Common backup battery types for CT power take-off devices

With the rapid development of the Power Line Communication and Smart Grid, current transformer power take-off technology has become the mainstream power supply method for line monitoring equipment due to its advantages of no additional wiring and convenient installation.

However, CT power has a flaw called the 'low current dead zone'. When the line load is extremely low or  power outage, the induced current is insufficient to maintain the operation of the equipment. Thus, a reliable backup battery becomes the key solution to ensure that data is not lost and communication is not disconnected.

Here, let's learn about the common types of backup batteries and their key parameters in CT power harvesting devices.

1. Why does the CT power take-off device have to be equipped with a battery?

The principle of CT energy harvesting:  obtain energy from high-voltage wires throgh electromagnetic induction. Its output power is proportional to the square of the line current.

• When the current is high: the energy is sufficient, even can battery charge.
• When low current or no load: the induced voltage dropping, once there is no battery support, the monitoring device will instantly power off and restart, resulting in loss of historical data and communication interruption.

Therefore, the backup battery is not only a "spare tire", but also a "stabilizer" for the system.

2. Main backup battery options

There are three main battery solutions available on the market to meet the working environment requirements of large temperature differences, difficult maintenance, long lifespan for outdoor power equipment:

A).  Lithium thionyl chloride battery (Li/SOCl2 Battery) - with a long lifespan, it is currently the preferred choice for high-end power monitoring equipment.

• Characteristics: Extremely high energy density and low self discharge rate (annual self discharge<1%).
• Advantages: Wide working temperature range (-55°C~+85°C), floating charging life >10 years, suitable for installation on high-altitude and difficult maintain lines.
• Disadvantage: Weak instantaneous high current discharge capability, usually requiring parallel capacitors to support the instantaneous transmission of wireless modules.

B). Lithium iron phosphate battery (LiFePO4 Battery) -- balance safety and cycling. If the CT power take-off device is frequent charging and discharging mode, this lithium iron phosphate is a excellent choice.

• Features: Long cycle life (over 2000 cycles), high safety, and good high temperature resistance.
• Advantages: Compared to ordinary lithium batteries, it is more resistant to overcharging and over-discharging. compared to conventional combinations (12.8V) and (14.4V),it has a more stable voltage platform (3.2V).
• Disadvantage: The low-temperature performance is relatively weak, and it needs to be used in conjunction with heating circuits in extremely cold areas.

C). Super capacitors (SC) -- Strictly speaking, they are not batteries. They have good instantaneous explosion effects,which often used as auxiliary energy storage in CT power harvesting.

• Features: Extremely fast charging and discharging speed, almost infinite cycle life.
• Advantage: Perfectly solve the high current demand of wireless communication module transmission.
• Disadvantage: Low energy density, unable to maintain device standby for a long time, usually combined with lithium-ion batteries to form a "hybrid energy storage system".

3. How to choose the appropriate battery?

When choosing a backup battery, you cannot only consider the capacity, but also the following four parameters:

A). Voltage platform and cut-off voltage

The CT power module requires a stable DC voltage (such as 12.8V or 14.4V).

Attention: Lithium ion batteries are rated at 3.6V and lithium iron phosphate at 3.2V. When selecting, it is necessary to confirm whether the power management circuit (PMU) supports this voltage range and whether the cut-off voltage will cause the device to shut down abnormally.

B). Capacity

 unit: mAh or Ah.

Calculation formula: Battery capacity>(device average power consumption x Min. power outage time)/ discharge depth.

For example, if the average power consumption of the device is 100mW and it is required to maintain operation for 24 hours after a power outage, considering the conversion efficiency, at least a high-capacity battery pack needs to be equipped.

C). Pulse discharge capability

Power monitoring equipment usually wakes up and reports data at regular intervals. At the moment of GPRS/4G transmission, the current may reach 2A.

Key point: The battery must be able to withstand this pulse without generating excessive voltage sag, otherwise it will cause the device to reset.

D). Wide temperature performance

Power equipment installed outdoors, the temperature inside the box can reach 70°C+ under direct sunlight in summer, and -30 °C in northern in winter.

Requirement: The battery must be able to charge and discharge normally within the range of -40 °C to+75°C, and must not leak or explode at high temperatures.

Application Suggestion: For CT power supply devices, the most ideal solution is often a hybrid mode of "lithium sub battery (lithium iron phosphate)+supercapacitor". Lithium ion batteries are responsible for long-term standby and low current loads, while supercapacitors are responsible for absorbing surge energy from CT and assisting wireless transmission.

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