Measuring the capacity of NiCd batteries

The capacity tester will give an 11.‘j accurate indication of the capacity of up to six NiCd (nickel-cadmium) batteries connected in series. It is very useful for monitoring the aging process of NiCd batteries, and for spotting, faulty batteries, which are always a source of trouble in battery-powered equipment.

Before the capacity of a NiCd battery can be ascertained, it must be fully charged with the usual charger. Next, the battery is connected to the capacity meter, which draws a constant discharge current. After the battery is connected, and the circuit reset, a counter is started and stopped again when the tester measures that the ‘discharged’ level of the battery is reached. The counter is clocked with oscillator pulses, and the number of ‘ticks’ recorded is an indication of the battery capacity in math (milli-ampere-hours). This measurement method works on the assumption that the ‘discharged’ voltage of the relevant battery is known and predefined as a switching threshold on the tester.

The battery is discharged by a constant current sinking circuit formed by T1 and ICS. A MOSFET Type BUZ10 is used as a power regulator because of its very low drain-to-source resistance (RD.s(on))- The discharge current is measured with the aid of R8 and held at a constant value of I A by comparator ICI. The discharge current is adjusted by preset P1. Transistor T2 enables the current sink to be switched on and off under the control of bistable IC3a• The current sink is switched off if IC3a is set.

The discharge cycle is started when the reset input of IC3a is low. The set input is driven by a comparator, IC2, which coma pares the battery voltage with the voltage at the pole of rotary switch S1. This switch selects the number of batteries connected, which can range from one to six. It ensures a supply of 0.5 V nominally per battery connected.

The battery voltage is divided: 2 by R9-RIO and then fed to the –input of IC2. When the voltage per battery drops below 1 V, the voltage at the -input of IC2 drops below the reference set with S1. Consequently, the comparator changes state. Its output swings to +5 V, sets the bistable, and switches off the current sink on the battery. Also, oscillator/counter IC4 is stopped via T3, so that the count pulses on connector K1 disappear. Although T1 is switched off at this point, the batteries are still being discharged at a rate of about 50 mA. This means that they should be disconnected as soon as possible after the test.

Measuring the capacity of NiCd batteries Schematic diagram

The pulses supplied by IC4 may be fed to any counter capable of accepting a digital input signal with a swing of 5 V. The prototype of the capacity tester was Connected to the 4-digit counter module described in Ref. I. Apart from dAc_ supply and clock’ pulse connections, the reset line is also connected directly. This allows the whole circuit (i.e., capacity tester and readout) to be reset by pressing a single key.

Preset P2 is adjusted such that a new battery produces a readout that corresponds to the capacity of the battery in mAh, e.g., ‘1200’ for a 1.2-Ah battery. Since a battery  capacity of 1 Ah equals 1,000 output pulses per hour, the oscillator clock frequency becomes

( 1,000/3.600)* 29 = 142.2 Hz.

which can be measured at pin 9 of IC4. If a frequency meter is not available, P2 may be adjusted until the display counts 100 pulses in 6 minutes.

The printed circuit board allows the tester and the 4-digit readout to be built in one go. The parts list of the readout module is repeated here for convenience. The readout board is fitted on top of the capacity meter board with the aid of four PCB spacers. This allows the pin header on the read-out board to be soldered at the trackside so that it can be inserted into the socket on the capacity meter board. The reset wire must be run separately between the two boards.

Measuring the capacity of NiCd batteries Schematic diagram

Measuring the capacity of NiCd batteries Schematic diagram

Measuring the capacity of NiCd batteries Schematic diagram

Parts list
METER SECTION

Resistors:
R1 = 220 Ω
R2 = 1 kΩ
R8 = 0.5 Ω, 1 W
R9 = R10,R12,R15 = 10 kΩ
R11 = 3.9 kΩ
R13 = 470 Ω
R14 = 100 kΩ
R16 = 470 kΩ
R17 = 47 kΩ
R18 =  22 Ω
P1 = 1 kΩ, preset (H)
P2 = 47 kΩ preset (H)
Capacitors:
C1, C2, C4 , C6 , C9, C11 = 100 nF
C3 = 10 nF
C5 = 1 nF
C7 = 39 nF
C10 = 100 uF, 16 V
C12 = 100 uF, 25 V
Semiconductors:
D1 = zener diode, 2.7 V,400 mW
D2 = LED, 3 mm
T1 = BUZ10
T2,T3 = BC547B
Integrated circuits:
IC1 = CA3160
IC2 = CA 3140
IC3 = 4013
IC4 = 4060
IC5 = 7805
Miscellaneous:
K1 = 4-way SIL socket
S1 = single-pole. 12-way rotary switch for PCB mounting
Heat sink for T1, e.g..
Fischer Type 1CK35/SA* PCB REF. 934085
READ-OUT SECTION
Resistors:
Ri , R4, R12 = 1.5 kΩ
R5, R11 = 56 Ω
R13 = 10 kΩ
Capacitors:
CI, C2 = 100 nF
Semiconductors:
T1-T4 = BC547B
LD1-LD4 = HD1107
Integrated circuit:
IC1 = 74C926
Miscellaneous:
S1 = push-button switch for PCB mounting
K1 = 4-way SIL pin header
*Dau (UK) Ltd, 70-15
Barnham Road, Barnham,  W.Sussex PO22 0ES.
Telephone (0243) 553 031
Reference:

  •  4-digit counter module.
  • Elektor   Electronics
  • December 1992.

 

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