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The impact of harmonics on battery-charging circuits


Battery-charging circuits are used to charge portable batteries from an external power source, mostly AC, which, as we know, are very venerable to harmonics. A simple definition of harmonics is additional noise frequency signals that affect the regular power source.

If the main power supply has a frequency of, say, 60Hz, then there’s a possibility that a small 60Hz noise signal leaks out to the output circuit, in this case our battery charger. The noise signal may be 60Hz or a multiple of 60Hz, from then on referred to as “nth-order harmonics”. For example, 2nd order harmonics is a 120Hz signal, whereas a 3rd order harmonics – 180Hz. Overcoming the harmonics is crucial when designing any power system.

Leaked noise

Here we will explore the impact of any leaked noise frequencies (harmonics) on a typical Li-ion battery-charging circuit; see Figure 1.

Figure 1: A typical Li-ion battery-charging circuit

Here, the charging process starts at a specific DC voltage from a power source, shown as V1. At this stage, there are no harmonics. The circuit starts to charge the load battery V2 at 3.7V; see Figure 2, which shows the relation between V1 and the charging current passing through the load battery, V2.

As the load battery (V2) charges above V1, the charging current that passes through it goes higher, too. This threshold voltage, or “breaking voltage”, represents the minimum voltage required for V1 to start charging the load battery, V2 (in our case, the Li-ion battery).

Figure 2: The relationship between V1 and the charging current passing through the load battery, V2

Experiment and results

In this experiment we assumed that:

  • The harmonics signal is 1V and zero phase shift.
  • The experiment was conducted using a 60Hz harmonics signal and its multiples, to 660Hz.
  • Ambient temperature is assumed fixed.

By observing the breaking voltage at various harmonics levels, there’s a corresponding shift in it, starting at a harmonics frequency of 480Hz, equivalent to the 8th harmonics order.

From 480Hz upward, the breaking voltage goes higher than the nominal 3.7V. This means that the voltage source (V1) needs more voltage to start the charging process when the harmonics signal is at 480Hz or above. In other words, the charging circuit performance will decline linearly at 480Hz and higher-degree harmonics; see Figure 3.

Figure 3: Relation between the applied harmonics and the breaking voltage required at V1

This leads us to conclude that the sample Li-ion battery-charging circuit shows a declining charging performance at 480Hz or higher harmonics. The minimum voltage required as a power source to the circuit V1 has to be higher than 3.7V to allow the circuit to start charging the load battery V2. This finding emphasises the need to use harmonics filters at power supplies connected to the mains, since battery chargers will show lower performance at higher harmonics frequencies.

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