Three-wire Voltmeter
Since you intend to operate the volt meters through a push button activate switch I would operate these cheap volt meters from a separate power supply such as a local alkaline battery pack. Momentary operation means that these batteries would last a long time. Use in this manner eliminates any concern about the state of the monitored batteries with respect to the meter operation. Such setup would be all similar to if you had a regular DVM that you turned on from time to time and probed your batteries with its leads.