Charging system problem?

I am trying to help out a friend with a 2004 Murray riding mower w/ an 18.5hp B & S. A shop recently replaced the flywheel in an attempt to fix a perceived charging system problem.The battery (it's new) keeps running down. How do I check the charging system? How much voltage should be measured at the battery when the engine is running? This one goes to about 13.5v after it runs a minute or two. I have 20 years experience as an auto mechanic, so I have some electrical ability but I'm not familiar with these. Thanks for any advice.