Battery Discharge Cut-off Control Schematic

Battery Discharge Cut-off Control

This circuit prevents over-discharge of a lead-acid battery by opening a relay contact when the voltage drops to a predetermined voltage (lower voltage threshold).
When the battery is recharged to a second predetermined higher voltage (upper voltage threshold), the relay contact automatically re-closes and power again flows to the load. Both lower and upper voltage thresholds are independently adjustable to the desired voltages. Preventing the over-discharge of batteries increases the service life significantly.

Battery Discharge Control Schematic

Battery Discharge Cut-off Control Schematic

Experiment of discharging a 6V 4.5Ah lead-acid battery

Relay contact of solid state switch

While the use of a solid state switch is perhaps a more advanced technology, it tends to introduce significantly higher voltage drop than a relay contact. This voltage drop is exacerbated by the high 20A load current rating. For instance, a very good MOSFET switch has a Rdson of 0.016Ω while a good relay offers only 0.005Ω contact resistance – the corresponding voltage drops at 20A are 0.32V for the MOSFET and 0.1V for the relay contact – the difference is significant!

Relay selection

The main drawback of using a relay is its coil power consumption – any power consumption increases the rate of battery discharge. As a result, I searched for the most cost effective power relay with low power dissipation. The result of the search was a Panasonic PCB automotive relay with 30A contacts, 0.005Ω contact resistance, low 38mA coil current and $5.02 price tag that is available from DigiKey – more details are noted on the schematic.

Not having this relay on hand, I experimented with an Omron G8P series relay – it does the same job, but runs at a higher coil current. Of course, the relay selection is yours – you may not need such a high current rating – just keep these tradeoffs in mine.

Further reducing relay coil power

To further reduce the relay coil current, I employed a relay economy technique described in a previous post. (fig 4). Adding two common components reduces the relay coil voltage and current to 70%. Also, when the relay drops out, the control circuit current drops to about 4mA thus minimizing battery drain.

The occasion of this circuit

If this circuit appears similar to previously posted circuits, you are observing correctly. The reason for this is that the requirements are similar and this basic circuit has exceptional utility, simplicity, straightforwardness and cost-effectiveness – it requires only one inexpensive IC and a few garden variety components.

Setting the voltage thresholds

Note that this is best set up using an adjustable DC power supply. If you do not have an adjustable voltage power supply, but are handy at math, you can measure the attenuation of the R5 & R6 accurately and then calculate the required voltages out of the potentiometers. Keep in mind that the upper threshold may not be set below the lower threshold or the circuit is dead in the water.

Setup using an adjustable DC power supply:

  1. Initially, set the lower limit CCW and the upper limit CW and apply power.
  2. Set the adjustable power supply to the desired upper limit voltage.
  3. Rotate the upper limit pot CCW until the relay picks up.
  4. Then set the power supply to the desired lower limit voltage.
  5. Rotate the lower limit pot CW until the relay drops.

Refer to the battery voltage table on the schematic for determining your best settings – there is no easy “one size fits all” setting – you must evaluate your situation and experiment. I believe that the potential fickleness caused by series battery resistance is the very reason why there is a limited commercial offering for such a product – see next section…

Note that limiting the battery discharge to 50% of depth of discharge will greatly extend battery service life. Increasing battery capacity or paralleling batteries is usually a good idea.

Effect of high battery source resistance – oscillation

Battery cut-off controls are subject to oscillation if the lower and upper limits are set too close to each other. What happens is that when the load is disconnected at the low battery voltage limit, the battery is no longer loaded and the voltage tends to rise – if the voltage rises to the upper voltage limit, the control again applies power to the load which again pulls the voltage down to the lower limit – so if the relay starts clicking and the LED flashing slowly, you will know what is happening. The are four potential solutions: 1. set upper threshold voltage higher, 2. reduce load current (if you have any control over it), 3. use better or new battery that has lower source resistance, 4. parallel batteries to reduce source resistance.

Load shedding

One interesting application is load shedding in which there are multiple loads – some that are vital to keep powered and others that are not. This may require more than one control, depending upon priority and number of loads. In this case, the most critical device would be permanently connected to the battery, but the non-essential loads would be subject to automatically disconnect as the battery discharges.

