Lead Acid Battery Charger with float

NOT designed for Gel type Lead Acid Batteries!


R1 = 0.1 ohm P1 = 2KR2 = 3.3 ohm P2 = 1KR3 = 220 ohm C1 = 100uF/40V (Electrolytic)R4 = 2K7 C2 = 100pFR5 = 1KC3 = 0.1uF (100nF)R6,R10 = 1K5 U1 = LM350R7,R9 = 4K7 U2 = LM334 R8,R13 = 1K8 U3 = LM301AR11 = 33Q1 = 2N3906R12 = 100 ohm D1 = 1N4001Led1 = Red D2 = 1N457 (NTE177)Led2 = GreenS1 = Toggle Switch, ON-ONS2 = Pushbutton, momentary ’ON’

The circuit furnishes an initial charge voltage of 2.5 Volt-per-cell at 25°C to rapidly charge a Lead-Acid battery.
The charging current decreases as the battery charges, and when the current drops to about 180mA, the charging circuit reduces the output voltage to 2.35 Volt-per-cell, floating the battery in a fully charged state.This lower voltage prevents the battery from overcharging, which would shorten its life.The LM301A (U3) compares the voltage drop across R1 with an 18-mV reference set by R2.The comparator’s output controls the voltage regulator, forcing it to produce the lower float voltage when the battey-charging current passing trhought R1 drops below 180mA.The 150mV difference between the charge and float voltages is set by the ratio of R10 and R12.The red and green Led’s show this state of the circuit.

At points A and B you would see approximately 18mV depending on the types of components used.At point C (in between) there is a current flow of approximately 2.3 milli-amps, again, depending on your choice of components and ofcourse the input voltage at ’Vin’ of the LM350 regulator.

Led1 is a red led, and for Led2 you can use a yellow or green one.Make sure that C1 is at least twice the input voltage. C2 is a small ceramic type.Vin is at least 18-20volt max about 30V.Switch S1 is a toggle or slide (ON-ON) switch of your choice.The regular on-off type will not work.S2 is a momentary ON pushbutton swith to start the charge.

This circuit is not recommended for GEL-TYPE batteries!

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