I pulled out the rest of my hair trying to understand why my new Ryobi drill battery wasn't accepting full charge. Long story short...
Ryobi one+ 18V battery Model P102 Measuring with a DVM gives lower than correct voltage. Battery acts like ~900K resistor in series.
Vbatt 20.4V
R-load Measured volts
10M 19V meter alone
3.5M 17.48 meter with shunt resistor
1.8M 20.4 meter with shunt resistor
Acts like ~900K in series until it turns on. Wonder why they did that?
So, measuring with a bare 10M input voltmeter gives incorrect indication of the actual state of charge.