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A 12V battery autocut controller built around an ATmega328P can monitor battery voltage and disconnect an external charger when a configured limit is reached. For a typical 12V lead-acid battery, an example cycle-charge cutoff is about 14.4V, with charging restarted at a lower, battery-specific voltage.
This is a voltage-based supervisory controller, not a complete universal battery charger. The external charger must already provide suitable current limiting and charging characteristics. The design below is intended primarily for specified 12V lead-acid batteries—not lithium-ion or LiFePO4 packs.
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What the controller does
The operating sequence is straightforward:
- An appropriate DC charger supplies the battery.
- A resistor divider reduces battery voltage to a safe ADC voltage.
- The ATmega328P measures and filters that voltage.
- When the configured upper threshold is reached, the controller de-energizes a relay or turns off a MOSFET.
- Charging resumes only after the battery falls below a lower restart threshold.
The separation between the cutoff and restart thresholds is called hysteresis. It prevents the switching device from chattering when the battery voltage fluctuates near one value.
The ATmega328P does not itself regulate charging current, perform absorption timing, compensate for temperature, balance cells, or identify battery chemistry. A dedicated lead-acid charger IC such as TI’s BQ24450, or a suitable commercial charger, is a better foundation for unattended or safety-critical charging.
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Choose the battery chemistry first
“12V battery” describes nominal voltage, not charging requirements. The example thresholds in this article are for a conventional 12V lead-acid system, subject to the battery manufacturer’s specification.
| Battery type | Can this design be used unchanged? | Reason |
|---|---|---|
| SLA/VRLA lead-acid | Only with the manufacturer’s limits | Cycle and float voltages vary by model and temperature. |
| AGM lead-acid | Only with the manufacturer’s limits | It may require different absorption, float, and temperature settings. |
| Gel lead-acid | Do not assume compatibility | Overvoltage can permanently damage the gel electrolyte. |
| Flooded lead-acid | Only with an appropriate charger | Ventilation, electrolyte maintenance, and possible equalization matter. |
| 3-series lithium-ion | No | It requires a chemistry-specific charger, cell monitoring, and protection. |
| 4-series LiFePO4 | No | It requires a LiFePO4 charging profile and suitable BMS. |
For one cited 12V lead-acid product family, Power-Sonic specifies approximately 14.1–14.7V for cyclic charging and 13.5–13.8V for float service at 20°C. Those figures are not universal; use the data sheet for the exact battery.
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DC charger positive -- fuse -- relay contact or MOSFET -- battery positive
DC charger negative ------------------------------- battery negative
|
controller ground
Battery positive -- resistor divider -- ATmega328P ADC inputMeasure as close as practical to the battery terminals. Measuring on the charger side of a relay or at the end of a long cable can include voltage drop and produce a misleading cutoff point.
For a first prototype, keep the switched circuit at the charger’s low-voltage DC output. Do not use a beginner relay circuit to switch mains casually. AC mains switching requires appropriate isolation, creepage and clearance, fusing, enclosure, strain relief, and compliance with local electrical requirements.
Parts list
Required
- Arduino Uno Rev3 for the easiest prototype, or an ATmega328P on a custom board.
- 5V regulated supply for the controller and relay, preferably from an efficient buck converter.
- 10kΩ resistor from battery positive to the ADC node.
- 3.3kΩ resistor from the ADC node to controller ground.
- 5V relay with contacts rated for the charger output current, voltage, inrush, and load type—or a correctly designed logic-level MOSFET switch.
- BC547 or another suitably rated transistor for the relay coil.
- 1kΩ base resistor.
- 1N4007 flyback diode across the relay coil.
- Battery-side fuse, reverse-polarity protection, wiring, connectors, and decoupling capacitors.
Optional
- 16×2 LCD using the Arduino
LiquidCrystallibrary. - Small capacitor from the ADC node to ground for noise filtering.
- Temperature sensor positioned to measure battery temperature.
- Hardware comparator or independent overvoltage cutoff.
- Manual service disconnect and status LED.
Voltage-divider design
The divider protects the ADC by scaling the battery voltage:
Vadc = Vbattery × R2 / (R1 + R2)
Vbattery = ADC reading × Vref / 1023 × (R1 + R2) / R2With R1 = 10kΩ and R2 = 3.3kΩ:
Divider multiplier = (10,000 + 3,300) / 3,300 ≈ 4.03
At 14.4V battery voltage: Vadc ≈ 14.4 / 4.03 ≈ 3.57VThat is below a nominal 5V ADC reference. The divider must nevertheless be checked for charger transients, wiring errors, resistor failure, and the highest voltage that can actually appear in the system. Add input protection where appropriate, and use 1% resistors when the cutoff voltage matters.
