Design advice / help for battery low voltage cut off

Thread Starter

dustin1706

Joined Apr 14, 2021
4
Hello,

I'm very much a complete amateur at circuit design, but find it fascinating. I am attempting to come up with a simple circuit for a low voltage battery cutoff. I've found several designs that exist (and would work fine, honestly), but haven't found one that is exactly ideal and it's bugging me that I can't seem to figure it out.

In short, I plan to use Dewalt 20v batteries to power a few different things as I have a ton of the batteries and chargers and this would be convenient. The Dewalt batteries contain no BMS of any kind as that circuitry resides in each tool / charger, so it's just a basic 5s Lipo. I do not want to use an off the shelf BMS board as I am adding a few different connections, 5v USB charging, etc. and would like to be able to have it all on a single veroboard or PCB for simplicity of mounting in a 3d printed case that attaches to the batteries.

My criteria for the low voltage cutoff are as follows:
  • As simple as possible.
  • Operates up to 22v and trips at ~16v.
  • Low power draw while on.
  • Deals with the potential hysteresis.
  • Don't care if it utilizes a single momentary button, individual on/off buttons, or a switch.
  • Ideally zero power draw once the low voltage cutoff trips, or extremely low at the very least.

The last requirement is the one that has been giving me trouble. A perfect circuit would trip, and then stop drawing any current at all. It's extremely unlikely that the low voltage would trip, then be left attached to the battery for months, but not inconceivable. It's certainly overkill, but seems to be an interesting design challenge.


A few of the solutions I've found and played with on a bread board:

Dave Jones soft latching power switch circuit - This is very clever, but in practice the momentary switch is a bit finnicky and you have to press it for the "right" amount of time to turn on or off depending on your resistor/capacitor choice. Also, it uses standard NPN BJTs which all seem to have an emitter to base voltage limit of ~6v.

David Watts LVCO - builds on Dave's soft latching circuit, but integrates a TC54 voltage detector for the LVCO portion. Same issues as above.

There exist many 555 timer and TL431 / TLV431 based circuits that I've found on this forum and elsewhere that are effective, but in each case there ends up being one or several paths from the positive of the battery though a few resistors to ground which creates some current draw when off.
ie: https://forum.allaboutcircuits.com/threads/need-help-tlv431-low-voltage-cutoff-circuit.172831/


So given all that information, my questions come down to:
  1. Is it possible/reasonable to design a circuit that draws no current once it trips but doesn't have the odd single pushbutton behavior? [Again, being fully aware that a draw of a few mA isn't really a big concern, but curious if it's possible.]
  2. What's the best way to deal with the 6v limitation of standard BJTs? Using a voltage divider for each seems awkward, using FETs is much more expensive, is there a simpler solution?

I've got a variety of BJTs, FETs, relays, etc. etc. on hand and can easily order more or less anything from Mauser or Digikey as I am in Canada.

Thanks!
 

crutschow

Joined Mar 14, 2008
29,765
Here's the LTspice simulation of a TL431 circuit that draws <3mA when active and only leakage current (<1µA) when the output is cut off:
Pot U2 allows adjustment of the cutoff voltage (yellow trace, showing tripping at 16.1V for a 50% pot setting).
The output stays cutoff even if the voltage rises back to normal, so the circuit can be reset back on by momentarily pressing PB S1 (red trace at 0.8s).

If you want to be able to turn it off manually, you can add another push-button between Vb and the MOSFET gate (G).
Thus the circuit can double as an ON/OFF switch.

1618462618357.png
 
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Thread Starter

dustin1706

Joined Apr 14, 2021
4
Thank you both very much for the replies. I had not come across that particular circuit nor the ICL7665S in my searching. I'm going to give both solutions a try.

Thanks again!
 

andaanda

Joined Aug 17, 2021
1
Hi, I have the exact same requirements. I want to use my collection of dewalt batteries to build small power packs for when I am on the road.

