e01c093.PDF

TEST &MEASUREMENT

Battery

Discharger/Capacity

Meter (2)

Part 2: keyboard, discharger and assembly

Design by B. Stuurman

To enable the control module, described in Part 1, to function as a bat-

tery discharger, a number of additional hardware components are

required. First, of course, is the keyboard for the entry of data and second,

a printed circuit board with the actual discharger itself. A nice design

aspect of this project is

that the hardware can

be tested with the aid

of a few diagnostic pro-

grams.

Keyboard

A small keyboard is required to

enable the entry of user information.

Our approach has been a ready-

made keyboard with 12 buttons.

Each key has an individual as well

as a common connection. Because

the control module can measure ana-

logue voltages, it suffices to ﬁt a volt-

age divider for each key. The

schematic of this is shown in Fig-

ure 5 . The table below the figure

lists the key symbols. The second

line indicates the resistor value for

each key and the third line shows

the resulting hex value that will be

obtained when the key is pressed.

12/2001

Elektor Electronics

TEST &MEASUREMENT

a length of 20 cm, the end of which

has a 3-way PCB socket. This socket

is connected to Input 0 of the control

module.

The proper functionality of the

keyboard can be tested with the aid

of the program ‘KEYTEST.HEX’,

which can be found on one of the

diskettes that belong with this pro-

ject (see the Parts List). After that

program has been loaded, it can be

started by typing ‘chip’. When no

key is held down, the display will

show the value ‘FF h ’ or ‘FE h ’; when

a key is held down, the displayed

value should be the same as the one

listed in the table. The actual value

may deviate a little, because only the

high nibble is used, the low nibble

should be between 5 h and A h .

+5V

analogue

GND

Hex value

010201 - 2 - 13

Figure 5. The keyboard with the necessary voltage dividers.

The resistors may be mounted directly on the

back of the keyboard. The common connec-

tion for the buttons is connected to Gnd. The

+5 V supply is connected to the common

divider resistor, which has value of

24k7. The other end is connected to

the individual key resistors. To the

keyboard we fit a 3-core cable with

Discharger

The discharger, the schematic if

which can be seen in Figure 6 , com-

prises three parts. The upper part

serves as the power supply and for

the on and off control of the cooling

fan. An unregulated mains adapter

(6-12 V/300 mA) may be used for the

power supply. The regulator (IC1) is a

‘low drop-out’ type, which will pro-

vide 5 volts even when the input

voltage is as low as 6 volts. If you

use a mains adapter with an

adjustable output voltage then you

can use this to set the speed of the

12-V fan. The mains adapter is con-

nected to K3, the cooling fan to K2,

while K1 is used for the power sup-

ply connector from the control mod-

ule.

The battery voltage is measured

at connector K6 (+aV and –aV). A

short length of wire links this con-

nector directly to the terminals for

the battery, so that the voltage drop

across the fuse and current carrying

wires does not affect the measure-

ments. The signal at K6 is ﬁltered by

R4 and C3 and is subsequently fed

to an adjustable attenuator with

three ranges: 10.2 V, 20.4 V and

40.8 V. The high value of each range

results in FF h (255 d ). The attenuator

is controlled with Out 0 and Out 1. A

reduction in (unnecessary) calcula-

tions was obtained by selecting

appropriate values for the attenuator

resistors. The program

‘DIVITEST.HEX’ on the diskettes is

used to test the attenuator.

IC1

LP2950CZ5.0

D 2

12V

1N4148

+5V

100n

220

25V

10k

BC517

+Batt.

100k

220 Ω

OUT2

INPUT2

OUT1

OUT0

SERVO1

(PWM)

3A15 F

100 µ

50V

BS170

+aV

–aV

22k

BUZ100

100n

25k

100n

R11

IC2

R10

10k

INPUT1

R13

R14

R15

R16

+5V

CA3130

R12

100k

470p

010201 - 2 - 14

–Batt.

Figure 6. Schematic of the actual discharger. Power-MOSFET T4 does the ‘heavy work’.

Elektor Electronics

12/2001

TEST &MEASUREMENT

COMPONENTS LIST

(Discharger board)

Batt.

Resistors:

R1 = 1k

aV +

Ω

R2,R10,R12 = 10k Ω

R3 = 100k

IC1

R13

R14

R15

R16

IC2

Ω

R4 = 220Ù

R5 = 16k Ω

R10

3A15/F

010201-2

+12V

R6 = 48k

R7 = 97k Ω

R8 = 18k

R9 = 22k

Ω

R11 = 56k Ω

R13-R16 = 1O, 0.5W

P1 = 25k

preset

P2 = 100k Ω

preset

Capacitors:

C1,C4,C5 = 100nF ceramic

C2 = 220µF 25V radial

C3 = 100µF 50V radial

C6 = 470pF

Figure 7. Copper-layout and component overlay for the discharger PCB.

