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What a battery cut-off circuit does
A cut-off circuit monitors a supply and disconnects a load when voltage falls below a chosen threshold. It is different from a low-battery indicator, which only warns you, and from a battery-protection circuit, which can also handle hazards such as overcharge, overcurrent, or short circuits.
- Undervoltage cut-off: Stops the load operating below a selected voltage.
- Load switch: Disconnects a load but does not necessarily measure battery voltage.
- Battery protector: Provides chemistry-specific protection functions; a simple low-voltage cut-off is not a complete Li-ion protection system.
- Restart behavior: Some circuits switch back on when voltage recovers; others latch off until reset or power removal.
Identify the battery before choosing a threshold
CR2032 and other primary 3V coin cells
A CR2032 is nominally 3V, but its usable voltage depends on load, temperature, age, and the minimum supply voltage of the device. Energizer specifies a typical 235mAh capacity to a 2.0V endpoint with a 15kΩ load; that is a particular test condition, not a promise of capacity in an arbitrary circuit. The same specification gives approximately 0.19mA at 2.9V and describes higher short-pulse current only under specified conditions. See the Energizer CR2032 data. Coin cells have appreciable internal resistance, so a current pulse can pull terminal voltage below the cut-off threshold even when the unloaded battery voltage is higher.
For a CR2032-powered device that needs to remain near 3V, 2.7–2.8V can be a starting threshold. A low-current timer or sensor with a lower minimum supply may start around 2.4–2.7V instead. These are design starting points, not universal battery limits. Duracell’s CR2032 data also specifies capacity to 2.0V under its test conditions; that endpoint should not be mistaken for the required cut-off for your device. See the Duracell CR2032 data.
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Single-cell Li-ion or Li-polymer
A rechargeable cell commonly described as “3.7V” is not equivalent to a 3V coin cell and can reach approximately 4.2V when charged. Use the cell maker’s voltage limits and a protector intended for that cell. TI’s BQ297xx family is designed for single-cell Li-ion/Li-polymer protection, including overcharge, over-discharge, overcurrent, and short-circuit functions. Do not treat a CR2032-oriented cut-off circuit as the only protection for a rechargeable lithium cell.
Two alkaline cells or another pack
Calculate the threshold for the number and chemistry of cells, and account for cell imbalance and voltage sag under load. A circuit intended for a 3V coin cell may not suit a higher-voltage pack.
Recommended circuit: supervisor and high-side P-channel MOSFET
A low-power voltage supervisor provides a defined trip point and often built-in hysteresis. A P-channel MOSFET in series with the battery’s positive lead disconnects the load while keeping its ground tied to battery negative.
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Battery + -------- Q1 P-MOSFET source
Q1 drain -------- Load +
Q1 gate
|
pull-up resistor
|
Battery + -----------------+
Battery + ---- supervisor sense input
Supervisor output -- inverter/driver as required -- Q1 gate
Battery - ---------------------------------------- Load -
This is a functional wiring diagram, not a pin-for-pin circuit: the supervisor’s output polarity and MOSFET gate drive must match the selected parts. For a P-channel MOSFET, gate near source means off; pulling the gate below the source turns it on. Many supervisors assert an active-low low-battery output, so an inverter, transistor, or alternate output configuration may be needed. Do not assume the supervisor can drive the gate directly.
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- Above the trip threshold, the supervisor enables the gate-drive path, pulling Q1’s gate below its source so the load receives power.
- Below the trip threshold, the supervisor changes state and the gate pull-up brings Q1’s gate close to its source, disconnecting the load.
- When battery voltage recovers, the supervisor’s recovery threshold determines whether the load restarts. A design with hysteresis restarts at a higher voltage than the voltage at which it shut down.
Why hysteresis matters
When the load turns off, battery voltage can rebound. If the restart threshold is the same as the shutdown threshold, the circuit may turn on again, pull the voltage down, shut off, and repeat. This chatter can waste energy or disrupt the load. Choose a supervisor with built-in hysteresis or design a comparator feedback network to create separate shutdown and recovery thresholds. TI’s undervoltage-monitoring reference design demonstrates hysteresis with a 2.00V low threshold and a 2.034V recovery threshold; those values illustrate the method and are not a recommendation for a 3V battery. Analog Devices describes low-power monitoring, hysteresis, timeout behavior, and low-battery outputs in its MAX6433 family. Microchip’s MIC2755 includes hysteresis and has approximately 2µA typical supply current.
Set the cutoff for the device, not the battery label
Before setting the threshold, establish the chemistry, the load’s minimum operating voltage, its maximum and pulsed current, whether brownout can corrupt data, and whether the goal is conservative shutdown or maximum usable capacity. Also decide whether the measured threshold should apply to voltage under load or to a no-load estimate. A detector connected across the battery sees the loaded battery-terminal voltage; a detector after a regulator sees a different rail.
