Chargestar Scenario2018-09-17T20:35:07+08:00

What follows is a scenario designed to demonstrate the issues that need to be considered by developers when choosing an electric vehicle charging station solution for large scale apartment developments.


The new Central Park development will have 400 parking spaces of which 40 will be reserved for electric vehicles. The power requirement for 40 7 kW single phase charging stations operating concurrently is 280 kW. However, the Development only has 50 kW spare during the day and 100 kW spare capacity during the night. The charging station solution must provide billing and load management facilities.

Electric Vehicle Charging Solution.

Isabella bought a brand new apartment in the Central Park development and was delighted to find an electric vehicle charging station in her allocated parking space ( Isabella bought a Renault Zoe ( which has a 40 kWh battery pack and a range of 300 km at Freeway speeds. Isabella drives around 50 km per day. The Zoe requires requires a top up of 10 kWh overnight except on the occasions when Isabella arrives back from a weekend in Margaret River with around 10 kWh left in the battery pack. On those nights the Zoe requires a charge of 30 kWh to fully recharge the battery pack. Isabella likes to have a fully charged battery pack in the mornings in case she has to make unexpected trips during the day.

Isabella’s charging station is rated to 7 kW. The Renault Zoe has an on-board AC charger rated to 22 kW which can charge the 40 kWh battery pack in around two hours. The Zoe communicates with Isabella’s charging station and detects that the station is rated to 7 kW. The car sets its charging rate accordingly and will fully charge from flat in around six hours. That’s fine because Isabella only drives around 50 km per day and the car charges over night.

The charging stations are connected to the THOR dynamic load balancing system ( which matches the amount of power the charging stations can draw with the spare power capacity of the building. The building normally has a spare capacity of 50 kW during the day and a 100 kW after 10pm at night. This was sufficient when Isabella moved in because she was the only occupant with an electric car and she was able to charge at 7 kW all the time. However, now there are 40 electric cars in the parking area and all the EV chargers are in use after five pm. THOR dials down the available power for each station to 1.25 kW until 10pm at night. The occupants in the complex start going to bed after 10pm and the available power in the building increases from 50 kW to 100 kW. There is 2.5 kW of power available for each charging station from 10pm until 6am when the building occupants get up for work and the spare capacity in the building reduces to 50 kW.

Isabella is happy with the current arrangement because the Zoe can still get its 10 kWh overnight at a rate of 1.25 kWh per hour. But, Strata Management have informed the occupants that they intend to install another two hundred charging stations in the car park and that the overnight charging rate will be 0.42 kWh per hour when the 240 charging stations are operating concurrently. A car charging for eight hours between 10pm and 6:00am at 0.42 kWh per hour will draw 3.36 kWh. This is not enough to provide the Zoe with the 10 kWh it needs to fully recharge. Isabella realises that the battery in the Zoe will deplete during the week unless she gets a chance to charge somewhere else such as at the new DC fast charger which has just been installed at the servo across the road. However, Isabella would prefer to get a full charge overnight as the power in the complex is much cheaper than the $0.50 cents per kWh charged by the Servo. Strata Management have informed the owners that it will cost half a million dollars to upgrade the grid connection so that electric vehicles can fully charge overnight. The owners will have to weigh up the expense of a grid upgrade against the cost of charging at commercial DC fast charging stations.

Charge Star Billing & Access Control.

The developers specified an embedded SATEC sub-metering system ( for the apartment complex and it was decided to connect the initial 40 charging stations to the sub metering system. Each charging station has its own circuit with a dedicated meter that records the power being used by the circuit. The SATEC meter is connected to the sub-metering system back office which records the data and generates the invoices which are then sent out to the apartment occupants. The occupants receive a combined bill with two line items, one for the apartment and one for the charging station. Access control is provided by Charge Star ( The charging stations are connected to the Charge Star network The owners are issued with Charge Star RFID cards or alternatively RFID cards provided by Strata Management. The ID of the RFID card given to the occupant is programmed into the charging station assigned to the occupant. Only that RFID card can operate that particular charging station.

Strata Management intend to install another two hundred charging stations. However, the cost of retrofitting meters to the circuits and connecting them to the sub-metering system is prohibitive. The meters alone cost around $600 each and labour costs are very expensive. Strata Management have decided to use Charge Star for billing as well as access control ( The new charging stations will have on-board MID certified meters which are compliant with NMI standards. The stations will be connected to Charge Star. Charging sessions will be activated by using the NEXTCHARGE smart phone application ( and/or the NEXTCHARGER RFID card ordered through the application. NEXTCHARGE will record details of the charging sessions and bill the user’s credit card stored on the smart phone application. No access control is required as anyone using a charging station is billed for the power. There is no need to assign a charging station to a particular occupant. Charge Star reimburses the charging session fees to the charging station provider periodically. The occupants who will use the two hundred new charging stations will not receive a combined bill because they will pay per charging session. Receipts are stored on the occupant’s NEXTCHARGE smart phone application and are also sent via email to the occupant’s email address.

System Installation.

The initial requirement is for 40 charging stations.

  • Install 40 wall mounted networkable Volt single phase charging stations rated to 7 kW.
  • Connect the Volt charging stations via CAT 5 network cables back to a network switch.
  • Install the Circontrol current clamp on the building input circuit.
  • Connect the Circontrol current clamp to the Circontrol CVM-MINI power analyser.
  • Connect the CVM-MINI power analyser to the TCP2RS+ RS485 (Modbus RTU) to Ethernet (Modbus TCP) converter.
  • Connect the TCP2RS+ to the network router/switch.
  • Connect the switch to a router with SIM card capability. The router is connected to the Charge Star VPN via the SIM card for remote monitoring.
  • Configure THOR for dynamic load monitoring.
    • Example. The building has 50 kW available. Two cars charging get 7 kW each. Forty cars charging get 1.25 kW each.
    • Example. The building has 100 kW available. Two cars charging get 7 kW each. Forty cars charging get 2.5 kW each.