Which design did you end up using?
 

LowQCab

Joined Nov 6, 2012
2,037
Both Circuits are very similar in their operation.
Both Circuits require a Push-Button to initially turn them "On".
My Circuit also includes an "Off" Push-Button.
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.
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LowQCab

Joined Nov 6, 2012
2,037
Maybe,
I don't have time for it right now.
Figure out if You can combine this
One-Switch-Push-On-Push-Off-Circuit with the UVLO-Circuit.
At first glance, It doesn't seem to be worth the extra Components required.
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.
Push Button 3 .png
 

LowQCab

Joined Nov 6, 2012
2,037
I prefer my Buttons / Switches to have only one function, and be clearly labeled.
I've got more important things to do than to try to remember Button-Press-Sequences
for ~35 different devices.
Additional Switches aren't that expensive unless you're into counting Pennies
down to 4 decimal-places in a mass-production-scenario, in a very competitive field.
.
.
.
 
Thank you for the help, additionally I can't find the IXTR170P10P-ND easily for a reasonable price, is there an alterative you would recommend?

The desire for a single power switch comes down more to space constraints than any cost saving measures. Would it be possible to use these circuits or similar with a latching power switch? Say when the circuit trips the low voltage detection, it requires the use to toggle switch off then back on to start it back up? Ideally a switch similar to this.

crutschow uses the fds6575 which is out of stock through JLC but I can pick them up on element14.

Additionally I would like to use this circuit to monitor another input and only provide power if the input is high.
I am wanting to use this for power tool batteries, they have a pin for the temperature cut-off, this pin is high (pack voltage (18V)) when the battery is safe to use and low (0V) when thermal sensor has kicked in.

Would it be as simple as using the temperature output to power this circuit while removing the connection as drawn above to the mosfet and then with the main pack 24V going through the mosfet?
 

LowQCab

Joined Nov 6, 2012
2,037
IXTR170P10P-ND
Is a huge ~100-Amp P-Channel FET that was used for a different project.
You can use virtually any P-Chan FET that has specifications that
can handle your particular Load, (which You have not specified).
This FET is likely to be able to handle anything You can throw at it,
(provided that it's mounted to an adequate Heat-Sink),
therefore, it's a safe-bet choice.
Batteries can create a serious amount of Current and easily smoke under-rated parts.

2-Push-Buttons and an LED Indicator,
take up less space than the Switch You linked to, and the cost is roughly the same.
And, it's easier to make 3- small round holes than to cut a perfect rectangular hole.
There are hundreds of Switch designs to choose from,
why does it have to be a Single-Throw-Illuminated-Rocker-Switch ?

The extra Over-Temp-Inhibit function just needs
a Diode to pull down the Voltage-Divider Voltage,
just like the function of the "Off" Button.
The Diode is only necessary because I don't know
what else the Sensor may be connected to.
You will also need to add a Current-Limiting-Resistor to the "On" Button
to prevent the Temp-Sensor from creating a Short-Circuit to the Hot-Rail if
someone should push the On-Button while the Sensor is active, (Low).
A 1K-Resistor will work just fine.
It should have been included in the original design anyway, just for extra safety.
.
.
.
 

Thread Starter

dustin1706

Joined Apr 14, 2021
4
I never could quite get the separate on/off switches or a single button for both working along with a latching low voltage cutoff at a range of voltage levels. I tried a variety of resistor and capacitor values, but just couldn't dial it in. I ended up with a compromise where there's an on/off switch, but also a push button to turn it on. But it functions as intended and latches off.

I used a TL431A and a 2SJ661, a DC-DC 5v converter with built in current/voltage/temp protection as a 5v source for a usb outlet (MEZD71202A-G ), and a generic aliexpress voltage display. Not my prettiest work, but it is functional and I sure prefer it to butane soldering torches for work in tight spaces / automotive / etc.




 
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