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You can charge a Ni-MH battery from solar power, but not by connecting a panel directly to the cell. The All About Circuits project described here is a useful supervised, slow-charge experiment for one known-capacity AAA cell—not a universal or unattended charger. Its 5 W panel, LM317 regulators, comparator, switching transistor and 555 timer illustrate the main design challenges: limiting current, deciding when to stop, and handling variable sunlight.
What this project is—and what it is not
The original project, published in 2016, targets one 1100 mAh AAA Ni-MH cell. It uses a 5 W solar panel specified at 22 V open circuit and 300 mA short circuit, and reports approximately 90 mA average charging current on a sunny winter day. Those panel figures are not the current delivered continuously to the battery: open-circuit voltage and short-circuit current are separate test conditions, and real output depends on sunlight, orientation, shading and the panel’s loaded operating point. The published project and schematic are the reference for its exact component connections.
Ni-MH cells have a nominal voltage of about 1.2 V, but their voltage changes during charging and varies with current, temperature and cell condition. A fixed voltage cutoff is not equivalent to a modern smart charger’s end-of-charge detection. Treat the original circuit as an educational prototype to test and supervise, not as a design proven safe for indefinite unattended charging.
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Do not substitute a lithium-ion charger board. Li-ion and Ni-MH use different charging profiles and termination methods. A solar charger explicitly designed for lithium-ion/polymer, such as the Adafruit bq25185 solar charger, is not a Ni-MH charger.
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Ni-MH current and charge time
Use the capacity printed on the actual cell, not its AA or AAA size, to estimate charging current. 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. Energizer describes 0.1C for 12–14 hours as a slow-charge approach and recommends maintenance charging below 0.025C, subject to the cell and application guidance. These are not universal guarantees; follow the particular cell manufacturer’s limits. See the Energizer NiMH handbook.
| Cell capacity | Approximate 0.1C current |
|---|---|
| 750 mAh | 75 mA |
| 1100 mAh | 110 mA |
| 1900 mAh | 190 mA |
| 2500 mAh | 250 mA |
The prototype’s reported 90 mA average is about 0.082C for its stated 1100 mAh cell. Dividing capacity by current gives an idealized estimate of 1100 mAh ÷ 90 mA, or about 12.2 hours. It is not a reliable outdoor completion-time prediction: solar variation, regulator losses, charge inefficiency, temperature, cell condition and circuit interruptions all matter.
Charging methods and their trade-offs
- Slow timer charging: Relatively simple, but it relies on a known cell capacity, suitable current and a dependable time limit. A timer that resets when clouds or night interrupt power can extend charging and risk overcharge. Energizer’s charger guidance explains this limitation.
- Smart charging: A purpose-designed charger may use negative-delta-V detection (recognizing the voltage peak and subsequent fall), temperature cutoff or rate-of-temperature-rise detection, and a backup timer. This is more appropriate for routine charging than a lone fixed voltage threshold.
- Maintenance charging: A low current may maintain a full cell in a validated application, but it is not a quick way to recharge an empty cell. Prolonged overcharge generates heat and can shorten life; Panasonic’s Ni-MH handbook cautions against assuming trickle charging is suitable without application-specific validation.
- Rapid charging: Requires capable termination and thermal monitoring, as well as sufficient power and control margin. A small, intermittently shaded panel is a poor match for uncontrolled rapid charging.
How the original circuit works
Think of the design as functional blocks rather than as a collection of parts to copy without checking values. Use the original schematic for wiring and component values; verify them against the exact parts and operating conditions before assembly.
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- Solar input: The high-voltage panel gives the control circuit room to operate, but its 22 V open-circuit rating is far above the voltage needed for one cell. The original author also wanted a panel potentially useful for a 12 V car battery; that is not necessary for this single-cell charger and makes regulation and heat management especially important.
- LM317 regulation: One LM317 is set to about 1.47 V as a reference/output arrangement, and another supplies a 12 V control rail. The usual approximate relationship is
Vout ≈ 1.25 V × (1 + R2/R1); the adjustment-current term is omitted here. Resistor values are not universally valid: check the schematic, tolerances, regulator dropout, input voltage and heat. The project’s 1.47 V value is a circuit threshold/reference, not a universal statement that every Ni-MH cell is full at 1.47 V. - Comparator cutoff: A comparator watches battery voltage against a reference and changes the charging state at a selected threshold. This is simpler than smart termination, but cell voltage depends on temperature and charging current. A fixed threshold is not negative-delta-V detection and may behave poorly with a damaged, reversed or unusual cell.
- Switching and indication: The project uses a 2N3904 for the status LED and an IRF840 MOSFET as the battery-current switch, with a series current-limiting resistor. The IRF840 is more capable than this low-current application requires; for a modern substitute, check gate-drive voltage, low
RDS(on)at that voltage, current and voltage margins, package and thermal performance. A high voltage rating alone does not make a MOSFET the right choice. - 555 timer: A 555 runs at about 1 kHz and an approximately 80% duty cycle to reduce average current; the pulsing also helps the LED remain visible in bright sunlight. PWM changes the current waveform, not the need for a reliable termination method. Peak current, average current, pulse duration, temperature and battery behavior still need checking.
The comparator and timer are educational control techniques, not proof of complete charge management. A cell’s charging response can vary, and a fixed threshold or timer can fail to stop charging at the right point.
Before you build: size and protect the system
- Identify the cells. Confirm Ni-MH chemistry, capacity, number of cells and the manufacturer’s current and temperature limits. This single-cell design should not be expanded by simply adding holders. Do not mix cells of different capacity, age, brand or state of charge; do not mix Ni-MH with NiCd, alkaline or other primary cells.
