Undervoltage battery protection with Relay

Thread Starter

horilyts

Joined Oct 22, 2020
9
I want to design an undervoltage protection circuit for my battery. The battery is 24V and I want the relay to cut the current when the voltage is under 19V but I have never used a relay before and I'm kind of struggling with using the relay and hot to set the under-voltage limit. Can anyone help me with it?
 

Ya’akov

Joined Jan 27, 2019
6,547
I want to design an undervoltage protection circuit for my battery. The battery is 24V and I want the relay to cut the current when the voltage is under 19V but I have never used a relay before and I'm kind of struggling with using the relay and hot to set the under-voltage limit. Can anyone help me with it?
Welcome to AAC.

Do you specifically want to build something? I ask because many variations on this are available for about what parts might cost you. It even has a nice DIN rail form factor.
 

LowQCab

Joined Nov 6, 2012
2,499
Well, everybody has to start somewhere .........
A Relay,
while it certainly could be made to work with some additional parts,
is usually not the best solution.

How much Current does your Load draw under the worst conditions ?

A P-Channel MOSFET Transistor, and a TL431-Voltage-Reference, and 3-Resistors,
would be a far more elegant solution, ( and usually much smaller ).
You would normally have to have a similar Circuit to drive the Relay in any case.

Are You ready to get your feet wet and smoke a few parts in the learning process ?
.
.
.
 

Ian0

Joined Aug 7, 2020
6,281
How do you want the circuit to reset? Willyou need to press a switch to turn it back on again? Or should it turn back on automatically? (Don’t forget that a battery voltage has a tendency to rise as soon as the load is removed)
Is it important that the circuit draws zero current after it has tripped? If you are disconnecting the battery to protect it, you don’t want it still to be drawing current and further damaging the battery just to run the protection circuit.
 

Dodgydave

Joined Jun 22, 2012
10,484
Here is a quick circuit diagram using a TL431 zener, (the resistors are 60K and a 10K preset ) set the preset to 19V or when the relay just drops out, RH is the hysteresis resistor, I've made it 1M , but you can lower it to give you a swing of 1 to 2v.
If you have a simulation software you can try it first.
IMG_20220720_190918.jpg
 

Thread Starter

horilyts

Joined Oct 22, 2020
9
Well, everybody has to start somewhere .........
A Relay,
while it certainly could be made to work with some additional parts,
is usually not the best solution.

How much Current does your Load draw under the worst conditions ?

A P-Channel MOSFET Transistor, and a TL431-Voltage-Reference, and 3-Resistors,
would be a far more elegant solution, ( and usually much smaller ).
You would normally have to have a similar Circuit to drive the Relay in any case.

Are You ready to get your feet wet and smoke a few parts in the learning process ?
.
.
.
It doesn't need to draw much current, 15Amps maximum. I actually searched for tlc431 but the window voltage detector applications outputs the Vref 2.5V in my simulation just as in the datasheet and I couldn't manage to output the 19 Volts from it...
 

Thread Starter

horilyts

Joined Oct 22, 2020
9
How do you want the circuit to reset? Willyou need to press a switch to turn it back on again? Or should it turn back on automatically? (Don’t forget that a battery voltage has a tendency to rise as soon as the load is removed)
Is it important that the circuit draws zero current after it has tripped? If you are disconnecting the battery to protect it, you don’t want it still to be drawing current and further damaging the battery just to run the protection circuit.
yes, I'd like to cut the battery current from the load completely.
 

crutschow

Joined Mar 14, 2008
30,760
Below is the LTspice simulation of DD's basic circuit in post #5.
It removes power to the load with the circuit staying connected to the battery which draws only about 1/4mA.

R1 is a lower value so the TL431's quiescent current doesn't turn Q1 on.
Q1 was changed to the PNP 2N3906 (the 2N2222 is an NPN although shown as a PNP).
There is a little over a volt hysteresis from the turn-off to the turn-on point as determined by R4.

1658855151792.png
 
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Ian0

Joined Aug 7, 2020
6,281
If you move the relay to between Vb and R2, then it will consume no current when disconnected. It will need a push button switch across the relay to restart it, but doesn’t need any hysteresis (R4)

It may also be worth adding a circuit to reduce the coil current whilst in operation - use a DPDT relay and use the second set of contacts to switch a resistor in series with the relay coil. As relays don’t drop out until the voltage is about 30% of the rated voltage, use a resistor that is about the same value as the coil resistance. That will reduce the power by half.
 

crutschow

Joined Mar 14, 2008
30,760
Here's the simulation with the added coil resistance R6, to reduce the operating current by using the extra contacts of a DPDT relay to short out the resistor when the coil is initially energized, as Ian0 suggested.
The 1kΩ resistor is close to the coil resistance of a typical 24V relay.
As can be seen, the relay coil voltage is reduced expect for a few ms when the relay is first energized (red trace).

1658864231093.png
 

Ian0

Joined Aug 7, 2020
6,281
Drifting slightly off topic here, because I realise that the spec is for a 24V battery, but if the supply were a bit lower, then there’s a neat 555 circuit.
D8A9AAC2-E94B-4D39-970D-5624A412E0BA.jpegThis will switch off at 10.5V and back on at 12V.
Either of the discharge or output pins can drive a low current relay connected to V+.
Alternatively, Pin 3 could drive a P-channel MOSFET instead of the relay - with a CMOS 555 it is a low power solution.
 

crutschow

Joined Mar 14, 2008
30,760
I realise that the spec is for a 24V battery
You could, of course, use a LM317 or similar to regulate the voltage to say 12 or 15V.
Only the 3 resistor string would be connected to the 24V, with values selected to get the desired trigger levels.

With that you likely wouldn't need the LM4040-5 since the regulator will provide a fairly stable reference voltage.
 

Ian0

Joined Aug 7, 2020
6,281
I think it only qualifies as "neat" when it's a single-chip solution. When it needs the regulator the TL431 circuit is neater!
Of course, if there just happened to be a 12V regulator somewhere else in the circuit. . . .
 

Thread Starter

horilyts

Joined Oct 22, 2020
9
The battery needs to be 24V since the motors are driven with 24volts, after the protection circuit there will be regulation but I specifically wanted to connect the protection just after the battery. Thank you for the answers!
The thing is, I actually managed to have the output I wanted without the relay using LTC4368 from analog devices, but when I make a dc analysis through LTSpice, there is always a voltage rise duration, where I get i.e 12Volts when applied 19.4Volts and it rises after. This can damage the battery as far I know so I want to cut the power exactly at 19Volts and I don't want to have a threshold for it. Am I doing something wrong or is it just the way these ICs work? That is why I thought using a relay would be convenient but I'm not sure.
 

Thread Starter

horilyts

Joined Oct 22, 2020
9
I don't mean the voltage rise time, I know it's natural.
Here in this DC analysis for example(don't mind the values please, the screenshot is from when I didn't optimize the voltages), I won't get 0Volts just right before 22Volts as you can see. I need to apply ~21 volts to get the 0Volts but what if the battery will output 21.4V? Then I can get ~16V from the IC . I don't want that, I want the voltage to be cut off directly
 
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