Clifford,
Nearly all li-ion battery cells are 3.5v nominal. 4.2v is hot off the charger. 16 in series is 56v nominal. Getting capacity up involves putting this string in parallel to increase storage amps.
There is likely bad cells in the pack. A BMS (Battery Management System) can monitor individual cells or packs made up of several cells. Most lithium battery systems have some sort of BMS and balancing feature.
Depending on how the pack is constructed you may be able to look at the actual pack construction and check cell voltages to determine where the issue is.
The other possibility is the battery pack is more damaged. This can happen if the battery pack low voltage cutoff fails and the pack gets totally run down. In that case, the whole battery pack may be ruined.
Given the pack size must be quite large to run AC units, hopefully you can contact VOLTA tech support for guides and assistance.