Technical specification overview for the Redflow ZCell / ZBM2 battery

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

Some key characteristics of ZCell are: 

• 48 Volt DC nominal battery (typical operating range 40-58 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:
  • Based on 75A sustained current output
  • Nominal 40V Disconnection point
  • Based on a ZBM2 module at 100% State of Health at room temperature
  • 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)
• 3.0-3.3kW continuous (preferred upper bound)
• 5kW peak output rate 
• Operating electrolyte temperature range of 15-50 degrees Celsius. Most energy efficient operating electrolyte temperature is 20-25 degrees Celcius.
  • Operation at ambient temperatures well 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.
  • For guaranteed 'cold start' for a newly installed battery (or one that is turned off for an extended period) in environments below a 15 C ambient, a conventional cabinet heater may be required (as it routinely is for other telecommunications equipment in such situations).
• 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 the 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. The BMS also logs operating data, provides web-based system performance graphs, and allows for secure remote BMS access via any Internet connection. 

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 that optimal, less than 10kWh may be obtained (again as per the indicative chart below).

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

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.