Make Circuit diagram for battery charge controller using schmitt trigger

[Siddhat]

I am planning to make a battery charge controller for my project.What I am planning to do is that I am going to make a 12-V lead-acid battery charger that cuts-off the power when the battery is fully charged and again starts charging when the charge in the battery goes below to a certain level(say 30%) using operational amplifier using schmitt trigger.But i could not make a proper circuit that works.Can someone help me out with the circuit diagram using schmitt trigger??

[Doctorandus_P]

There is a lot of magic smoke inside battery chargers that you do not want to let escape.
- shor circuiting the battery,
- over / under charging...

And you will make mistakes during the design / build of your circuits. Everybody does.
A good Lead-acid battery charger is also a 2 step process:
1). Charge with limited / constant current untill the voltage reaches the maximum.
2). Charge with that voltage until the current drops below a threshold.

The second part is also important to fully charge the battery.

A schmitt trigger can be a part of a battery charger to start / stop the charge process
(Although I woud personally prefer a microcontroller, which also keeps track of (dis) charge currents etc).

Schmitt triggers can be made with even a simple single transistor, but it is easier to understand with an opamp. Start by making a few simple oscillator circuits with opamps. They almost always use a schmitt trigger of some kind. You can generally recognize this by any connection between the output and the non-inverting "+" input of the opamp.

The book "Opamps for everyone" slod006b is a free reference book from Texas Instruments and it is a very good refererence for everything related to opamps.
https://www.eevblog.com/forum/beginners/opamps-for-everyone-pdf/

Then later on in your design you will need some voltage references for the thresholds.
TL431 is a popular choise, but the LM317 (small TO92 ) is also a good candidate.
Chips like these are about a few bucks in a bag of 100 from Ali / Ebay.

[digsys]

.. or just use something like the BQ24450. It handles ALL battery charge / conditioning, simple to use and able to go to 5A plus.
There are several all-in-one SLA / (other chemistry) charger ICs out there as well. I used to use micros, but this is so much easier.
Plus you can start to learn of all the chemistry etc from application notes.

[rstofer]

That BQ24450 is a terrific device for valve regulated lead-acid batteries (SLA batteries).

There are thousands in stock at DigiKey at around $8 but the device is not recommended for new designs.  I imagine there is a replacement.

There is an evaluation board in stock (3) but it's pricey at $102.

[Siddhat]

Check out this circuit that I made earlier which only cuts-off the power when the battery is fully charged. I used a L7812CV voltage regulator , 12V zener diode, a BC547 transistor, a relay and some resistors to make the circuit.
But I seem to have no idea how to use a Schmitt trigger.

[Siddhat]

I am strictly focusing on Schmitt triggers using the op amp for this project.A friend of mine suggested me this op amp circuit but i donot know to implement it in a circuit.
Could you help me with the circuit by making one?
Appreciate all of your help.😀

[rstofer]

That schematic is just an op amp used as a comparator with feedback to give some amount of hysteresis.  It is a single supply circuit so that's why the + input comes from a voltage divider and the - input has a voltage divider to get the input into range.  The feedback resistor upsets the bias point of the + input causing hysteresis.

In effect, the op amp is a Schmitt Trigger.  It has a threshold in each direction due to the hysteresis created by the positive feedback.

Page 5 here:

http://www.analog.com/media/en/technical-documentation/application-notes/AN-849.pdf

The circuit assumes that the op amp output is capable of charging a battery and it might.  But it will do a particularly poor job when compared to commercial charging circuits and the real requirements for lead-acid batteries.

The BQ24450 datasheet gives a complete description of the manner in which batteries are supposed to be charged.  It is generally a 3 step process and particular care needs to be taken with deeply discharged batteries.

You mentioned discharging to 30% but you didn't say 30% of what.  Here is some charging information.  The site has a lot more battery related information.

http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery

The open circuit voltage of a Power-Sonic battery is 2.16 V/cell when fully charged and 1.94 V/cell when completely discharged.

Page 8 here:

http://www.power-sonic.com/images/powersonic/technical/1277751263_20100627-TechManual-Lo.pdf

That's not a very large voltage swing before the battery is no longer capable of delivering useful energy.

[Siddhat]

Yes, that is just an schematic of an op amp used as a comparator.But what I didnot understand is how do i actually use a op amp like that in a battery charger that could do the things I am planning to do?

I actually have not fixed it yet that 30% of what. I mean the the charge up to 30%. I guess.

[rstofer]

Maybe Google for 'op amp comparator battery charger'  There are over 1/2 million hits.
Here is a video on the subject but I haven't watched it