Is alternator overcharging the battery?

I've searched this forum for an answer, but no luck:  My B&S engine 283H07 0399-E1 has the Dual Circuit alternator wiring.  I put in a new 12V battery, and it started right up.  I checked to see if my charging circuit was working, holding my meter on the pos/neg posts of the battery, engine idling, and it slowly climbed up to 17-plus volts DC.  Is this normal?  Is my alternator overcharging the battery?  There's no regulator, just a diode.  Do I need a new alternator?  Can I disconnect the battery after I start the engine, and let it run off the magneto, without ruining something?  (I'd have to bypass the fuel shut-off solenoid).  I don't want to cook my battery. 

I couldn't track down the fault source, so I went around it.  I went over every connection, conductor and ground; no luck.  So, I removed and capped off the float bowl fuel solenoid, started the engine and removed the battery ground connection.  It ran fine without hesitation or stalling.  I ran the mower around, with the blades, etc under load, and no difference in engine performance.  I'll pick up one of those battery cut-out switches next time in town, and go that route.  Putting the battery on a trickle charger occasionally is no big deal for me; I have another battery for an 'old timey' car I do the same with.    Side note: I've read about this overcharge issue on other websites, and it's not uncommon.  Sadly, the only 'fix' I've seen is one fellow found a dead short somewhere on the harness.  You're right, there are only a few components that can fail, but when they all check out good, it's frustrating.  I suppose I could rip out and re-wire the mower, but I'm not about to do that.  Thanx for the reply!