MagiCube, anti-theft battery enclosures, reduce base station energy consumption.

The MagiCube is a patented precast concrete enclosure developed by Poynting to protect base station batteries against theft. It comprises a pre-cast reinforced concrete cube capable of holding up to 28 typical 200Ahr 12V batteries. The MagiCube reinforced concrete walls, roof and floor are about 200m thick with no visible door or opening mechanism providing a high level of security. MagiCube was primarily developed to allow operators to remove batteries out of base station containers thereby not only protecting batteries against theft but also to remove the incentive to damage and break open the containers filled with valuable equipment. Often the additional costs in terms of repairs, vandalism to RF components and lost revenue due to site downtime could be larger than the cost associated with replacing stolen batteries.

One of the real concerns has been that most base-station containers have air conditioners to regulate temperature. These are often set at around 25 degrees C since battery life is compromised above these temperatures as explained below. Poynting was concerned that storing them outside in a concrete enclosure will be detrimental to battery lifetime and we hence focused design to minimise the temperature inside the MagiGube using passive materials and techniques. These turned out to work remarkably well.

Temperature control equipment accounts for around 45% of energy consumption of mobile base stations. The different types of equipment (radio’s, batteries etc) has different temperature requirements, with the battery temperature being very critical to the life and the electrical performance of the battery. When the temperature is not controlled battery damage rate is high, which greatly influences its service life. In some high-temperature areas, the batteries can be replaced every six months.

As example the life of a vented lead acid battery decreases to its half with an increment of 10°C. For a VRLA battery (valve-regulated lead-acid battery), more commonly known as a sealed battery (SLA) or maintenance free battery this is even worse; the life is reduced by 50% when the temperature increases by 8°C.

Temperature Tests

Our solution is a cement enclosure that is painted with a ceramic paint to stop the sun from heating the cube. We recently did tests in Johannesburg, South Africa from the 7th of December to the 1st of January 2016. Note that this period is one of hottest experienced in Johannesburg in recent history.

The data have been measured using 2 temperature monitors and Intelligent Data Loggers. In the graphs below, the measured temperatures next to the batteries is shown in green, and the ambient temperate outside is shown in orange.

MagiCube

As you can see the outside temperatures varied between 15℃ at night to 38℃ during the day, which gives a 23.9℃ average, very close to the measured battery average of 24.0 ℃.

Mathematical model

A mathematical model for the temperature change characteristics of the cube is also given in the report. The implications of having a mathematical model is immense: We can now predict battery temperatures in different locations based on available climatic data for most locations on earth.

The model illustrates the simple principle which is one of the major advantages of the cube:

The average battery temperature will equal the average temperature of the location.

This seems trivial but really is effective since the average temperature (over a short term) at any site is determined by both the minimum (night time) and maximum temperatures. Even in very hot conditions this day/night average is substantially lower than the maximum temperature viewed in isolation. Inside a container the temperature is regulated and roughly remains constant since a typical container contains equipment which generates heat and, even at night, therefore does not cool down. Since batteries in standby mode does not generate heat the fact that they experience both daytime highs and night time lows while their “thermal mass” ensures that they do not heat up or cool down during daily variations, but remain within a degree or two at the average of the two extremes.

Cost saving due to controlled temperature

By moving the batteries into the ceramic painted cube, there are primarily three areas where costs are saved:

  • battery live is prolonged,
  • no temperature control equipment is required
  • and additional to this is the reduced electricity consumption which can be close to R14000 per year (Assuming a 1.2 kW air conditioner, that operates 24 hours a day for 30 days @ R1.60 / kWh)

This cost saving alone justify the additional cost of the concrete enclosure.

 

-Andre Fourie