TL431 auto off for battery charger

The TL431 could be thought of as a high gain transistor with a (something like) 1.8V zener in series with the base.

When the reference pin is at or above 2.5V, the device switches on; less than 2.5V and it's off. It will try and short out the supply to hold the ref pin at 2.5V.

As shown in the thumbnail (I'm not watching the video) the relay will activate when the input voltage reaches a certain threshold.

That could disconnect the battery via the relay contact, at which point the charger input voltage may increase further, holding the reference higher still and locking the relay on, until power is removed.


However - it's a really bad idea for charging a battery.

For a normal 12V lead-acid, the charge voltage should be set to 14.8V for cyclic charge/discharge use, with a current limit so the charging power supply is not overloaded when a flat battery is connected.

Charging power should only be disconnected when the charge current drops down a fairly low level - which happens long after full voltage is reached. Alternatively, the voltage can be reduced to 13.8 and left at that on float charge.

For other battery types the principle is generally the same, though with different voltages etc:

Bulk charge at constant current, then the charge completes at constant voltage, with current reducing as the battery nears full capacity, then cut off totally when the current drops to a low threshold.

(13.8V is float charge voltage for a 12V lead acid, that can be connected permanently - no cut-off needed).

See the "Battery University" info on charging batteries of whatever type you are using.

To start with it's NOT a circuit, it's a useless drawing from someone who knows nothing about electronics.

In general these non-circuits are either non-functional or just useless, the Internet is full of them, and you should never try and follow one.

In this case there's no monitoring of the battery charge at all, so it doesn't work, end of story, it's just a random collection of components.

Assuming this is a 12V lead acid (car) battery your 13.58V supply is too low to charge it anyway, the nominal voltage of the battery is 13.8V, and you need probably 15V or more to charge it.

General charging considerations :


https://www.ti.com/lit/an/slyt769/slyt769.pdf

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rjenkinsgb said:

The TL431 could be thought of as a high gain transistor with a (something like) 1.8V zener in series with the base.

When the reference pin is at or above 2.5V, the device switches on; less than 2.5V and it's off. It will try and short out the supply to hold the ref pin at 2.5V.

As shown in the thumbnail (I'm not watching the video) the relay will activate when the input voltage reaches a certain threshold.

That could disconnect the battery via the relay contact, at which point the charger input voltage may increase further, holding the reference higher still and locking the relay on, until power is removed.


However - it's a really bad idea for charging a battery.

For a normal 12V lead-acid, the charge voltage should be set to 14.8V for cyclic charge/discharge use, with a current limit so the charging power supply is not overloaded when a flat battery is connected.

Charging power should only be disconnected when the charge current drops down a fairly low level - which happens long after full voltage is reached. Alternatively, the voltage can be reduced to 13.8 and left at that on float charge.

For other battery types the principle is generally the same, though with different voltages etc:

Bulk charge at constant current, then the charge completes at constant voltage, with current reducing as the battery nears full capacity, then cut off totally when the current drops to a low threshold.

(13.8V is float charge voltage for a 12V lead acid, that can be connected permanently - no cut-off needed).

See the "Battery University" info on charging batteries of whatever type you are using.

Click to expand...

Thank you for your sincere assistance. Look, I am 1 month away from being 66 years old. When I was 18-19 I went to school to "attempt" to start my BSEE degree - never happened. Then, I did a lot of self-study and learned a little bit. Younger people may say "just enough to be dangerous" - I hope not. Anyway, I spent a considerable amount of time trying to understand this. I breadboarded just the divider part (using a 5K trim pot) instead of the 2.2K resistor, the TL431, another 1K pullup resistor connected to the anode of an LED. Using the adapter as my source, when the reference voltage was less than 2.5 volts, the LED was lit - but dimly. I understand that this "might" be because there is a slight leakage current through the TL431. Yes, you are correct - when I increased the reference voltage to 2.495 volts the TL431 properly shunted and the LED lit normally. Now, I remove the 1K pull up and the LED, but left the reference voltage at 2.495 volts. Then I placed my meter between the positive (+) source and the cathode (pin3) of the TL431. When I powered up the "circuit",

rjenkinsgb said:

The TL431 could be thought of as a high gain transistor with a (something like) 1.8V zener in series with the base.

When the reference pin is at or above 2.5V, the device switches on; less than 2.5V and it's off. It will try and short out the supply to hold the ref pin at 2.5V.

As shown in the thumbnail (I'm not watching the video) the relay will activate when the input voltage reaches a certain threshold.

That could disconnect the battery via the relay contact, at which point the charger input voltage may increase further, holding the reference higher still and locking the relay on, until power is removed.


However - it's a really bad idea for charging a battery.

