Simple battery charging circuit. On/Off Switched Float Charger

Simple battery charging circuit

This is a Smart Battery Charger, with fully automatic 12V battery charge indicator. It is an extremely simple charger that anyone with little experience can assemble.

The input voltage depends on the power supply you are using, 110Vac or 220Vac. The charging time depends on the power supply you are using and the type of battery you are charging.

To calculate the closest charging time, we can do a simple and objective quick calculation that does not take into account the variations of the resistance factors of the battery, the variations of the charger, the chemical depreciation factor of the battery and so on.

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The calculation is quite simple. Let us take an example: you have a UPS 12 volt battery with 7 amps, your power supply is 3 amps.

So we know that the battery charge is more or less 7 Amps Hour, which means that to fully charge the battery, 7 constant amps are required. Since the power supply is 3 amps, we need to divide 7 amps by 3 amps of power supply.

To charge a battery without damaging it usually requires a current of 10% and a maximum of 20% of its current. For example:

If our battery is 7 amps, the most we need is a 1.4 amp charger, and normal charging consumption is 700 milliamps. This 10% is normally used in devices that are directly connected to the power, such as alarm systems, backup devices and others.


Charge C lasts 2:33 hours

Two hours and thirty-three minutes to charge

If your PS is different, no problem, look at its supply current and put in the formula to see the approximate result.

Figure 2 shows the complete electrical schematic of the small circuit.

It is important that the power supply delivers 20% more than the battery voltage, e.g. if your battery is 12 V, the power supply must be 14,4 V. You can vary a little, e.g. 13.2 V, which is 10% of the battery voltage, but you cannot use a 12 V power supply to charge a 12 V battery as there will be no potential difference.

Here is how to use the charger:

When all the assembly is done, carefully check for wrong parts, reverse polarity diodes and shorts in the terminals after checking everything.

Connect the positive terminal of the power supply to the input of the Vcc circuit and the negative terminal of the power supply to the ground circuit of the charger.

With the potentiometer or trimpot you can regulate the output voltage of the charger, for example you have a 12V battery, normally 12V batteries UPS are charged at 13.2V at 14.4 volts.

Then use a multimeter on the DC volt scale, it depends on the multimeter, on the output of the charger and set the voltage to the most desired, ie the maximum voltage for it to fire and the green LED will light up.

You can now use your new charger, insert the battery and let it charge until the charger reaches the voltage you set, limit voltage, it will trigger the relay and turn on the green LED, indicating that the battery has been charged.

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A good tip: If you have a spare 12v buzzer, or if you really want to add an audible indicator to your circuit, you can connect these buzzers, which you can easily find in electronics stores and are cheap, to the output where the green LED is lit, which is the full charge indicator, and you can turn on the positive buzzer on the relay output and the negative buzzer directly on the output.

Material List:

We offer for download the necessary materials for those who want to assemble with PCI. Printed Circuit Board, the files in PNG, PDF and GERBER files for those who want to send for printing.

Need for Battery Charger relative to Power Electronics?

The need for low power battery charger systems has shown a considerable increase over time due to the fact that the use of portable appliances and communication equipment is rapidly increasing with time. And so, charging mobile devices has emerged as a challenge and to deal with this challenge, battery chargers are used.

Now, you must be thinking about how a battery charger can supply power to a battery.

simple, battery, charging, circuit, switched

In general, a battery charger supplies the electric current to the battery so that the cells within the battery can store the energy which is getting passed through it. For a battery, there are basically two types of charging modes.

The first is fast charging, which is applied to new or unused batteries. While the other is floating charging, which is applied to in-service batteries where providing supply to the load is necessary for compensation of the small charge which the battery losses during its service period.

SCR based Battery Charger

An SCR-based battery charger makes use of the switching principle of the thyristor in order to get the specific output. The circuit includes a transformer, rectifier, and control circuit as its major elements.

As we have already discussed in the beginning that a small amount of ac or dc input voltage is needed for the purpose of charging the battery. So, the elements of the circuit help to provide the desired voltage to charge the battery.

Working of Battery Charger circuit using SCR

The figure below represents the circuit of a battery charger incorporating an SCR:

Here, an ac voltage signal of value 230 V, 50 Hz is applied as input and the load is a 12 V battery that is required to be charged.

Following are the circuit elements:

  • AC supply
  • Step down transformer
  • Rectifier circuit
  • SCR
  • Zener diode as a voltage regulator
  • Battery to be charged

Let us now understand how the above-given circuit operates.

So, initially, 230 V ac supply is provided to the step-down transformer which converts the high voltage given at the input of the primary winding into a low voltage which is obtained at the output of the secondary winding. So, here the voltage obtained at the other side of the transformer is 15V with respect to neutral.

From the circuit, it is clearly shown that the transformer forms connection with the rectifier circuit, hence the output of the transformer will be provided to the rectifier circuit. As we have an ac input signal, so let us understand how the rectifier circuit operates when the two halves of the ac signal are applied.

