9v NiMH Battery Charger Circuit

Everyone uses NiMH batteries. These batteries can be charged quickly, conveniently, and safely. In this post, we’ll examine a DIY NiMH battery charger circuit, including how it functions and the calculations used to select the best parts. This is a simple electronic circuit that is not difficult to construct. This circuit includes an Opamp, a transformer, a voltage regulator chip (LM338), several transistors, and capacitors and resistors.

Components Required

  • Step down transformer (0-12V AC)
  • Bridge Rectifier module or (1N4007 X 4)
  • Transistor TIP125 (PNP)
  • LED = 2
  • Resistors 47Ω, 10Ω, 2KΩ, 1KΩ each one
  • Capacitors 330µF = 2
  • 9V NiMH battery

NiMH Batteries: 

NiMH batteries, also known as nickel-metal hydride batteries, need to be charged slowly and at a low current; overcharging or overflowing the battery causes it to heat up.

Each NiMH battery is constructed from individual NiMH cells that are connected in series or parallel to produce higher output voltages or currents. 

The nominal voltage of one NiMH cell is roughly 1.2V. A fully charged NiMH cell will have 1.6V across its terminals. Eight cells are often connected in series to form a 9V NiMH battery. The output voltage as a result of this will be 1.2 x 8 = 9.6V. A NiMH battery needs to be charged at a voltage that is equal to its fully charged voltage, which is 12.8V (1.6V x 8) for an effective charge.

Along with the charging voltage, the battery’s charging current must be taken into account. A NiMH battery can be safely charged at 0.5C, or half its present capacity. A battery with a 9V/300mAH current capacity will be used in this circuit. As a result, we must charge the battery with 12.6 V and 150 mA, or 0.5 C.

9v Nimh Battery Charger Circuit:

This NiMH battery charger circuit is powered by a 220 volt AC wall outlet. The voltage level is raised to 18 volts using a step-down transformer. To convert AC signals to DC signals, four diodes are placed in a bridge rectifier design. A bridge rectifier integrated onto a chip is an alternative. Capacitor C1 receives the DC output voltage and smooths the DC signal. The LM338 IC receives the smoothed DC signal.

The resistors R2, R4, and R5 in the circuit above allow the LM338 voltage regulator chip to be configured to produce a certain output voltage. The output voltage is determined by the equation

Vout = 1.25 ( 1 + R2 / ( R4 + R5 ) ) + IAdjR2

The output voltage is set to 12.8V using this formula, which is the charging voltage for 9V NiMH batteries. Up to 5A can be delivered to the load using the LM338. A heat sink is required for usage with the LM338 voltage regulator.

The battery is then supplied with the output voltage via an 80 Ohm current-limiting resistor. In order to securely charge the battery, this resistor restricts the output current to 150mA. Given that this resistor must withstand a lot of currents, go with one with a high wattage, like 10 Watts.

Charge Indicator Unit:

In order to inform the user when the battery is fully charged, an Opamp was employed as a charging indicator. Here, Opamp is designed to function as a comparator. R4, R5, and R6 were built up as voltage dividers to supply a reference voltage to the Opamp LM301. The combined resistance of R5 and R6 resistors is higher than R4’s, therefore the voltage reference set to the Opamp’s positive input measures about 12.7 volts, which is about equal to the voltage of a fully charged battery.

Let’s assume that the circuit is connected to an uncharged battery. Reference voltage to the Opamp’s negative terminal will be less than 12.7 volts when the battery is undercharged. In contrast, a voltage reference of 12.7 volts will be applied from the voltage divider network to its positive terminal. Opamp’s output is compelled to a high state by this. As a result, current flows to the battery and charges it while the PNP transistor is kept in the OFF state.

The voltage across the battery begins to rise as the battery begins to charge. Voltage in the inverting terminal of the Opamp will be higher than in the non-inverting terminal when the battery is completely charged. Opamp then shifts to a low state for its output. The PNP transistor is now ON as a result.

Now that the battery voltage is roughly comparable to the output of the LM338 and the Opamp output pin offers a conduit for current flow, the incoming current from the LM338 will be sunk into the output. The LED will light up since the PNP transistor is ON, signaling to the user to remove the battery from the circuit as the battery has finished its charging cycle. The battery must now be unplugged from the charger circuit.


I hope all of you understand how to design a 9V NiMH battery charger circuit. We MATHA ELECTRONICS will be back soon with more informative blogs.

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