What are the technical specifications of a ZCell battery?


The ZCell battery core is a Redflow ZBM2 zinc-bromide flow battery

Some key characteristics of ZCell are: 

  • 48 Volt DC nominal battery (typical operating range 40-57 Volts)
  • Nominal 10kWh energy output per daily discharge cycle (see diagram below)
  • No reserved battery capacity requirement - 100% energy discharge capable with no potential for battery damage
  • No cycle depth limitations - battery performance and lifetime is not sensitive to cycle depth
  • Charge rate limit of 50A per battery (obtained at 57 Volts)
    • Near-linear charge rate from completely empty to 100% full
    • Configure all charger voltage limits/phases identially (flat charging profile at full voltage from 0->100%)
    • Battery blocks further charging automatically when full (embedded self-protection)
    • Configure charger for 56 Volts maximum to obtain a 40A charge current limit (most energy efficient)
    • Configure charger for 57 Volts maximum to obtain a 50A charge current limit (fastest charge time)
    • Battery self-disconnects/self-protects when charge exceeds 50A
  • Discharge rates:
    • 3.0-3.3kW continuous (preferred upper bound)
      • Based on 75A sustained current output
      • Nominal 40V Disconnection point
      • Based on a ZBM2 module at 100% State of Health at room temperature
    • 5kW peak output rate 
      • Duration at this rate before voltage collapse depends upon State of Charge
      • Based on initial 100A output rate (full battery) rising to 125A limit as battery depletes
      • Battery self-disconnects/self-protects above 125A discharge rate
      • Nominal 40V Disconnection Point
      • Sustained discharge rates above 60A are substantially less energy efficient (see diagram below)
  • Operating electrolyte temperature range of 15-50 degrees Celsius. Most energy efficient operating electrolyte temperature is 20-25 degrees Celcius. Operation at ambient temperatures outside of this range (typically 0-55 C) can  be maintained for extended periods due to high electrolyte thermal mass and via automatic temperature regulation with the on-board battery controller. The controller uses a speed-controlled fan to automatically leverage differences between electrolyte and ambient temperature to heat or cool the electrolyte as required - see this FAQ for more details. 
  • On-board battery management, control and monitoring system. Monitors electrolyte fluid and outside ambient temperature, cell voltage, charge/discharge current, and includes two fluid leak detectors.
  • Battery can suspend operations automatically if safe limits are exceeded (to self-protect). Where appropriate, ZCell will return to normal operation automatically when conditions improve.
  • ZCell enclosure includes secondary electrolyte containment to accommodate the unlikely event of electrolyte leak from the ZBM2 battery core.
  • Bundled with ZCell BMS - a WiFi/Web based configuration, control and monitoring product interfacing one or more ZCell batteries to a range of energy inverter/charger/rectifier products. Logs operating data and provides web-based system performance graphs. 

 

The ZBM battery technology exhibits a relatively high internal resistance (circa 0.1 Ohms). This results in output voltage drop of circa 0.1 Volts per Amp of current drawn, compared to the battery Open Circuit Voltage (OCV). For instance, drawing current from the battery at 50 Amps implies a voltage drop during discharge of circa 5 Volts.

 

In terms of Open Circuit Voltage (OCV), typical electrode stack voltages are:

  • 53 V OCV at 100% State of Charge (SOC)
  • 50 V OCV at 25% SOC
  • 40 V OCV at 0% SOC

 

Energy output during a discharge cycle

The 10kWh nominal output energy expectation per discharge cycle for ZCell varies in practice, depending on the rate at which energy is being drawn from the battery and upon the operating temperature of the electrolyte fluid.

Where the battery is discharged in its 'sweet spot' (as per the chart below), and at typical 'room temperature'. it is possible to obtain more than 10kWh per full discharge cycle.

Toward the operating temperature limits and/or at lower or higher discharge rates, less than 10kWh may be obtained (again as per the indicative chart below).

 

Typical Energy - Capacity tradeoff curve for a new ZBM2 operating at room temperature (21 C)

 

Redflow ZBM2 sample energy-discharge curve

 

Example of  battery operation efficiency as a function of electrolyte temperature

Note that electrolyte temperature is controlled toward a target 20-25C temperature automatically by the onboard battery controller. This is achieved through a high electrolyte thermal mass and an automatic temperature optimisation system. This system optimises electrolyte temperature by using differences between ambient and electrolyte temperature opportunistically throughout each daily ambient temperature cycle. 

 

ZBM example battery efficient vs electrolyte temperature

 

More detailed technical specifications of the ZCell battery (based on the underlying Redflow ZBM2 core) are as per the ZBM2 Datasheet [PDF]

More detailed installation, operating and safety information for the underlying ZBM2 is contained in the ZBM2 installation and operation manual [PDF]

 

More detailed information, Understanding the Redflow Battery, ZBM2 White Paper [PDF]

 

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