Battery Test Centre, page-6

The low efficiency reported for the ZCell may have more to do with this from page 6 of the report;

"ITP have worked to incorporate the pack’s maintenance cycles into their cycling regime, but its characteristic requirements fundamentally differ from the lithium-ion batteries."

Perhaps the ZCell is not being cycled properly or the maintenance cycle is being run from near full rather than near empty.

From

https://faq.zcell.com/content/1/42/en-us/what-are-the-challenges-inherent-in-zcell-technology.html
"Periodic self-maintenance cycles

The battery needs to periodically undertake a 100% discharge and to spend a couple of hours offline doing self-maintenance. This process is the secret to the long life and 100% discharge capability that we have - each time this happens, the battery is returned to service pretty much 'like new!'.

In typical home solar self-consumption scenarios this occurs around once a week (depending on the operating duty cycle of the battery). The worst case (if the battery is operating at 100% duty cycle - meaning that it is charging or discharging at a significant rate on a 24x7 basis) the maintenence cycle occurs once per four days.

Where multiple batteries are installed, these maintenance cycles can be automatically controlled to maximise overall system availability by sequencing maintenance cycles in an appropriate manner (and in a timing regime configured and controlled by the BMS). Even with a single battery, in most applications, the BMS makes the maintenance cycle practically invisible by causing it to trigger at the most appropriate point in the daily usage cycle of the energy system concerned (e.g. at a point where the battery is at or near a state of complete discharge anyway).

This self-maintenance cycle, its implications, and its management (via the ZCell BMS) is explained and discussed in much more detail in another FAQ."