[JimD]

Harbor Freight sells an inexpensive battery load tester, I think it is about $20. It has a big resister that you can put in circuit with the battery for a few seconds. It is just a voltmeter but the scale is in green, yellow or red to tell you if your battery (or alternator, can be used multiple ways) is good.

Unless the car wash corroded a connection (the timing seems odd) my guess is battery. They do go bad quickly sometimes. That is an excuse for why I am still using a 9 year old battery.