- Choose a conservative current. A rough slow-charge starting calculation is 0.1 times capacity in amp-hours. The cell maker’s instructions take priority. Do not assume that a current suitable for one AAA cell is suitable for another AAA, or for an AA cell.
- Characterize the panel under load. Find its loaded voltage and current in realistic sun, partial shade and cloud. A 5 W label is a nominal maximum under specified test conditions, not guaranteed field output. Verify that the regulator and control circuit retain adequate voltage headroom as illumination changes.
- Calculate regulator heat. For a linear regulator, approximate dissipation as
Pheat ≈ (Vin − Vout) × I. At 20 V input and 0.1 A output to about 1.5 V, that is roughly(20 − 1.5) × 0.1 = 1.85 W—substantial for a small package, especially inside a sunlit enclosure. Check the regulator’s thermal limits, use an appropriate heatsink if needed, and test actual enclosure temperature. Also check resistor power ratings. - Add fault protection. Consider reverse-polarity protection, reverse-current blocking to prevent battery discharge into the panel at night, a fuse or resettable overcurrent device, battery temperature sensing, and a defined no-charge state if control power fails. Prevent insertion of primary cells; use insulated, secure contacts and an enclosure that does not trap excessive heat.
Bench test before sunlight
Do not make a battery the first test load. Use a current-limited bench supply in place of the panel and verify each stage before outdoor operation:
- With no cell connected, check the regulator reference and control rail against the schematic.
- Verify the comparator’s switching threshold and the MOSFET’s on/off behavior using a controlled voltage source or suitable test setup.
- Measure charge current directly with a known-good cell and confirm it is within the cell maker’s limit. Check pulse peak as well as average current if the circuit is pulsed.
- Test behavior when input power disappears and returns, including whether charging restarts and whether a timer resets.
- Check reverse polarity and reverse-current behavior without risking a valuable cell.
- Monitor the cell and regulator temperature. Stop if either exceeds its permitted operating range or the battery becomes unusually warm.
Only after these checks should you test with the panel outdoors, starting with direct supervision.
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Outdoor validation: measurements that matter
Record panel voltage under load, battery current, cell temperature, sunlight conditions and time. Repeat in direct sun, cloud and partial shade. Check whether the circuit continues, stops or restarts after an interruption and whether the battery discharges into the panel after sunset.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchMeasure cell voltage after it has rested, not only immediately after charging. The original project reports average post-charge voltages of 1274 mV for cells charged by its solar charger and 1295 mV for cells charged with a Duracell charger. That comparison does not establish equal capacity, cycle life, safety or equivalent state of charge: voltage alone is not a capacity test. To assess delivered capacity, use a controlled discharge test with appropriate equipment and the cell maker’s discharge limits.
When not to use the design unchanged
- For unattended charging or leaving a cell connected indefinitely.
- When capacity or cell condition is unknown, or the cell is damaged.
- For multiple cells in parallel or series without a charger architecture designed for that arrangement and appropriate monitoring.
- When the panel can deliver substantially more current than the design allows, or the outdoor enclosure can overheat.
- For rapid charging or equipment where battery failure could create a safety hazard.
Individual cells are easier to monitor than a pack. In parallel, cells with different state of charge, resistance, age or capacity may not share current evenly. In series, a weak or already-full cell can be overcharged while total pack voltage still seems acceptable. Pack charging needs pack-appropriate control and, ideally, cell-level monitoring. The original circuit is explicitly a one-cell prototype.
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Troubleshooting
The battery gets hot
Disconnect the panel and let the cell cool in a safe place. Excess current, failed cutoff, high ambient temperature, a damaged cell or inadequate ventilation may be responsible. 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 incorrect comparator threshold, a higher-capacity cell than the design assumed, poor contacts or a damaged cell. Measure panel voltage and current under load, then measure battery current. Do not blindly raise the cutoff voltage.
The charger cuts off too early
Check the threshold and measurement point, cell temperature, wiring drops, cell resistance and comparator hysteresis. Let the cell rest and compare its behavior with a known-good charger; recalibrate only after confirming what voltage is actually being measured.
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The battery loses charge overnight
The circuit may lack reverse-current blocking. Add a correctly oriented blocking diode or a suitable MOSFET-based reverse-current stage, accounting for voltage drop and verifying its behavior at low panel voltage.
Charging restarts after clouds or sunset
A timer that loses power may restart from zero, extending charge time and increasing overcharge risk. Options include a nonvolatile charge-time record, temperature monitoring, a dedicated Ni-MH charge-management IC, or a design that defaults to no charge after an abnormal reset. Do not rely on a low maintenance current unless it is validated for the cell and application.
The LED is hard to see in sunlight
The original PWM stage was partly intended to keep the LED visible, but an LED does not prove a known battery current or a full charge. A separate power-present indicator and a clearly defined charging/full indication can improve usability; size the LED and resistor for the actual circuit.
Build or buy?
Build this circuit if the goal is to learn how solar input, regulation, switching and charge control interact—and if you can test and supervise it. For routine household AA/AAA charging, a commercial smart Ni-MH charger with independent cell monitoring, automatic termination and temperature or timer backup is generally the more practical choice. Select one that explicitly supports Ni-MH and follow its instructions.
A dedicated Ni-MH controller or a carefully engineered DC/DC front end can improve a custom solar system, but neither removes the need for suitable battery-specific termination and protection. A lithium solar charger board is not a substitute. For a solar deployment, size the panel from usable loaded output, then design the battery charger around the exact Ni-MH cell or pack.
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