Initially, when the positive half of the ac input signal is applied then the diode D1 in the above-given configuration will be forward biased and will conduct however, D2 will be in reverse biased condition thus will not conduct. Conversely, when the negative half of ac input is applied then D1 will not conduct but D2 will be in conducting state, this is clearly shown in the waveform representation given below:

So, the rectifier circuit will provide rectified output i.e., the dc voltage at terminal P.

Here we have used a Zener diode with breakdown voltage of 10 V as a voltage regulator to regulate the voltage level of the circuit. Therefore, terminal Q will be at 10 V due to the presence of the Zener diode.

As the terminal voltage at P which is nothing but the rectified voltage is comparatively more than at terminal Q thus, this makes the SCR forward biased, allowing it to conduct and due to this current will start flowing through the load i.e., the 12 V battery. And we have already discussed in the beginning that when current flows through the battery then the cells present within it stores the energy. In this way, the battery gets charged.

However, in case, the rectified voltage is less than that of terminal voltage at Q then automatically SCR will come in a reverse-biased state, getting it turned off and no flow of current through the battery will take place further.

Thus, we can say that here the SCR is acting as a switch that controls the voltage fed to the battery. Now, the question arises, once the battery gets fully charged how the circuit will operate.

So, basically what happens within the circuit is as the rectified voltage here is 15 V, so once the battery gets fully charged (suppose it reaches 14.5 V) then the remaining value of the voltage at terminal P will be insufficient to cause further conduction through the SCR because now the rectified voltage will be less than the voltage at terminal Q. This will not allow current to reach the battery further and resultantly the charging circuit will get deactivated.

Basically, this comparison between rectified voltage and the charging potential is made using a comparator circuit. Once the charging potential falls below a certain value then the charging circuit will automatically get activated and again the charging of the battery will take place.

It is to be noted here that the value of the breakdown voltage of the Zener diode and the transformer in the circuit depends on the charging potential of the battery. Thus, the potential at which the battery will be charged will decide the value of these two circuit parameters.

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Drawbacks of Battery charger circuit using SCR

  • This charging is a quite time taking process.
  • The rectifier circuit for ac to dc conversion, do not remove ac ripples as the filter circuit is absent here.
  • The process of charging and discharging is slow due to the presence of a half-wave rectifier.
  • This is only suitable for charging the batteries with a low to medium ampere-hour rating.

This is all about a battery charging circuit using SCR.

Battery Charger Types | Trickle Float Charger Working

The purpose of a battery charger is to charge a battery without overcharging it. The simplest type of charge controller for renewable energy systems monitors the battery voltage and turns the charging current off or reduces it when the battery voltage exceeds a specified level.

It turns the charging current back on when the battery voltage falls below another specified level; the charge controller turns the circuit on (closed) to resume the charging.

Trickle Charger Working

The simplest type of battery charger is the continuous trickle charger, which charges the battery at its self-discharge rate by applying a constant voltage and current, regardless of whether the battery is fully charged.

Because a simple trickle charger must be turned off manually after a period of time to prevent overcharging the battery, it is not generally used in renewable energy systems, although it could be used in a small home system.

The charging current can be preset to the trickle charge requirement for the particular type of battery, which is usually some percentage of the battery’s rating.

A portion of the total current from the source is diverted through the shunt control, and the rest of the current trickle charges the battery.

The current through the shunt control is set at a value that establishes the desired charging current to the battery. This charger provides the same current to the battery regardless of the charge state of the battery. This causes the battery to overcharge and potentially damages the battery once it is fully charged.

The circuit diagram in Figure 1 illustrates one possible configuration of a continuous trickle charger.

In this configuration, the source output voltage must be compatible with the battery voltage; if is not, a regulator must be used in series with the source to regulate the module voltage down to a battery-compatible level.

The diode prevents the battery from discharging back through the source should the module output drop below the battery voltage.

Figure 1 Continuous Trickle Charger Circuit Diagram

Float Charger Working

The float charger provides a relatively constant voltage, called the float voltage, that is applied continuously to a battery to maintain a fully charged condition.

In its simplest form, the float charger is a trickle charger with an automatic on/off switch (usually a thyristor or a transistor). This charger senses when the battery voltage reaches a preset reference level (VREF1), which corresponds to full charge or float charge and shuts off the current to the battery.

When the battery discharges down to a second preset level (VREF2), it turns the current to the battery back on.

The sense resistor, R, is used to isolate the battery voltage from the output of the switch so that it can fluctuate independently of the source voltage.

The voltage sensing circuit compares the battery voltage to each of the two reference voltages and turns the shunt switch on or off accordingly. This setup overcomes the problem with the continuous trickle charge of having to turn it off manually.

It is not an ideal way to charge a battery, but it is better than continuous trickle charging that can overcharge the battery. The circuit diagram in Figure 2 illustrates one possible configuration of a float charger.

Figure 2 Switched Shunt Float Charger Circuit Diagram

In another type of float charger, the electronic on/off switch is in series with the source and load.

A regulator is used to set the current and voltage. Figure 3 shows the concept of a series float charger.