Semiconductors:

D1 = 1N4148

D2 = LED, red, high efficiency

T1 = BC517

T2,T3 = BS170 (or BSN10A)

T4 = BUZ100

IC1 = LP2950CZ5.0

IC2 = CA3130

Miscellaneous:

K1,K5 = 3-way SIL pinheader

K2,K6 = 2-way pinheader

K3 = 2-way PCB terminal block,

lead pitch 5mm

K4 = 5-way pinheader

F1 = fuse 3.15 A (fast) in PCB mount holder

PCB, order code 010201-2 (supplied

together with control board 010201-1)

Diskettes, project software, order code

010201-11

Figure 8. This is what the completed discharger PCB looks like.

The remainder of the circuit is for

the adjustment and regulation of the

discharge current. The Servo 1 out-

put has a PWM signal that is ﬁltered

by R9 and C4 and is then applied to

the gate of the power-MOSFET (T4).

The source connection contains the

current-measuring resistors R13-R16.

The voltage drop across these is

ampliﬁed by opamp IC2 and routed,

via Input 1 on K5, to the control mod-

ule. P1 adjusts the gain and P2 is

used to compensate for the offset

voltage. A variable is used to set the

operating range of the PWM. It also

shows if the current is stable. It is

possible that the MOSFET is unable

to provide the desired current when

the voltage is lower than 1 V. In this

event, the current variable ‘goes

through zero’ and this is then indi-

cated.

There is an anti-parallel body

diode between the drain and source

of the power-MOSFET. If, by acci-

dent, a battery is connected the

wrong way around, fuse F1 will blow

and limit the damage. It is important

to use a fast fuse here.

The printed circuit board layout

and component overlay for the dis-

charger are shown in Figure 7 . The

assembly of the board is self-

explanatory. All the connectors are,

again, male PCB headers, with the exception

of K3 (screw terminals) and those for the cur-

rent carrying connections, for which PCB pins

are used. Resistors R13-R16 are mounted

4 mm above the board to improve their power

dissipation.

With the prototype, the printed circuit

board traces that go to the battery terminals

were tinned with extra solder in order to

reduce their resistance. Figure 8 shows a cor-

rectly assembled example of the discharger

PCB.

Assembly

The prototype was successfully housed in a

Teko sloping enclosure type 362, but many

12/2001

Elektor Electronics

TEST &MEASUREMENT

other kinds of enclosures are also suitable, of

course. The title photograph gives a good

indication of how the various components

have been ﬁtted inside the enclosure.

The rear panel has a large opening for the

airflow from the cooling fan and in the front

there are 8 holes, dia. 14 mm, for the air inlet.

Both printed circuit boards are mounted to

the bottom of the enclosure with the aid of

stand-offs. Suitably sized apertures need to

be cut in the front panel for the keyboard and

the LCD. Additional holes are required for the

two slide switches and binding posts (wan-

der sockets). The RS232 connector was

mounted on the left-hand side of the proto-

type enclosure.

The battery discharger is now nearly

ready. All that remains are a few intercon-

nections. In order to prevent mistakes, the

complete schematic of the necessary wiring

is given in Figure 9 . The lead from the mains

adapter is connected to K3 (check the polar-

ity!). Solder three wires to the power MOSFET

(use heavy wire for the source and drain con-

nections) with push-on terminals at the other

end; these are connected to the terminals for

T4. Use the same heavy wire to make the con-

nection between the battery connections on

the PCB and the terminals on the front panel.

All other interconnects employ connectors,

which are cut from a strip of turned pins for

PCB mounting. The cable for K6 has two ring

terminals at the other end, which (together

with the other ring terminals) are bolted to

the binding posts. The cooling fan is con-

nected to K2 and the plug from the control

module power supply lead goes to K1. A 3-

core cable, with a 3-way female header at

both ends, is used for the connection

between K5 and Input 1 of the control mod-

ule. This cable also carries the +5-V and 0-V

lines. A 5-way female header with 5 individual

wires, each with a 1-way female connector at

the other end, is connected to K4. These are

connected to Out 2, Input 2, Out 1, Out 0 and

PWM (Servo 1) respectively. This prevents

ground loops.

The ﬁnal wire that is required, connects K5

on the control module to the slide switch

‘Chip-Auto’ on the control panel.

switched on.

A program can now be up-loaded

by pressing Alt+l. At the bottom of

the screen you are prompted for the

name of the program. The extension

is always ‘HEX’. The program is

loaded after confirming with enter.

By loading the program ‘KEYTEST’

the keyboard can be tested. The pro-

gram can be executed with the com-

mand ‘chip’ or by setting the switch

‘Chip-Auto’ to the ‘Auto’ position and

switching the discharger on.

