Not sure how you measure the charging? If you are just measuring the voltage, this will tell you very little about the actual battery charging.

The only thing that can tell you about the battery charging is an ammeter. Current is charging a battery, not the voltage, this is why battery capacity is given in Amp-hours. You can have high voltage and little to no current at the same time.

If you expect big charging current you can use an ammeter with a shunt. There is also hand-held clamp on ammeters available.