge. But if set on a block of wood it will hold charge.

Is this battery setting in a plastic tray, upon some shielding/insulation material or resting directly on the frame which may be resting directly on the ground?

I wonder could it be this simple?

bknipfer; your starter gear is not engaging the engine gear, may have broken spring on starter or rusty? perry