The voltage divider has to be

tested as well. For this purpose we

load the program ‘DIVITEST’ and run

it. We connect a voltage of 5 Vdc to

the battery terminals. The value on

the display should be 125 (5/10.2 ?

255) with an attenuation of 2. At an

input voltage of 15 Vdc the value

should be 188 and the attenuation 4.

And ﬁnally 25 Vdc; the value should

be 156 and the attenuation 8. The

measured values may deviate by

about 2 or 3 units, but if it is more,

and the resistor divider values are

correct, then it is possible that either

T2 or T3 is leaky.

Once all the tests have been com-

pleted successfully, the program

‘NICADIS’ may be loaded. The dis-

charger is now ready for use, with

the exception of calibrating the cur-

rent. Connect a battery pack to the

discharger, with an ammeter in

Analysis

of a discharge voltage curve

The curve shown below is the discharge voltage of a 7-cell NiCad battery with a

nominal capacity of 1900 mAh. The discharge current was set to 190 mA (0.1 C)

and the discharge voltage to 1 V/cell. The battery was fully discharged after 8

hours and 4 minutes. The measured capacity was 1532 mAh. A little bit low per-

haps, but then the battery wasn’t new any more either.

The curve is as expected, but there is still something strange. At a voltage of 8.4 V

the battery pack is empty, the voltage declines rapidly – this much is normal. At

7.1V however, the voltage ﬂattens out. Because the voltage drop has been about

1.3 V it appears that 1 cell has reversed polarity and is accepting a charging cur-

rent in the wrong direction. Shortly afterwards the end voltage has been reached.

Because the discharge current was 0.1 C, it would have been better if the switch-

off voltage was set to 1.1 V/cell. In this case no polarity reversal would have taken

place.

Lead-acid batteries are more sensitive to polarity reversal than NiCad or NiMH-

cells. It is usual to select a switch-off voltage of around 70% to 80% of the nomi-

nal voltage, but this is also dependent on the discharge current, of course.

9.50

9.25

9.00

8.75

8.50

8.25

8.00

Software

The battery discharger has two modes of

operation: the command processor can be

active or a program can be running. To com-

municate with the device, or to upload a pro-

gram, ‘CHIPTERM’ or ‘VBTERM’ (Windows

version) has to run on a PC and the dis-

charger connected to the serial port. The

switch ‘Chip-Auto’ on the discharger has to

be set to the Chip position before it is

7.75

7.50

7.25

7.00

100

150

200

250

300

350

400

450

Min

010201 - 16

Discharge curve for a 7-cell NiCd-battery pack.

Elektor Electronics

12/2001

TEST &MEASUREMENT

series. Set the number of cells and

the voltage per cell as appropriate.

Set the current to 2500 mA. Using

P1, calibrate the actual current to be

2500 mA. Now set a current of

100 mA, and use P2 to set the actual

current to this value. Because of the

control loop the readout may ﬂuctu-

ate a little; therefore adjust to a best

possible average.

carried out to smooth the curve. At

the same time, the average voltage

and discharged energy are calcu-

lated. The generated .DAT file can

be read by the plot program and be drawn as

a discharge curve. The file PLOTR.ZIP con-

tains all the necessary information.

(010201-2)

LCD

display

Generating discharge

voltage curves

While discharging, a voltage sample

is stored in memory every minute.

Memory locations 600 h -7FF h in the

EEPROM are used for this. Address

600h contains the attenuator value

and 601 h the current value. Once the

discharge cycle is completed, these

values can be retrieved and stored

in a ﬁle. To do this, go the command

processor and open a .LOG file

using Alt+o. By giving the com-

mand ‘prog 600’ the address 600h is

displayed on the screen followed by

two data bytes. By pressing the ‘+’-

key successive values will appear.

Continue until the data byte is equal

to 00 h . This indicates that the end of

the sample log has been reached

and the .LOG ﬁle can now be closed

with Alt+c. Using the program

‘LOGDAT.EXE’ the sample values

are converted to voltages and num-

bered. In addition, interpolation is

chip

auto

analogue

+5V

GND

off

COMPONENT LIST

(mechanical)

keyboard

(with p.d. resistors)

Batt.

– enclosure, e.g., Teko desk type

362

– heatsink for T4: Fischer

SK132/37.5/SA

– fan, 40 x 40 mm, 12 Vdc

– mains adaptor, 300 mA, 6-12

Vdc,

– binding post, red

– binding post, black

– keyboard, 12 keys, 1 common

connection, 12 individual

connections, e.g., Display

Electronics # 03.52.1252 or

Conrad Electronics # 19-55-61

– miniature slide switch, 2 x

changeover

– turned pin socket strip, lead

pitch 2.54mm

aV +

3A15/F

+12V

6 - 12 V DC

mains

adaptor

fan

BUZ100

010201 - 2 - 15

Figure 9. Wiring diagram for the battery discharger. There are quite a few connections!

12/2001

Elektor Electronics

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