I have a14.5 h.p. briggs and stratton riding mower . Ihave changed the battery,starter, silonoid and diode .the battery will not hold a charge .it has 15 to16 volts going to the key switch but from there nothing .could the switch be bad .if so how do you test it . I have to charge battery before every use . Please help me

Have to run real quick, 3 things come to mind, Bad Battery, Not Charging, or a Drain/short of some kind. Charging can be seen with a voltage increase when running. A drain can be seen with ampmeter or a bulb placed between the battery terminal and the battery cable.
If the bulb is lit, you have a drain, if it is off, the short/drain is cleared.

Try charging the battery, let it sit for a full day and then turn the key. If its dead you either have a short or a dead battery. Disconnect the negative from the battery and then lightly tap it back on the terminal, see any sparking?

Hi,
You have a short somewhere, sounds to me. Disconnect battery leads, negative first, and check the resistance between the leads. Should read infinity. My guess is you will read about a 1 k ohm, about the resistance it takes to run a battery dead in about a week. Do not leave the battery in a discharged state or it will quickly go bad.
Tell me what you find. If I guessed right, please provide the model info. Oh, you say you changed the diode; put the stripe on the correct end?
Peace,
Clarke