Battery Discharge Control Photos

Undocumented words and idioms (for our ESL friends)

dead in the water –idiomatic phrase –maritime expression referring to a ship with a dead propulsion system –in electronics, it simply means dead or inoperative


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  • Mark p

    Good day sir,
    What component do I need to change so it can handle higher currents, or at least 30A?

  • Mars67

    I built this circuit and it works very well! Thank you mr Keith. This is now the third circuit of yours that I have built that works well.


    Yes you may as long as the VOLTAGE under load is higher than the battery it will charge. The current is controlled with a contact so the voltage drop is zero

  • Willail974

    Hello Mr Keith,

    The circuit works, i have checked it on a adjustable DC power supply. I try to find a best relay, because the one used has a big consumption.

    Thank a lot for your help !!

    PS : Do you think that i can use your 12V LDO solar charge controller circuit for cut off the voltage at 14V for a 12V battery charger?

    • Jim Keith

      The LDO solar charge control is intended for DC applications–while it may work OK with a high AC ripple, it was never intended or tested for such–as a result, I recommend a large input capacitor to keep the ripple low. Another issue is short circuit current–most battery chargers have a loosely coupled transformer (low mutual inductance) to help reduce short circuit current, or max current into a dead battery–your transformer may not perform well for this application so you can test the circuit accordingly. Keep an eye on the MOSFET temp rise as it may get quite hot when the battery begins to top off (absorb phase)–it may require a larger heatsink.

      Since you already have bridge diodes, you can eliminate the series blocking diode in the LDO regulator.

      Caution: There is no reverse polarity protection–If you accidentally connect the battery reverse, something will die…
      Check out my SCR battery charger:

    • Willail974

      I want to add the LDO solar charge controller circuit on a transformer 12V 2A with a bridge of diodes. Is it possible?


    • Jim Keith

      This is a good idea if your battery charger voltage tends to creep up at low current. Many battery chargers are poor in that the output voltage is a function of line voltage that may vary as much as ±10%.

  • Will

    Thank you Mr KEITH for your answer.
    I’m sorry but i made a mistake when i said that the relay didn’t pick up. In fact, when i powered the circuit, the relay pick up and after setting the lower limit voltage to 11,8V, the relay didn’t drop by rotating the lower limit pot CW.
    I checked the voltages in the gate of the transistor BS170. By setting the pot CW and CCW, i found 12V and 0V but the relay did’nt drop. Did i make a mistake on the wiring of the BS170? Please could you confirm that the source pin is connected to the the ground, the drain pin to the resistor R11 and the gate to the output N°2 of LM339N?
    Do you think that the relay i have use is correct for this circuit? Fixing in first the upper limit should not damage this circuit?


    • Jim Keith

      Yes, the source goes to gnd and the drain to the relay coil via R11. If wired correctly, the MOSFET may be defective (shorted).

      The relay is a non-critical element. Misadjusting he controls cannot damage the circuit.

  • Will

    Hello Mr Keith,

    Good work !! I had made your circuit, but it doesn’t work. Please can you help me!! I don’t understand your settings.
    “Keep in mind that the upper threshold may not be set below the lower threshold or the circuit is dead in the water.

    Setup using an adjustable DC power supply:
    Initially, set the lower limit CCW and the upper limit CW and apply power.
    1/Set the adjustable power supply to the desired upper limit voltage
    2/Rotate the upper limit pot CCW until the relay picks up=> it doesn’t work
    3/Then set the power supply to the desired lower limit voltage.
    4/Rotate the lower limit pot CW until the relay drops.

    What component are destroyed if you set in first the setting of upper limit CW? LM339 and 2N7000?
    In the circuit, i use a BS 170 instead of 2N7000 and i respect the right pining.

    Thank you

    • Jim Keith

      The BS170 will work, but pinout is opposite the 2N7000.
      Misadjusting the pot setting should not damage anything.

    • Jim Keith

      First determine if the relay picks up by grounding the low side.
      (jumper R11 if necessary as that is in there only to reduce relay current)

      Then determine if the MOSFET will pick up the relay by applying 12V to the gate, and drops when the gate is grounded.

      Then prove that the Set /reset latch circuit functions.

      If you still have problems, read my “Tips on Troubleshooting” and make no assumptions that this or that is OK without actually checking or experimenting…


    Honestly do I wish I had this thirty five years ago! Still very interesting circuit

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