CONSTRUCTORS have often been advised that it is unwise to charge a sealed 12V lead/acid battery directly from a simple “cartype” charger which usually consists of a transformer, bridge rectifier and a meter that gives some indication of the charging current.

There are good reasons for this, including the fact that a simple car battery charger is not suitably current limited and can quickly sizzle a badly discharged sealed lead/acid battery.

It is recommended that the charging current for a sealed lead/acid battery is limited to 25 per cent of the battery’s Ah (Ampere- Hour) rating. For example, an 8Ah battery can supply one amp over an eight hour period, four amps over two hours and eightamps over one hour and so on.

It is unlikely that the output voltage from a standard charger is suitable for charging a 12V sealed lead/acid battery. A constant andstable voltage of between 2·4V and 2·5V per cell is required for cyclic charging which equates to 14·4V to 15·0V for a 12V battery.However, the circuit diagram of Fig.1 shows a method of charging sealed lead/acid batteries using a basic car battery charger with the aid of an L200 voltage/current regulator chip. The off-load output voltage from a typical basic car battery charger is 13·0V as measured, which is taken to a 2,200μF 50V electrolytic capacitor C1. This smoothes the charger output and increases its available d.c. voltage to just over 20V, providing enough “headroom” to overcome the voltage drop across the L200 regulator and diode D1.

On The Limit

The value of the current limiting resistor (R1 to R6) is determined by measuring the open circuit voltage across pins 2 and 5 of theL200 (with power applied to its input). This is the reference voltage and should be in the region of 450mV which is divided by therequired output current (2·0A maximum). For example, Vref/required current = 0·45/0·2 (200mA) = 2·25 ohms.

The output of the charger has an adjustable current limit, consisting of six low value resistors wired to a 1-pole 6-way wafer switch. This enables the current to be reduced, enabling a good range of sealed lead/acid batteries to be charged. The resistor/switch combination is connected between pin 2 and pin 5 using short leads.

The diode D1 prevents any current flowing from the battery being charged, through the potential divider (R7 and VR1) should thecharging source be removed with the battery connected. With the selected current limit resistor in circuit and power applied to theinput of the regulator IC1, adjust VR1 for a voltage of between 14·4V to 15·0V as measured between the cathode (k) of diode D1and 0V line.

When the above adjustments are complete the battery may be connected and the charger switched on and left as the battery will automatically draw less current as it reaches its charged state. A full charge should take about 10 to 14 hours.

This can be monitored by the ammeter of the battery charger, but a more accurate method is to monitor the voltage across the current limiting resistor using an external voltmeter. The actual charging current can then be determined by the application ofOhm’s Law, i.e. the voltage across the switched resistor network / the value of resistance in ohms.

(Readers wanting to know more about the L200 should check Andy Flind’s feature article “Using The L200CV’’ in EPE July 1998 –
ARW).

Article reproduced by permission of Wimborne Publishing. www.epemag.com