The graph in Figure 4 illustrates the idea of the series switched float charging. The noncharging portions of the time scale are compressed in order to show more than one on/off cycle.

The time during which the battery is not charging (discharge) is typically long compared to the time the battery is charging.

Figure 3 Switched Series Float Charger Circuit Diagram

Figure 4 Typical Series Switched Float Charging Curve

Three-Stage Float Charger

A characteristic of lead-acid batteries is that you charge them by applying a constant voltage and let the battery draw whatever amount of current it requires until it is fully charged.

A good way to charge a lead-acid battery is in three charging stages. These stages are

(1) Bulk stage (or constant current),

(2) Absorption stage (or topping or acceptance), and

(3) Float stage.

Figure 5 shows the charging stages for a typical battery.

The stage of battery charging where the battery voltage increases at a constant rate is the bulk stage.

When a three-stage charger is applied to a battery that is significantly discharged, there is a maximum charge current to the battery.

The charger is set to the maximum battery voltage, which is typically 14.4 V to 14.6 V for a lead-acid battery at 25° C.

The battery voltage starts from the discharged level (VDISCH) and increases to VMAX at a nearly constant rate during this bulk stage, as shown in Figure 5.

Figure 5 Charging Curves for a Three-Stage Charger with Switched Float

The stage of battery charging after the battery voltage reaches the maximum and the current through the battery begins to decrease is the absorption stage.

When the battery voltage is 75% to 80% full, the current through the battery begins to decrease, marking the beginning of the absorption stage.

During this stage, the voltage is held at the maximum value while the current decreases. The decrease in current is not limited by the charger but by how much the battery can absorb; getting the correct rate for absorption is important for the longest battery life.

This absorption stage continues until the current through the battery decreases to a few percents of IMAX. At this point, the battery is fully charged and the battery current is much smaller.

After the current to the battery reaches some lower level, the charger enters the float stage.

The float stage is the final maintenance or trickles charge stage with the purpose of offsetting any self-discharging of the battery. Typically, the float voltage is from 13.2 V to 13.8 V at 25° C.

During the float stage, the float current can be pulsed to keep the battery fully charged. During all three stages, the charger controls the voltage and supplies the current that is required to optimize battery life.

The circuit diagram in Figure 6 illustrates this process for charging the battery. The minimum current sensor issues a signal to the feedback control when the battery becomes fully charged. The feedback control then causes the regulator to decrease its output voltage to the float level.

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Figure 6 Function of a Three-Stage Charger

Review Questions

  • What is a disadvantage of a trickle charger compared to a float charger?
  • What does float charging mean?
  • What are the stages of a three-stage charge controller?
  • What happens during the bulk stage?
  • What happens during the absorption stage?
  • The trickle charger must be turned off manually when the battery is charged or it may overcharge it.
  • It is charging a battery in a manner that keeps it at its fully charged condition.
  • Bulk, absorption, and float
  • During the bulk stage, the battery is charged at its maximum rate.
  • During the absorption stage, the battery continues to charge but at a rate determined by battery capacity.

How it works

Primarily, the IC’s pins 2 and 6 are responsible for controlling the lower and upper voltage limits, respectively. Thus, connecting a discharged 3.7 V Lithium Ion Battery will prompt the IC’s pin 2 to detect the low voltage level and set it high. It initiates the charging process.

When the battery archives its threshold full charge capacity, Pin 6 will change the output to low. Thus, this will limit further charging. Note that your transformer’s voltage shouldn’t exceed 6V while its current rating should be about a quarter of the battery’s Ah.

TP4056 Based Lithium Ion Battery Charger Module

A TP4056 Lithium Ion Battery Charging Board

You’ll find two forms of the TP4056-based Li-ion charger breakout board in the markets. One has a battery protection circuitry, while the other lacks one. The kind offering protection has three modules responsible for the task. They include:

  • A battery protection IC- DW01A
  • A Dual N-Channel Enhancement Mode Power MOSFET IC
  • TP4056 IC.

Hence, the kind with a protection feature has three ICs while the one without has only the TP4056 IC. Notable, the TP4056 is common to both types thanks to its key features that are as follows:

  • Firstly, it guarantees constant current and constant voltage
  • Also, it has an SOP package and a relatively low external component count. Hence, its best suited to DIY charging applications.
  • Besides, it is compatible with USB supplies and wall adapters.

How it works

For best results in charging a 3.7 V Lithium-ion battery, apply a constant current of approximately 20 to 70 % of its capacity. You should do this until it reaches 4.2 V. Afterwards, charge the battery at a constant voltage until there is a 10% drop in the initial charge rate. The TP4056 is responsible for facilitating the above process.

Several Li-Ion batteries

It is important to be cautious when charging Li-ion batteries as they are subject to heating during charging. Still, lithium-ion batteries are easier to charge at higher rates than lead-acid batteries. It is thanks to their capability to be charged at a 1C rate.

Of cardinal importance when charging Li-Ion batteries is keeping the temperature at bay. Hence, a precise temperature sensor circuit is handy.

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