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| Application | Possible starting threshold | What to verify |
|---|---|---|
| CR2032 logic that must stay near 3V | 2.7–2.8V | Confirm reliable operation and voltage sag at actual load. |
| Low-current CR2032 timer or sensor | 2.4–2.7V | Check the device’s minimum supply and pulse behavior. |
| Single-cell Li-ion protection | Use the cell and protector specifications | Do not substitute a CR2032 threshold. |
| Regulated 3V rail | Depends on whether monitoring the rail or regulator input | Input and output thresholds are not interchangeable. |
Comparator alternative with a resistor divider
A comparator and reference are useful when the trip point needs adjustment or when you want to build the threshold from basic blocks. Select a comparator that works below the lowest expected battery voltage, has a suitable input common-mode range and output type, and behaves predictably during startup and brownout. Keep input leakage in mind if the divider uses high-value resistors. Microchip’s comparator application material illustrates low-battery detection using a divider, reference/diode network, comparator, and transistor or MOSFET stage.
For a divider with RTOP from battery positive to the sense node and RBOTTOM from that node to ground, the trip point is:
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Rearranged: RTOP/RBOTTOM = VTRIP/VREF − 1. For a nominal 0.615V reference and a 2.70V target, the ratio is about 3.39. A 340kΩ top resistor and 100kΩ bottom resistor yield about 2.71V before tolerances and other errors. Treat those values as an example calculation, not a universal parts recipe; the selected IC’s specifications govern the final values.
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A permanently connected divider draws current whenever the battery is connected. In a coin-cell application, divider current may be comparable to the supervisor’s own consumption. Include the divider, pull-up, indicator, and off-state load leakage in the battery-life budget.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose the switch and topology carefully
High-side P-channel versus low-side N-channel
A high-side P-channel MOSFET switches the positive supply and is usually the better default for a standalone small load. A low-side N-channel MOSFET is often easier to drive and may have lower resistance, but it disconnects ground. Signal wires, programmers, USB connections, or sensors can then provide unintended return paths and partially power the supposedly disconnected circuit.
Check the MOSFET at the actual gate voltage
Choose a part whose RDS(on) is specified at the gate-to-source voltage your driver can provide, such as 1.8V or 2.5V. A low gate-threshold voltage does not mean the MOSFET is fully enhanced at that voltage. Also check current and pulse ratings, leakage, body-diode direction, package dissipation, and voltage rating above the battery’s maximum. TI discusses the low-gate-drive selection issue in this MOSFET application discussion.
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Test the cut-off before connecting the battery
- Use a current-limited bench supply in place of the battery during initial testing.
- Set the supply above the intended turn-on threshold and confirm the load runs normally.
- Lower the supply slowly while measuring supply voltage, detector output, MOSFET gate-to-source voltage, and load voltage. Record the shutdown point.
- Raise the supply slowly and record the restart point. If hysteresis is intended, verify that restart occurs at a higher voltage than shutdown.
- Repeat at the real maximum load current and with pulsed loads if the application uses them.
- Test with the intended battery and check that load sag does not cause unwanted cycling.
- Measure off-state battery current with an instrument suitable for the expected low current, and test external signal connections for back-power paths.
Measure gate-to-source voltage, not just gate voltage relative to ground: the source of a high-side P-channel device moves with the battery.
Troubleshooting common failures
| Symptom | Likely cause | What to check |
|---|---|---|
| Load repeatedly turns off and back on | Too little hysteresis or voltage rebound after disconnection | Shutdown and recovery thresholds; behavior with the actual load. |
| Cut-off trips during a pulse | Coin-cell internal resistance causes terminal-voltage sag | Pulse current, battery condition, and threshold at battery terminals. |
| Battery drains while load is off | Supervisor, divider, pull-up, indicator, or another circuit still draws current | Measure total off-state battery current and inspect signal connections. |
| Load gets a brief power pulse at startup | Undefined supervisor startup state or poorly biased MOSFET gate | Ensure the gate has a defined pull-up/down and check startup timing. |
| Load remains partly powered when switched off | Back-power through GPIO, USB, serial, or another attached circuit | Disconnect signal paths or provide isolation where required. |
| Cut-off voltage varies from expectation | Reference/resistor tolerance, comparator offset, leakage, temperature, or PCB leakage | Measure actual threshold and review all component tolerances. |
| Battery inserted backward damages or powers the circuit | Undervoltage cut-off is not reverse-polarity protection | Add a suitable series diode, ideal-diode stage, or reverse-polarity protection circuit. |
When a discrete circuit or dedicated protector makes sense
A transistor-only threshold can reduce part count, but transistor variation, temperature dependence, uncertain hysteresis, leakage, and gate-drive limits make its cutoff imprecise. It may be acceptable for a noncritical hobby indicator or load shutdown where the trip point is not important. For a defined threshold, prefer a supervisor or comparator designed for low-voltage operation; an ordinary op amp is not automatically suitable at a 3V supply.
For rechargeable lithium cells or safety-sensitive designs, use a protection IC or complete protection system specified for the exact cell and required functions. A low-voltage cutoff alone does not address charging, overvoltage, overcurrent, short-circuit, or temperature protection.
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