Battery tester

Lead Acid Battery Condition Checker Circuit Diagram

To make sure that a lead-acid battery (whether standard or sealed) is always fully charged, a constant voltage should be applied to it. Chargers that check whether the battery is still charged and only commence charging when it is (nearly) flat, do not guarantee that it is fully charged when it is required in an emergency.

The charger described monitors the battery voltage constantly and tops up the charge if and when required. This has advantages and drawbacks. The battery is always fully charged, but it becomes sluggish, that is, it does not take the charge well. This effect disappears after normal use (a few duty cycles). If the voltage is not too high (≤ 13.8 V), the battery is charged and has a long life.

It is not advisable to constantly apply a high voltage of say, 14.4 V across the battery (this is in any case not necessary because at 13.8 V it remains fully charged). The only reason for applying a high voltage (temporarily) is that this charges a flat battery quicker.

Always use a commercially available charger: this meets safety requirements and is normally relatively cheap. A partly charged battery may be connected to such a charger without any precautions.

When the battery is fully charged, do not leave it connected to the charger, because that provides quite a high voltage. It is then that the circuit shown here comes in handy. The voltage at which charging should be stopped is preset with P1. Turn the preset fully anti- clockwise and connect a voltmeter across the battery. The LED will not light. Turn P1 slowly clockwise till the LED just lights: the battery is then being charged. When the battery voltage has reached the required value, turn P1 slightly anti-clockwise until the LED goes out. This happens fairly gradually, so it is best to do this in semi-darkness if possible.

Lead Acid battery Conditioner Circuit Diagram:

The 723 is powered by the battery by the battery via D2 (current about 10 mA). The internal reference voltage is reduced to 2.2 V by R1 and R2. This voltage is compared with the divided battery voltage at the wiper of P1. If it is lower, the output (pin 10) goes high, whereupon D1 lights and the optoisolator switches ON the thyristor. This results in the charger output being applied to the battery: the level of the current is determined by the charger. Since the output of the charger is an unfiltered, rectified alternating current, the thyristor will switch OFF at every zero crossing, but immediately switch on again if the battery voltage is not high enough.

It is quite feasible to leave the monitor between the charger and the battery. Charging will then take a little longer because there is a loss of about 1 V across the thyristor.

The thyristor must be mounted on a heat sink if the current exceeds I A: the maximum current must not exceed 5 A.

The circuit may be built on the PCB. A large part of the board is occupied by the heat sink for the thyristor. Use flat connectors that can be screwed on to the board for the four terminals that carry fairly high currents.

Parts list:


R1 = 4.7 kΩ

R2, R4 = 2.2 kΩ

R3, R7 = 10 kΩ

R5, R6 = 1 kΩ

P1 = 1 kΩ preset


CI = 1 nF

C2 = 100 µF, 25 V


D1 = LED, 3 mm, red

D2 = 1N4001

Th 1 =TIC 106

Integrated circuits:

IC1 = CNY17-1

IC2 = CA723


Heat sink for Th1, e.g., SK59 4 flat connectors with screw fitting

1 case 95x60x24 mm (33/8×23/8x 1 in)

PCB Ref. 934033

Front panel foil Ref. 934033-F


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