Battery discharger circuit

Thread Starter

CineDroid

Joined Jan 25, 2020
20
Hi all,

I'm working on a circuit that automatically discharges batteries I have up to a certain voltage - let's call it Vcut. I've opted for biasing a Mosfet with a zener diode. That Mosfet in turn regulates the current of a Relay, switching between a load resistor and a "job done" LED. I've attached a rough schematic and the .txt file generated by Falstad Circuit Simulator.

I have three questions:
- I'm still super new to electronics, so anybody seeing blatant mistakes, please comment.
- Second, when a battery is disconnected from the load, the voltage at the terminals gradually rises back up. In this case, that would mean the circuit would endlessly go from open to close to open to close when reaching Vcut. Any leads on how to address this in a simple way?
- Third, as simulated, it appears the relay is connected to neither poles between battery voltages 21.85V and 21.68V. Any suggestions on how I can clean this up?

Thanks.
 

Attachments

KeithWalker

Joined Jul 10, 2017
1,857
The discharge LED will turn on and indicate when the circuit is discharging the battery. It will turn off when the discharge is complete. It is not really necessary to have the second LED.
Bearing that in mind, you can have the relay disconnect the battery completely at the end of the cycle. This will solve your other problems. I have attached a modified schematic for you.
Regards,
Keith
New-1.jpg
 

crutschow

Joined Mar 14, 2008
27,459
In your circuit, the cutoff point depends upon the gate threshold voltage of the MOSFET as well as the Zener voltage.
For more accurate and stable control, you could use a voltage reference, such as a TL431, as the comparator.

What is the voltage range you want for Vcut?
 

Thread Starter

CineDroid

Joined Jan 25, 2020
20
such as a TL431, as the comparator.
I'll look into that

What is the voltage range you want for Vcut?
It's 19V in one configuration and 8V in the other.

you can have the relay disconnect the battery completely at the end of the cycle.
In this configuration, the resistance of the circuit is too great to get any significant current through the load resistor. I'll try and tinker with DPDT relays maybe so that the load resistor and the coil don't have to be in series.
 

crutschow

Joined Mar 14, 2008
27,459
It's 19V in one configuration and 8V in the other.
There's a problem with that wide range if you are powering the relay from the battery.
Either the relay won't work at the lower voltage or it will overheat at the high voltage.
It might be better to use a power MOSFET instead of a relay.

What is the maximum battery discharge current?
 

Thread Starter

CineDroid

Joined Jan 25, 2020
20
There's a problem with that wide range
It's not really a range. The two cut off points are user selectable.
the 8V cut-off discharges the battery through a 130Ω load resistor (180mA slow discharge / deep discharge operation)
the 19V cut-off discharges the battery through a 30Ω load resistor (0.8A "working" discharge / nominal discharge operation)
Ultimately they might be two different circuits.
 

crutschow

Joined Mar 14, 2008
27,459
Ultimately they might be two different circuits.
My point was that one relay won't handle such a wide voltage range.
Each circuit would need a different voltage relay.

Below is the LTspice simulation of an example cutoff circuit using a TL431 programmable shunt regulator as a comparator.
The 431 turns on when the Vref voltage is above 2.5V and turns off when it drops below 2.5V.
The U2 pot gives a Vcut range of about 20V to 6.4V.

1580360831150.png
 

Thread Starter

CineDroid

Joined Jan 25, 2020
20
@crutschow I think I understand what you're saying. In the theoretical and simplified world of Falstad the relay works with any voltage and we can connect the coil directly to the battery terminals no problem - while in reality have coil voltage specifications, and just looking at Amazon quickly seems like 5V is fairly common.
 
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