Electronics › Electronics › Auto Cut Off Battery Charger with LM317 & BD139 › I don’t know if you already
I don’t know if you already solved this problem, but a quick look at the schematic shows that the zener diode is shown the WRONG side of the resistor R4.
That R4 value is stated as 100 Ohms, I would think that should be nearer 10 Ohms with the zener diode connected to a point closest to the battery lead. This connection then picks up the voltage drop across R4 and when the battery is charged, the diode conducts and switches on TR1. Where the original schematic shows this as connected to the side of the resistor that will always be the ‘fully charged’ voltage, which of course it isn’t, is it!
In the way this is shown, the green LED is merely connected from the output of the LM317 to 0 volts and is an indicator that the circuit is powered, not neccesarily charging, you should not connect the charger to the mains until AFTER you have connected the battery. Sparks look pretty but not in this circuit.
As any silicon device is the ‘fastest fuse on the planet’ I would suggest adding a fuse in the output lead rated at the maximum value for an LM317 which according to the datasheet is 2 Amps but not for very long! A 2 amp slo-blo fuse is cheaper than new chips in the event of a short circuit or a seriously dead battery.
The red LED should be on when T1 is switched on as the battery terminal voltage reaches the required potential. If you are charging a lead-acid battery to full capacity, this would be 14 volts approx depending on the battery charging requirements. if you wish to retain a ‘float’ charge, that voltage should be reduced to 13.2 volts
That voltage is set by VR1 with TR1 on the off state. To set this situation up, simply disconnect the zener diode and adjust VR1 for 14.2 volts at the junction of D3 – R4 (or 13.2 for float charging), then reconnect the zener diode (to the juncton ot R4 – battery terminal !) and with no load, TR1 will immediately turn on, the red LED will light and the output voltage will drop. Because there is little or no current flowing through R4, both sides will be at the same potential.
Now if you charge a battery, it will work correctly and indicate the state of charge.
I should point out that this charger is capable of 1.5 Amps and as a LA battery should be charged at 1/10 of the Ah rated, this charger will work for batteries up to 15 Ah if they are used in cyclic mode (heavy discharge) OR as a float charge for any LA battery.