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You can charge a Ni-MH cell from solar power, but you should not connect a panel directly to the battery. Ni-MH cells need controlled charging, and the original All About Circuits project is best understood as a supervised, slow-charge learning circuit for one known-capacity AAA cell—not a universal or unattended charger.
This guide explains what the published circuit does, how to estimate current and heat, how to test a build, and where its voltage-cutoff approach falls short of a modern smart charger.
What the project builds—and what it does not
The project published by All About Circuits uses a solar panel, linear regulators, a voltage-comparator control stage, transistors, and a 555 timer to charge a single 1100 mAh AAA Ni-MH cell. It is a useful electronics project for learning about power regulation and charge control. It is not a certified charger, and its fixed-voltage cutoff does not provide the same end-of-charge detection as a smart Ni-MH charger.
A solar panel’s output changes with sunlight, shading, orientation, and load. A cell’s voltage also changes as it charges and with temperature. The charger must limit current and stop charging appropriately; a panel’s nominal wattage or an LED turning on cannot establish that the cell is charging safely or is full.
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Do not substitute a lithium-ion charger board. Lithium-ion and Ni-MH batteries use different charging methods and termination rules. A charger intended for lithium-ion/polymer is not a Ni-MH charger; for example, the Adafruit bq25185 solar charger is explicitly for lithium batteries.
Ni-MH charging basics: capacity, current, and termination
A Ni-MH cell is commonly described as a 1.2 V nominal cell, but its actual voltage varies with charge state, load, temperature, and rest time. Do not interpret 1.47 V—the reference/cutoff design value in this project—as a universal “full” voltage.
Choose charge current from the cell’s rated capacity, not its AA or AAA size. C-rate expresses current relative to capacity: for a 1100 mAh cell, 1C is 1100 mA, 0.1C is 110 mA, and 0.025C is 27.5 mA.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstall| Capacity | Approximate 0.1C current |
|---|---|
| 750 mAh | 75 mA |
| 1100 mAh | 110 mA |
| 1900 mAh | 190 mA |
| 2500 mAh | 250 mA |
Energizer’s Ni-MH handbook describes 0.1C for 12–14 hours as a slow-charge approach and recommends maintenance charging below 0.025C. These are manufacturer guidelines, not a guarantee that every cell tolerates the same current or duration. Follow the cell maker’s limits where available.
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- Slow timer charging: relatively simple, but depends on known capacity and reliable timing. If a timer resets when clouds interrupt power, accumulated charge time may be lost and overcharge becomes possible. A current that is too low or a duration based on the wrong capacity can also leave a cell undercharged.
- Smart charging: may use negative-delta-V detection (ending charge after voltage peaks and falls), temperature cutoff or rate-of-temperature-rise sensing, and a backup timer. Energizer’s charger handbook describes these protections and the risk of a timer restarting after an interruption.
- Trickle or maintenance charging: intended to maintain a charged cell at sufficiently low current, not quickly recharge an empty one. Prolonged overcharge can heat cells and shorten life. Panasonic cautions against trickle charging without application-specific validation.
- Rapid charging: needs more capable termination and temperature monitoring. A small panel with fluctuating output is a poor setting for rapid charging unless the system has suitable power and control margin.
How the published circuit is arranged
The original prototype’s functional blocks are easier to understand than a component list alone:
- Solar input: a 5 W panel specified as 22 V open-circuit voltage and 300 mA short-circuit current. These are different operating conditions; the panel does not deliver 300 mA continuously to the battery. Neither number alone gives the panel’s actual operating current or power under load.
- Voltage regulation: one LM317 is set to approximately 1.47 V for the battery-side reference/output arrangement, while a second makes a 12 V rail for the control circuitry. The usual approximate LM317 relationship is
Vout ≈ 1.25 V × (1 + R2/R1); it omits adjustment current and is not a substitute for checking the actual circuit, resistor values, dropout voltage, and thermal conditions. - Voltage comparator: monitors battery voltage against a reference and changes the charge state at the design threshold. A fixed threshold is simpler than negative-delta-V detection, but it does not reproduce it. Cell voltage depends on current and temperature, and a damaged or abnormal cell may not behave as expected.