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The ATmega328P has a 10-bit ADC, so its code ranges from 0 to 1023. It supports AVCC, an internal nominal 1.1V reference, and an external AREF reference. Using 5.0 in software assumes the 5V rail is exactly 5.000V. Measure the real rail with a trusted multimeter, use a precision reference, or apply a calibration factor. If switching to the internal reference, discard the first conversion after changing the reference because it may be inaccurate.
Battery thresholds: cutoff is not full-charge detection
| Function | Illustrative value | Qualification |
|---|---|---|
| Lead-acid cycle-charge range | 14.1–14.7V at 20°C | Example range from one battery family; follow the exact manufacturer data. |
| Example upper cutoff | 14.4V | A plausible absorption/cutoff value for some lead-acid applications. |
| Lead-acid float range | 13.5–13.8V at 20°C | Float charging is a different operating mode, not simply repeated cutoff. |
| Illustrative restart | 12.8–13.0V | Must be validated for the battery, load, and charger. |
| Original project example | 10.8V | Project-specific; not a general charging restart recommendation. |
Voltage changes with charging current, temperature, cable resistance, surface charge, battery age, state of health, and connected loads. Consequently, “battery full” is too strong a description for a voltage-only decision. A more accurate status is cutoff threshold reached.
Do not confuse the restart threshold with a low-battery load disconnect. A 10.8V reading may indicate severe discharge under load. Waiting for that voltage before restarting can leave a battery deeply discharged. Conversely, a restart value set too high can cause frequent charge cycling. Select both values from the battery documentation and validate them under the real load and charging conditions.
Relay driver circuit
ATmega328P output pin -- 1kΩ -- BC547 base
BC547 emitter ------------------ GND
BC547 collector ---------------- relay coil negative
Relay coil positive ------------ regulated 5V
1N4007 cathode ----------------- relay coil positive
1N4007 anode ------------------- relay coil negativeThe ATmega328P must not power a relay coil directly. Confirm the relay coil voltage and current, transistor ratings, base drive, and contact ratings. The contacts must handle the charger’s maximum DC voltage and current, including inrush and inductive effects. Decide whether normally-open or normally-closed contacts produce the safer failure state.
A MOSFET can be preferable for low-voltage DC: it is silent, efficient, and has no contact wear. It requires correct polarity, gate drive, heat management, and attention to body-diode behavior. A failed MOSFET may fail short, so it is not automatically a fail-safe replacement.
Power supply and protection
If an L7805 linear regulator is fed from a 12–15V source, its heat is approximately:
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Pheat ≈ (Vin − 5V) × IloadThat can become significant when the MCU, LCD, relay, and indicators run continuously. A buck converter usually wastes less power, although its switching noise may require careful ADC routing and filtering.
- Place a fuse close to the battery positive terminal.
- Add reverse-polarity protection before the divider and switching hardware.
- Use local bypass capacitors at the ATmega328P and relay supply.
- Keep high-current charger wiring separate from the ADC trace.
- Provide a manual disconnect for servicing.
- Where unattended operation matters, add an independent hardware overvoltage cutoff or use a certified charger.
Improved ATmega328P firmware
This example averages 16 samples, uses hysteresis, starts with charging disabled, rejects implausible readings, and makes the relay polarity explicit. It is a design example, not a verified drop-in implementation for every schematic.
