Watch the Charging Voltage.
But it would be easier to use the OBDFusion App to Log the data while you drive.
It is also wise to have this do Freeze Frame and Pxxxx code display.
$30 or less.
Android $3.99 App, interface around $12 for a total of about $16
http://www.amazon.com/Veepeak-Blueto...peak+bluetooth
iProducts $9.99 App, interface around $18 for a total of about $28
Amazon.com: Veepeak Mini WiFi OBD2 OBDII OBD II Scanner Scan Tool Adapter Check Engine Light Diagnostic Trouble Code Reader for iOS iPhone iPad and Android: Automotive