I would start by buying a volt meter. A multi tester would include volts, ohms, amps and several other measurements, but volts and ohms are what is most often measured. A cheap one will work fine.
Check the volts across the battery terminals with the engine running in neutral. The charging system should be supplying around 13.6 to 14.6volts to the battery depending on the engine speed. If the reading is lower, say 12.6 or less, the charging system is not charging the battery.
Last edited by GoMotor; 07-15-2018 at 12:26 AM.