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const uint8_t relayPin = 12;
const uint8_t sensePin = A0;
const bool relayActiveHigh = true;
const float cutoffVoltage = 14.40;
const float restartVoltage = 13.00;
const float vref = 5.00; // Replace with measured/calibrated value
const float dividerRatio = (10000.0 + 3300.0) / 3300.0;
bool chargingEnabled = false;
void setCharger(bool enabled) {
chargingEnabled = enabled;
bool pinState = relayActiveHigh ? enabled : !enabled;
digitalWrite(relayPin, pinState ? HIGH : LOW);
}
float readBatteryVoltage() {
const uint8_t samples = 16;
uint32_t total = 0;
for (uint8_t i = 0; i < samples; i++) {
total += analogRead(sensePin);
delay(2);
}
float adc = total / (float)samples;
return adc * vref / 1023.0 * dividerRatio;
}
void setup() {
pinMode(relayPin, OUTPUT);
setCharger(false); // Safe startup state
analogReference(DEFAULT);
delay(100); // Allow reference and input to settle
wdt_enable(WDTO_4S);
}
void loop() {
wdt_reset();
float batteryVoltage = readBatteryVoltage();
if (batteryVoltage < 0.5 || batteryVoltage > 18.0) {
setCharger(false); // Battery/sensor fault: do not charge
} else if (chargingEnabled && batteryVoltage >= cutoffVoltage) {
setCharger(false);
} else if (!chargingEnabled && batteryVoltage <= restartVoltage) {
setCharger(true);
}
delay(1000);
}The original project uses A0, relay pin 12, 10kΩ/3.3kΩ divider values, a 14.4V cutoff, a 10.8V low/restart value, and a one-second loop. The improved example deliberately uses a battery-specific-looking restart value only as an illustration. Change it after consulting the battery data sheet.
For a finished controller, replace blocking delays with timed sampling, require the voltage to remain above or below a threshold for a defined period, and display a distinct sensor-fault state. Add a hardware pull-up or pull-down so the relay driver remains disabled while the MCU is resetting. The ATmega328P watchdog, with selectable timeouts up to 8 seconds, can recover from firmware lockups—but it cannot repair welded relay contacts or failed power hardware.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.LCD status
A 16×2 LCD is useful during development but is not required for control. It might display:
Battery: 13.72 V
Status: CHARGING
Battery: 14.41 V
Status: CUT OFFOmitting the LCD reduces wiring, power consumption, and software complexity. A status LED and serial output may be sufficient for a workshop prototype.
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Assembly, calibration, and testing
- Prototype the firmware on an Arduino Uno. The Uno provides the ATmega328P, 5V operation, USB programming, 16MHz clock, and six 10-bit analog inputs.
- Build only the sensing circuit first. Power the controller from a current-limited supply and verify that the ADC node never exceeds the reference.
- Calibrate the divider. With the charger disconnected, measure battery-terminal voltage using a trusted multimeter and compare it with the displayed or serial value.
- Adjust the calibration constant. Correct
vrefor apply a multiplier, then verify at a second voltage near the intended cutoff. - Test with a bench supply or safe substitute. Sweep slowly through the restart and cutoff thresholds before connecting a battery.
- Test the relay without a live high-current load. Confirm that the chosen contact is open when charging is disabled and that reset leaves it in the safe state.
- Connect the charger and battery only after verification. Use the fuse and confirm polarity.
- Repeat with charger ripple and the intended load. Check that filtering prevents chatter and that the voltage at the battery terminals—not merely at the charger—is being used.
Useful fault tests include a disconnected divider wire, missing battery, power interruption, charger removal, reverse-polarity protection, and a simulated sensor reading outside the valid range. The recommended response to an invalid or missing measurement is to disable charging, not to guess that the battery needs charging.
Limitations and safer alternatives
A relay cutoff can reduce the risk of continued charging above a programmed voltage when the electronics, relay, charger, and battery are all functioning correctly. It cannot guarantee protection against a welded contact, failed transistor, drifting reference, excessive charger current, high battery temperature, or incorrect chemistry.
Use a dedicated charger or charge-controller IC when the system needs controlled current, bulk/absorption/float stages, storage mode, temperature compensation, fault handling, or dependable unattended operation. Commercial multi-stage chargers such as the Victron Phoenix Charger illustrate the difference: they manage charging stages rather than simply opening a relay at one voltage.
For learning, an Arduino Uno Rev3 is the simplest route. For a custom board, a bare ATmega328P reduces size and cost but requires its own clock, reset circuit, decoupling, programming connector, and power design. Microchip currently marks the ATmega328P as “Not Recommended for new designs,” so a new commercial product should evaluate current AVR or other MCU alternatives before committing.
Final design position
This project is best treated as an educational supervisory controller for one specified 12V lead-acid battery and one suitable current-limited charger. Use the battery manufacturer’s voltage and temperature limits, calibrate the measurement, implement hysteresis and fault lockout, fuse the battery connection, and keep the beginner version on the low-voltage DC side. If the requirement is a reliable charger rather than a demonstration of voltage-controlled switching, choose a battery-specific commercial charger or dedicated charge-controller architecture.
Reference material: matching ATmega328P project, Microchip ATmega328P documentation, AVR ADC reference guidance, Power-Sonic battery specifications, and Victron charging-stage information.
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