Why AC can’t Directly Charge Batteries

It is no longer news that only DC can be stored in a battery and not AC. Well, for now, that is how it stands.

Now, there is a reason for that. The reason cuts across the nature of AC, DC, and the battery itself.

So, before we go into the subject matter, let’s briefly look at some terminologies that will strengthen your understanding of the subject matter easier.

What is an AC

AC is an acronym for Alternating Current. It is the current that alternates its direction many times in a second. It can take the form of a sinusoidal (sine), square, triangle, sawtooth, and so on. The figure below shows the waveforms of some form of AC.

AC (Alternating current) isn’t only used to describe current. It is also used to describe electrical signals like voltage and power.

In an AC circuit, electric current and voltage alternate the positive side and negative side of its waveform. You see where the ‘alternating’ comes from right?

Alternating voltage and current periodically go through a cycle (I.e. Positive to negative cycle).

The number of this cycle that is made in 1 second is called Frequency.

For example, If an AC is 50Hz, it means the AC sine wave makes 50 times of the cycle in 1 second. In other words, the voltage or current wave completed a cycle 50 times in 1 second.

So, alternating current moves from positive half-cycle to the negative half cycle, then to the positive half cycle, and so on. That is, at one-half cycle, AC moves in one direction, and in the other half cycle, it moves in the opposite direction. This why alternating current (AC) is referred to as Bidirectional.

Key points: Alternating Current (AC) is the current that alternates its direction many times in a second. It is called bidirectional because it moves, periodically, in two directions. The frequency of Alternating Current (AC) is the number of cycles the AC makes in one second.

What is a DC

DC is an acronym for Direct current. In a DC circuit, the current or voltage does not alternate the positive and negative side of the waveform. Unlike in AC, it remains on one side of the waveform.

So, it doesn’t make any cycle, right? If it doesn’t make any cycle, would it have a frequency? Certainly not, right? It wouldn’t have a frequency. The frequency is zero.

So, in a DC circuit, current goes in one direction. That is why it is called unidirectional. And the frequency of a DC is 0 Hz.

Key points: Direct Current (DC) is the current that flows in only one direction. That is why it’s called unidirectional. And, its frequency is zero.

What is a Battery

A battery is simply a storage device that is used to store electrical energy. It stores electrical energy in the form of chemical energy. This chemical energy can be converted back to electrical energy.

Some Batteries are rechargeable while others aren’t.

Batteries come in various shapes and capacities. But there are two things you should know about them: All batteries have connected cell(s) and the cell(s) have two electrodes.

One of the electrodes is called the anode (negative polarity) while the other is called the Cathode (Positive polarity)

When a battery is sourcing (discharging), the conventional current enters the battery through the Anode (positive terminal) and leaves the battery through the Cathode (positive terminal).

But, when a battery is sinking (Charging), conventional current enters it through the Cathode (positive terminal) and leaves through the Anode (negative terminal).

Key points: A battery stores electrical energy in the form of chemical energy. Basically, it has a cell(s) and two electrodes (Anode and Cathode).

Now, take a look at how DC charges a battery without any problem.

How DC Charges a Battery

As mentioned earlier, DC is unidirectional. That is, it flows in one direction.

For a DC source to be applied to a rechargeable battery, the positive terminal of the DC source is connected to the positive terminal of the battery while the negative terminal of the DC source is connected to the negative terminal of the battery. This forms a complete circuit. And you know what happens to any complete circuit? Yea, current starts flowing.

The conventional current enters the battery through the Cathode of the battery. Thereby, charging the battery. It continues charging because the polarity of a Direct Current remains the same. Unidirectional.

Note: Conventional current is a reference current that is assumed to flow in the direction of a positive charge carrier.

Key points: DC can charge a rechargeable battery because it flows only in one direction

Now, how would it look like when an AC source is applied to a battery?

Well, take a look.

The Reason AC can’t be stored in Batteries

First off, electrical energy is not stored directly in a battery. The battery stores electrical energy in the form of chemical energy. Ok. Let’s proceed to why AC can’t be used to charge a battery.

Imagine that an alternating current (AC) is supplied directly to a rechargeable battery, with the negative terminal of the AC source connected to the negative terminal of the battery and the positive terminal of the AC source connected to the positive terminal of the battery. The current starts flowing. But something goes wrong.

Alternating Current (AC) changes its polarities (i.e. it flows in two directions). That is, in the one-half cycle of the AC waveform, current flows in one direction, but flows in the opposite direction during the other half cycle.

During the positive half cycle of the AC, conventional current flows to the battery through Cathode, and it gets charging. But when the AC gets to the negative half cycle, the current direction reverses, thereby leading to a discharge of the battery. And so on.

In the end, the positive half cycle of the AC that is charging the battery cancels out the negative half cycle that is discharging the battery.  So, the battery does not get charged.

In fact, if the Alternating Current (AC) continues to supply the battery, it could damage it.

To prevent the discharge of the battery caused by the negative half cycle of the AC, the battery terminals would have to be changed as the AC alternates from Positive to Negative half cycle. That is, the battery terminals would have to change as the AC changes. This will have to be done at the same frequency as the AC.

I mean, if the frequency of the AC is 60Hz, the battery terminals would have to be changed 60 times in one second. Do you think this is possible? Not at all. Or probably not yet.

So, because AC changes its direction of flow, thereby leading to charging and discharge of a battery, It can’t charge a battery directly.

Key points: Alternating Current (AC) cannot charge a rechargeable battery because its positive half cycle charges the battery whilst the negative half cycle discharges it. In the end, the battery does not get charged.

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