Very Simple Automatic 12 volts battery charger[Designed but need modification]

Thread Starter

ShayanFiroozi

Joined Nov 18, 2015
41
Hi,
i have a very simple automatic 12 volts battery charger and working very well in a real application.
it's perfect to charge 12volts 12AH SLA batteries.
i didn't designed this circuit(as i'm pretty beginner in Electronics !) but i draw this circuit diagram from a real PCB

now i need some modification , so hope experts help me :

1-i need to add 2 LED's to this circuit first LED to indicates charging and second LED to indicates battery full charge

2- when the main fails or unplug for a long time(for example 1 or 2 months !) the circuit will drain the battery , i think we can not use diodes because circit can not detect battery voltage (because diode will block battery current to come back to the circuit)so its automatic behavior will not work.

any solution and modification will be appreciated.

sorry for Bad English and Thank all of you.
 

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Thread Starter

ShayanFiroozi

Joined Nov 18, 2015
41
But I,m using this charger from 1 year ago to now!! Working well and it stops charging when battery get full
Also I didn't designed this circuit,it,s a real circuit which I bought and then draw the schematic from it
 

k7elp60

Joined Nov 4, 2008
561
I can show you some modifications that will let you put a diode in the output so the charger will not discharge the battery if the AC fails, and a led will come on when the charge current decreases to an amount that the battery is charged. Also ad a LED to show power is on. Both of these modifications require a good DVM to make the adjustments. If you have a good digital volt meter let me know and I will post the circuit and the adjustment proceedure.
 

AnalogKid

Joined Aug 1, 2013
8,259
Alec isn't saying the circuit doesn't work, he is saying that it doesn't work the way you think it does, and that adding the LEDs you want is not a simple change.

First, the circuit doesn't monitor the battery voltage directly. It makes an output voltage, measures the current, and adjusts the output voltage to keep the current at or below a 1.25 A. Adding an output diode in series with the output will not affect the current monitor circuit, and is the right way to prevent the circuit from discharging a connected battery when unpowered. This will decrease slightly the maximum output voltage, with will increase slightly the time it takes to recharge a dead battery. If this is a problem, R3 can be increased to compensate for the diode forward voltage drop. Your maximum output voltage now is 13.75 V, and the diode forward voltage will be around 0.8 V to 1.0 V at 1.25 A. Increasing R3 to 2.2K should work.

To add the two LEDs, you need one or two comparators, depending on how accurate you want the LED indications to be. This will take some perf board, one chip, and about 8 to 10 resistors, capacitors, and diodes.

ak
 

Dodgydave

Joined Jun 22, 2012
8,683
Since its a constant voltage charger, i would use an op amp comparator like lm3914 to monitor the current change across the series resistor, as the current drops the battery voltage rises, simply adjust the comparator reference to suit the voltage range.
 

Thread Starter

ShayanFiroozi

Joined Nov 18, 2015
41
Thank all of you friends
So I forget about Leds!!
In current circuit can I attach my sla battery permanently?? Or it will damage my battery??
 

k7elp60

Joined Nov 4, 2008
561
I think a diode is needed on the output lead in series with the 3A fuse, this is in case the AC power goes off. Yes you would have
increase the charge voltage to compensate for the drop across the diode. I would make the total value of R3 2.1K ohms.
My calculations also the current output is 13.75V. This is within the range of float chargers for 12V lead acid batteries.
I calculate the current limit to be very close to 1.4A. The LM317 data sheet read indicated the automatic current limit is
between 1.5A and 2.2A.
At the current charge voltage of 13.75V the battery is fully charged when the charging current drops to AH x .01. In this case
with a 12AH battery that is 120Ma. So a led to indicate fully charged would be helpful.
 

AnalogKid

Joined Aug 1, 2013
8,259
Is the current limiting circuit really necessary? That is about what the LM317T is set to anyway, isn't it?
Yes, but it is far more accurate, more repeatable across multiple builds, and adjustable by changing the shunt value. Overall, I'd keep it. Plus, there is the circuit mod where a higher value pot is placed across the shunt to make the current limit truly adjustable

ak
 

hp1729

Joined Nov 23, 2015
2,304
Yes, but it is far more accurate, more repeatable across multiple builds, and adjustable by changing the shunt value. Overall, I'd keep it. Plus, there is the circuit mod where a higher value pot is placed across the shunt to make the current limit truly adjustable

ak
Super! Thank you.
 

Thread Starter

ShayanFiroozi

Joined Nov 18, 2015
41
Leave it connected, it will just go into trickle mode.
Thank you man !
but i'm using this circuit for a battery backup so my 4.5 AH battery should always be connected to the this charger , also i'm using 600 milli ampere 16 volts transformer , is battery safe in such situation ?
if not what is your suggestion

thank you again :)
 

Dodgydave

Joined Jun 22, 2012
8,683
Don't think it will work, the transistor will pull the adjust pin higher increasing the out voltage and current, i think the idea is to pull the adjust pin to ground to stop it.

Also there is no protection if a 6v battery is put on 12v...
 

Thread Starter

ShayanFiroozi

Joined Nov 18, 2015
41
Don't think it will work, the transistor will pull the adjust pin higher increasing the out voltage and current, i think the idea is to pull the adjust pin to ground to stop it.

Also there is no protection if a 6v battery is put on 12v...
So it seems my first circuit is better for a battery backup , i'm going to make it
but still afraid to plug a battery permanently to it , i know you told me there's no problem,but many others told me it's not safe and not efficient :(
 

Dodgydave

Joined Jun 22, 2012
8,683
ALL sla chargers are set to 13.8v for a 12v battery in intruder alarms, these are left on permanently connected to the battery, they wont blow up your battery.
 

Thread Starter

ShayanFiroozi

Joined Nov 18, 2015
41
ALL sla chargers are set to 13.8v for a 12v battery in intruder alarms, these are left on permanently connected to the battery, they wont blow up your battery.
Thank you for helping me sir
but my first circuit in this Thread seems to be not automatic charger , so if i leave it permanently with battery , is it possible to be dangerous or reduce my battery life ?

my battery is 2.2 AH 12 volts and i'm using 200 milliapmere 16volts transformer

Thank you again , i'm really confused
 
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