# Help With LiPo Battery Circuit

Discussion in 'The Projects Forum' started by FedgeSniper, Dec 29, 2015.

1. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0

I am wondering if I could use 4 of the TL431 to measure the voltage from a 1s-4s LiPo battery. I would like I to cutoff around 3.2v per cell. I don't know what size resistors I would use to accomplish this. I am new to this sort of thing. The extent of my knowledge is wiring LEDs in series and a set value voltage regulator. I am wondering if it would be easier to use a POT?

2. ### Colin55 Member

Aug 27, 2015
204
21
The circuit consumes current ALL THE TIME.
You can put one of these across each battery and change the value of R3

3. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
So R3 is the only resistor I need to change? Is there a way to add a relay that will cut the power off?

4. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,142
768
Tl431 is a programmable zener diode, it conducts when the voltage across pins 1,2 gets to 2.5V, then pin 3 is pulled low, so the leds will be off until it reaches 2.5V.

5. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
What do you mean by "Programmable"? How would you program it?

6. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,142
768
By the resistor ratios of R2/R3,

The strike voltage is 2.5*(1+R2/R3)

Which makes R3 16.7K for a 3.2V supply

7. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
Its the basic equation for a voltage divider.

The TL431 is basically a comparator with a built in 2.5V Vref. If you take the input above that value; the cathode will clamp down to about 2V. Your circuit has a lot of nfb, which pretty much runs the comparator in linear mode - the cathode will stabilise the voltage at the top of the 2 series resistors at whatever value puts 2.5V at the tap.

The TL431 is commonly referred to as a; "programmable zener" - you programme it by calculating the ratio of the 2 resistors.

The circuit posted isn't suitable for driving LEDs because it supplies constant voltage and not constant current.

8. ### Colin55 Member

Aug 27, 2015
204
21
"The circuit posted isn't suitable for driving LEDs because it supplies constant voltage and not constant current."
What are you talking about ????????????????

9. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
I am not looking to power LEDs I am looking for a simple way to create a LiPo cutoff circuit.

10. ### Colin55 Member

Aug 27, 2015
204
21
This is NOT a cut-off circuit. It is a "power 2 LEDs" circuit.
A cut-off circuit is much more complex and much more expensive.
It is better to throw out the \$2.00 cells.

11. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
Than what would you suggest I use to achieve a cut off circuit?

12. ### Colin55 Member

Aug 27, 2015
204
21
Look up cut-off circuits on the web. I am not your nanny.

13. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
Thank you for your time. I wonder if there is anyone else that can help me with this circuit. I have researched cutoff circuits and have found a lot of dead ends.

Thank you

14. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
I have been using a TL431 shunt regulator for a couple of years in an e-cigarette 18650 charger. If you want to charge multiple cells - you *MUST* give each cell its own TL431.

The TL431 can handle 100mA max, dissipation isn't a problem at only 4.2V. With the cell disconnected, the TL431 has to handle all the current set by the supply voltage and the current limit resistor.
If you run the TL431 close to its limit, you must allow for the possibility that it could fail and short the cell - my solution is to include a diode in series to prevent back flow, the shunt voltage has to be calibrated to take the diode Vf into account.

You can boost the TL431 current handling by adding a PNP emitter follower in the manner of a Sziklai pair - the transistor must be inside the nfb loop.

15. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
Thank you for the reply. I really don't need to charge the battery I need to protect it from over discharging.

16. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
There is a voltage threshold detector circuit in some versions of the TL431 appnotes - simply use that to control a power management switch.

My e-cig has no over discharge protection whatsoever - using 2 cells in parallel for the high current draw, has made the difference between a few months life and over a year.

Any detriment caused by over discharging, has so far been insignificant compared to cell loading effect.

17. ### FedgeSniper Thread Starter New Member

Dec 19, 2015
15
0
if you look at page 28 there are some system examples. I am wondering if any of these schematics are close to what I am trying to achieve?

18. ### ian field Distinguished Member

Oct 27, 2012
4,447
791
Couldn't find the circuit example I was thinking of in that document.

The 431 is produced by a large number of manufacturers, most published pretty much the same version of the appnotes, but there's about 2 or 3 versions in circulation.

TL is basically a TI prefix - the 431 base number can turn up with various prefixes, KA431 is one I can think of off hand.

Dec 19, 2015
15
0