There are several misconceptions in this thread.
1. Volt meters do not put a load on a battery therefore the voltage read does not indicate its charge or its capacity in the case of non rechargeable batteries. A 10Kohm resistor across the battery terminals will provide the maximum load a focus motor or similar will put on the battery. Its normal for batteries to drop a tenth of a volt under load. The color code for a 10 K ohm resistor is brown, black, orange, and the 4th band is the tolerance which is not important for battery testing. Read the colors from the end with the color band closest to the end of the resistor. A 100 K ohm resistor can be used for non AF cameras, its colors are brown, black, yellow.
2. Camera electronics use solid state devices such as diodes, transistors, and IC chips. Solid state devices are current dependent and will operate within a voltage range, usually 5% of the stated nominal voltage.
In post 1 the OP links to a replacement battery he purchased that has a 650mAh rating. In post 3 the linked battery pack uses 1600 mAh batteries. In post 5 the link to the factory original shows 1250 mAh batteries are used and the new batteries to be 650mAh.
Replacement batteries must equal or exceed the factory original in current rating or they will not operate the camera correctly or the operation time will be very short. The replacement batteries must be the same voltage as the originals or modern equivalent.
Rolleiflex USA shows a rebuild service for the batteries using high power NiMH batteries as well as a charger for the batteries.
https://rolleiflex.us/collections/r...l-2-medium-format-slr-cameras-and-accessories
The USA link may not show up in other countries in which case use your favorite search engine for Rolleiflex your country.
A charger rated for lower current than the battery will take longer to charge the battery while one rated for higher current will charge it faster. Heat is a destructive force on batteries. Extended charging times or fast charging may over heat the battery causing damage which will shorten its useful life.
Good luck getting batteries and charger for these beast.