- Switching and indicator: a 2N3904 drives the status LED; an IRF840 MOSFET switches battery current, with a series resistor limiting current. The project notes that the IRF840 is more than required. When choosing a modern substitute, select for low on-resistance at the actual gate voltage, suitable voltage and current ratings, package, and thermal performance—not voltage rating alone.
- 555 timer: runs at about 1 kHz with roughly 80% duty cycle to reduce average current and help keep the LED visible in strong sun. PWM changes the current waveform; it does not make the circuit smart or provide charge termination by itself. Peak and average current, pulse duration, temperature, and a reliable end-of-charge strategy still matter.
The original project selected a higher-voltage panel partly because it could also be useful for a 12 V car battery. That is not needed for charging one AAA cell, and the large voltage drop makes regulator heat an important design issue.
Estimate current, time, and regulator heat before building
The original article reports about 90 mA average current on a sunny winter day. For its stated 1100 mAh cell, that is about 0.082C:
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A simple capacity/current estimate gives 1100 mAh ÷ 90 mA ≈ 12.2 hours. That is only an idealized estimate, not a promised solar charging time. Actual time can be longer or less predictable because of changing sun, panel operating point, regulator losses, cell temperature and condition, charge inefficiency, and the timer’s duty-cycle reduction. Record actual current rather than calculating from the panel’s 5 W label.
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Linear regulators dissipate the voltage they drop as heat. Estimate it with:
Pheat ≈ (Vin − Vout) × I
For example, dropping 20 V to about 1.5 V at 0.1 A gives (20 − 1.5) × 0.1 ≈ 1.85 W. That is substantial for a small package, especially in a sun-warmed enclosure. Check the regulator’s datasheet limits, use appropriate heat sinking if needed, and test in the intended enclosure. Also check the current-limiting resistor’s dissipation and rating.
Build and test in stages
The published project provides its own schematic and part arrangement. Use that schematic for exact connections and values; do not infer resistor values from the functional summary here. Before assembly, confirm that all components and the circuit suit the exact cell, panel, temperature range, and enclosure.
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- Identify the battery. Confirm it is Ni-MH, record its capacity and maker’s limits, and decide whether you are charging one cell or a designed pack. Do not mix different capacities, ages, brands, states of charge, or Ni-MH with NiCd; never put a primary alkaline cell in the charger.
- Choose a current target. For a slow-charge design, 0.1C is a useful starting reference only when supported by the cell maker and the chosen termination method. A 90 mA current is below 0.1C for a 1100 mAh cell; it is not automatically right for another cell.
- Check the panel under load. Obtain or measure open-circuit voltage, short-circuit current, and voltage/current at the intended operating point. Test realistic direct sun, partial shade, and cloud conditions. Nominal wattage is a test-condition maximum, not field output.
- Add protection and define failure behavior. Consider reverse-polarity protection, reverse-current blocking to prevent overnight discharge through the panel, a fuse or resettable overcurrent device, a cell temperature sensor, and a control failure state that defaults to no charge. Prevent insertion of primary cells. Provide insulation, secure contacts, and ventilation appropriate to outdoor heat.
- Bench-test before using sunlight. Replace the panel with a current-limited bench supply. With no battery, verify the regulator outputs and control rail. Check comparator switching and charge current using a known-good cell; verify cutoff behavior with suitable controlled test equipment. Test loss of input, polarity protection, and component temperature. Monitor the cell temperature throughout.
- Test outdoors under supervision. Record panel voltage, battery current, and cell temperature in direct sun, cloud, and shade. Check what happens at sunset and when input returns. Stop if the cell heats abnormally. Do not leave an unvalidated prototype charging unattended.
- Validate the charge result. Record cell voltage after it has rested for several hours, and, if you need to assess capacity, use a controlled discharge test. An immediate post-charge voltage is not proof of full charge or recovered capacity.