For a normal 12V lead-acid, the charge voltage should be set to 14.8V for cyclic charge/discharge use, with a current limit so the charging power supply is not overloaded when a flat battery is connected.

Charging power should only be disconnected when the charge current drops down a fairly low level - which happens long after full voltage is reached. Alternatively, the voltage can be reduced to 13.8 and left at that on float charge.

For other battery types the principle is generally the same, though with different voltages etc:

Bulk charge at constant current, then the charge completes at constant voltage, with current reducing as the battery nears full capacity, then cut off totally when the current drops to a low threshold.

(13.8V is float charge voltage for a 12V lead acid, that can be connected permanently - no cut-off needed).

See the "Battery University" info on charging batteries of whatever type you are using.

Click to expand...

Ok, I am going to try to type everything I said again. I want to thank you for your sincere reply. I had originally said that I am a month from becoming 66 years old. When I was younger, I attempted (never finished) a BSEE degree. Since then, I have had several jobs and continued to learn things on my own. I spent some considerable time trying to understand this. I didn't tell you (or anyone) that I am trying to use lithium ion batteries. I fully understand the dangers, but I just cannot seem to find anything else small and inexpensive. My whole project is a simple emergency lamp project. It uses "another relay" to keep 12 vdc LED lamp OFF. I accomplish what I do with DPDT switches and the adapter. The original adapter I was using was not 13.58 volts, but the adapter was listed as having an output of 12 vdc - the no-load voltage was 15.78 volts. I was "charging" the 2- lithium battery packs (in parallel to last longer) by the straight up voltage - switched - directly to these battery packs. I never let it run for more than about 1/2 hour, I never had any issues. Now, I breadboarded just part of the circuit - the voltage divider (* I replaced the 2.2 K resistor with a 5K trim pot), the TL431, another 1K pullup resistor and an LED. When the reference voltage was less than 2.5 volts, the LED was lit, but dimly. I had "guessed" that this was because there was a slight leakage current through the TL431. When I increased the reference to 2.5 volts, the LED was lit normally. I left the reference set at 2.5 volts and disconnected the power. Then removed the 1k pull up and the LED. I then put my meter between (+) and the cathode (pin 3) of the TL431. When I first powered up the circuit, my meter read 12.26 volts. As I dialed-down the reference - the voltage dropped to 11.99 volts and stayed (no matter how low the reference continued to drop). The relay that I attempted to use in this "circuit" has a must-activate (pull in) voltage of about 9 volts. This might be why when I was attempting to do something with this using my bench power supply set to 14 volts, the relay chattered crazily. I was "hoping" that this would work good enough that the charging would fully stop, light the charged LED so that I could then switch the lamp back to stand-by use (power the lamp relay to keep the LED light bulb off). If anybody else is reading this, then you have signed on to this forum to help people. Maybe originally you did - but not now. I am asking you to either 1) continue to be helpful while you want to keep coming back to this forum 2) delete or remove your membership 3) stop making rude and unnecessary / unwanted replies to people's posts here. The people that post here usually have honest / legitimate questions about things they want to understand. If you think about this - at one time YOU DID TOO. Best wishes for 2025 EVERYONE!

oldNewbie7632 said:

Ok, I am going to try to type everything I said again. I want to thank you for your sincere reply. I had originally said that I am a month from becoming 66 years old. When I was younger, I attempted (never finished) a BSEE degree. Since then, I have had several jobs and continued to learn things on my own. I spent some considerable time trying to understand this. I didn't tell you (or anyone) that I am trying to use lithium ion batteries. I fully understand the dangers, but I just cannot seem to find anything else small and inexpensive.

Click to expand...

One word - DON'T - charging of Li-Ion is absolutely critical, attempting to use a non-working drawing (presumably by a child?) of a supposed lead acid battery charger is suicidal. Li-Ion batteries can easily set on fire if not correctly charged, don't attempt charging with anything other than a correctly designed (and constructed) charger.

Basically you're throwing shotgun shells on an open fire, and hoping they don't explode.

This is a link to a site with an Arduino based charger, it describes what's required, and the software listing shows how he implemented it - notice that much of the code is for safety checks. If nothing else, read (and follow) the paragraph "CC-CV Charging"

oldNewbie7632 said:

The output voltage of the adapter is 13.58 volts.

Click to expand...

So at what voltage do you want to stop the charging?

Instead of blindly trusting random YouTube videos you should instead download the Texas Instruments data sheet for the TL431.
At the very least…Fully read AND UNDERSTAND the whole section 9.2.1 but I would read the whole data sheet.

If you're interested, I’ve made an animated video explaining how the TL431 works and its practical applications. It might help clarify things for you. You can watch it here: