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ELECTRONICS LABORATORY

EXPERIMENT 2

BATTERY CHARGER

Objective

The objective of this experiment is to practice design with bipolar transistors and obtain

something that is quite useful for household electricity needs.

Introduction

With the widespread use of portable electronic devices, rechargeable batteries have been the

center of attention. There are various kinds of rechargeable batteries including lead-acid

 batteries, nickel-cadmium batteries, and nickel-metal hydrate batteries. Each type of battery

has its own characteristics.

For the Electronics Engineering students, a battery charger circuit could be an interesting

application of transistors. The purpose of this experiment is to have the students familiarized

with battery charger circuits in addition to having them practice the design of transistor

circuits.

There are two main approaches in battery charger circuits: Constant current battery chargers

and constant voltage battery chargers. (See Figure 1 and 2)

Batteryto be

Charged

RE

Vcc

R1

R2

+

 _

 Figure 1. Constant Current Battery

Charger. The charging current is adjusted

by R E .

Battery

to be

Charged

Vcc

R1

R2

 A  currentmonitor 

+

 _

 Figure 2. Constant Voltage Battery

Charger. The charging voltage is adjusted

by R2.

1. The Constant Current Battery Charger.

The circuit in Figure 1 may be used as a constant-current battery charger. In this circuit, the

voltage divider, which is made of resistors R 1  and R 2, sets a constant base voltage for the

transistor:

VB = VCC R 2/(R 1+R 2)

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The base-emitter voltage of the transistor is almost constant. The resistor R E sets the emitter

current:

IE ≈ (VB – VBE)/R E 

The emitter current can approximate the collector current. The power supply voltage VCC and

the base voltage determine the maximum battery voltage. The transistor must stay in the

active mode. (VBE ≈ 0.7V and VCESAT ≈ 0.2 V for silicon BJT.)

VBATmax = VCC – (VE +VCESAT)

VBATmax = VCC – (VB – VBE + VCESAT)

VBATmax ≈ VCC – VB 

2. Constant Voltage Battery Charger

The circuit in Figure 2 may be used as a constant-voltage battery charger. In this circuit, the

resistors R 1 and R 2 sets a constant base voltage for the transistor:

VB = VCC R 2/(R 1+R 2)

In this circuit, the emitter current depends on the battery voltage:

IE = (VB – VBE – VBAT)/R TH 

where

R TH = R 1||R 2 

As the battery is charged, its voltage raises, and the charging current gets smaller. When the

 battery voltage reaches (VB – VBE) the charging current practically stops. The maximum

emitter current in this circuit may be calculated by taking VBAT = 0.

The emitter current can approximate the collector current. The power supply voltage VCC and

the base-emitter voltage VBE determine the maximum battery voltage. For this, R 2 is open-

circuited.

VBATmax = VCC – VBE 

Preliminary Work 

1. Search for the cell voltages for the following batteries:

Lead-acid cell: _________

 NiCd cell: ___________

 NiMH cell: ___________Li-ion cell: ___________

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(A possible source: www.energizer.com)

2. Find information on “memory effect” and “lazy battery effect.”

3. Check your local battery retailer on the availability of the following rechargeable batteries:

Battery Type Capacity Price

“Dry” Lead-Acid _______________ ____________

 NiCd (medium capacity) _______________ ____________

 NiCd (high capacity) _______________ ____________

 NiMH (medium capacity) _______________ ____________

 NiMH (high capacity) _______________ ____________

4. Get two NiCd or NiMH rechargeable battery cells. Put them in series

5. Design a 200mA constant-current battery charger for the batteries you have. Calculate

 power dissipated in each component. Determine the time it requires to charge the empty

 batteries completely.

6. Design a constant-voltage battery charger for the batteries you have. Calculate the

maximum charging current and the power dissipation in each component at this current level.

7. Select the components for the experimental work. Make sure that the resistors and the

transistors can handle the required currents and power dissipations. Here is a list of

transistors that may be suitable for your design:

BC337 (NPN, 500 mA 45 V)

BD135 (NPN, 1.5 A, 45 V)

BD137 (NPN, 1.5 A, 60 V)

BD139 (NPN, 1.5 A, 80 V)

Experimental Work

1.  Build and test the constant current battery charger that you have designed. Measure

the battery voltage once every third minute for 30 minutes. Graph the voltage versustime.

2.  Build and test the constant voltage battery charger that you have designed. Measure

the battery current once every third minute for 30 minutes. Graph the current versus

time.