What the original test does—and does not—show
The project reports testing four batteries with its solar charger and comparing them with a Duracell charger. It gives average voltages of 1274 mV for the solar-charged batteries and 1295 mV for the Duracell-charged batteries. That comparison does not establish equal delivered capacity, long-term safety, cycle life, or performance in variable weather. Voltage alone is not a reliable capacity test.
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Where the design needs caution or improvement
- Fixed-voltage cutoff: 1.47 V is a project reference value, not a universal full-charge threshold. A comparator watching one voltage threshold does not detect the voltage peak-and-fall behavior used in negative-delta-V control, nor does it replace temperature safeguards.
- Solar interruptions: if control power disappears and a timer resets, the circuit may charge for a fresh interval when sunlight returns. A cell that is already full can then be overcharged. A nonvolatile time/state record, temperature monitoring, a robust charge controller, or a safe no-charge reset state can reduce this risk.
- Nighttime reverse current: without blocking, the battery may discharge into the panel or control circuit after sunset. Add a correctly oriented blocking diode or an appropriate ideal-diode MOSFET stage, accounting for voltage drop.
- Thermal conditions: regulator heat combines with direct-sun enclosure heating. Validate the actual assembly at its expected ambient temperature, not only on an open bench.
- Scaling to multiple cells: the prototype is for one cell. Parallel cells can share current unevenly if they differ in charge state, capacity, age, or internal resistance. Series packs can hide an overcharged weak cell behind an acceptable total pack voltage. More holders require redesigned current capacity, thermal management, and suitable per-cell or pack-level monitoring—not simply more battery slots.
Troubleshooting
The cell becomes hot
Disconnect the panel. Excess current, failed cutoff, high ambient temperature, a damaged cell, or poor ventilation may be responsible. Let the cell cool in a safe place. Do not reuse a cell that has leaked, vented, swollen, or been physically damaged.
The charger never cuts off
Possible causes include weak sunlight, panel voltage collapsing under load, regulator dropout, an incorrectly set comparator threshold, poor cell contact, a larger-than-assumed capacity, or a damaged cell. Measure panel voltage and current under load, then measure battery current directly. Do not raise the cutoff voltage blindly.
The charger stops too soon
Check whether the threshold is too low, the cell is hot, the measurement point includes unexpected wiring or resistor drop, the cell has high internal resistance, or comparator hysteresis is inadequate. Let the cell rest and assess it with a known-good charger or controlled capacity test before recalibrating.
The cell loses charge overnight
Suspect missing or failed reverse-current blocking. Measure whether current flows from the battery toward the panel/control circuit with no solar input, then add or repair a suitable blocking stage.
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Charging resumes after clouds or sunset
This can happen if a power interruption resets the timer while the cell remains partly or fully charged. Treat it as a design limitation, not a reason to extend the cutoff threshold. Use a controller that preserves state or defaults safely, add temperature protection, or choose a dedicated Ni-MH charge-management solution.
The LED is hard to see in bright light
The original design uses PWM partly to keep the LED visible, but the LED is not a current meter and cannot show that charge termination is correct. A high-efficiency LED, correctly sized resistor, or separate power and charge-status indicators can improve visibility.
Build it or buy a smart charger?
Build this kind of circuit when the purpose is education or a supervised custom low-current experiment and you can validate current, cutoff behavior, and temperature in the actual setup. For routine household AA/AAA charging, a commercial smart Ni-MH charger with independent channels, charge termination, and temperature or timer backup is generally the more practical choice. Check that it explicitly supports Ni-MH and that its charging method suits your cells.
A solar panel can be the power source for a Ni-MH system, but it does not change the chemistry’s charging requirements. A dedicated Ni-MH charge controller or a properly designed intermediate power stage is preferable to adapting a charger board intended for lithium-ion. For additional guidance, see Panasonic’s Ni-MH technical handbook and Energizer’s charger handbook.
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