Auto Cut Off Battery Charger with LM317 & BD139

[Irfan]

I am looking for a battery charger circuit with auto cut off functionality,

 

Transformer : 18-0-18 1.5Amp

 

Battery Lead Acid 12v 7.2AH

 

I wish to use majorly two components namely :

LM317

&

BD139

 

Circuit posted by D MohanKumar is not working.

 

[AJISH ALFRED]

Hi Irfan,

Please post the link from where you find the circuit.

[Irfan]

http://www.engineersgarage.com/contribution/inverter-battery-charger-with-cut-off

 

This is where I found the circuit

 

Transformer used is a 18-0-18 1.5 Amp

 

Battery is an Artheon Make 12v 7.2Ah

 

[Irfan]

http://www.engineersgarage.com/contribution/inverter-battery-charger-with-cut-off

[Irfan]

The Circuit

 

Transformer I have used is a 18-0-18 1.5Amp

Battery is 12v 7.2 Ah

wysiwyg_imageupload:7773:

[Irfan]

Transformer I have used is a 18-0-18 1.5Amp

Battery is 12v 7.2 Ah

wysiwyg_imageupload:7773:

[AJISH ALFRED]

What is the status of RED and GREEN leds? What is the voltage with refernce to ground at the point just before D3 and after D3?

Make sure that you connected the zener diode in the reverse bias direction itself.

[Irfan]

 

The LED’s (both) are always lit!!!!

 

Zener is connected as depicted

 

Vario used is 5K and not 10K

 

I have also tried removing the resistance R4, but nothing happens

 

Voltage measurements that you require are with

1. the battery connected and transformer on, 

or, 

2. with battery connected and transformer off, 

or, 

3. with transformer on and battery not connected.

 

you may mark a mail to my gmail that would be irfanalavi at gmail

[AJISH ALFRED]

Hi Irfan,

Sorry for the delayed response,

I need the volatge readings with the battery connected and transformer ON.

[Irfan]

Thanks for the response

 

Volatge is

 

Before diode 14.95 & 14.65 after diode

 

Please be updated that all diodes used are 5402 & not 4107

 

please if u can mail me

[Savio]

Just wondering! won’t the zener be in breakdown condition  @ 8V causing red LED to be always lit up.

[Pete Braven]

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.

[Irfan]

I have not yet been able to solve this freaking issue, all i seek is an auto cut off charger

[Pete Braven]

Hi Irfan,

 

I am actually building a few SLA chargers at the moment, both with LM317 and also a 5 amp version with LM338T which works in a very similar way. I have adjusted the schematic to show more clearly what it should look like in order to work correctly.

wysiwyg_imageupload:9948:height=285,width=723

 

 

 

 

 

 

 

 

 

 

For clarity, the green LED connection is shown not connected to the base of T1 as some people have mistaken that in the original.

R4 has been made 10 Ohms and ZD is now connected via a link to the battery side of R4 where it will sense the voltage on the battery terminal as is required. I notice that R2 is maybe a little low compared to the datasheet and I would increase it to the 240 Ohms suggested as original spec.

Setting up is now a matter of having the whole circuit powered up with no load and adjusting VR1 to give 14.2 volts (or what is suggested on the datasheet for your particular battery) at the positive battery terminal. To check the cut-off works, simply connect the LINK and if a fuly charged battery is now connected, TR1 will pull the adjustment pin low and the output from the regulator will fall.

With a battery under charge, the current flowing through R4 will give a voltage drop, as the battery becomes charged, so the voltage across the resistor is less and ZD will eventually conduct, switching off the charge current. The red LED will stay lit because it is being fed by the current from the adjustment pin but the drain on the battery is really tiny.

I have not actually built this circuit yet but have a few very similar that work well.

As Savio has noticed, the 8.2 volt value for ZD does seem a little low to get a switch-off at 14.2 as shown here. You may need to increase that and a quick guess would be 1.2 volt across the LED then another volt base-emmitter on T1 is only going to give you a little more than 10 volts at best before the zener kicks in and shuts the thing down. You may find it needs to be 12 volts to work to a full charge but as I say, I haven’t tested this yet.

Let me know how you get on with these changes.

[Irfan]

Thanks for the response,  Please guide as to where the negative bus is running and I intent to be usig a 18-0-18 transformer for the 12V7.6Ah SLA battery, in the above circuit the negative bus is not very clear to me.<br> I shall be building this in the coming week starting from scratch and since this is a busy week of festivities may be delayed, however I appreciate your efforts and promise to update you on the status of the circuit.

[Author]

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