You no longer need to own both a battery tester and a battery charger - the TOPDON TB6000Pro does both! The TB6000Pro is able to test your battery ... when the alternator output varies.

Ralph’s simple battery charger is essentially this simple voltage regulator circuit. He chose the voltage-setting resistors R1 and R2 to give him 2.5 V on the TL431 when the output sees 3.6 V ...