Simple but reliable car battery tester - Electronics-Lab.com
Arduino - Car Battery Voltage Logger
Amazon.com: Goupchn 100A Battery Monitor 2.4" Ultra Clear Display with Shunt 0-120V Support APP Control Wireless Voltage Current Meter DC Voltmeter Coulometer for RV, Car, Golf Cart, Boat Battery Meter : Everything
Arduino-Controlled 12V Automatic Battery Charger | Full DIY Project