Power & Energy
Build a solar power bank: charge, hold, run and cut off
A small solar power bank has four jobs: charge its cell from a panel, stop before the cell is overcharged, run a load from the cell after dark, and cut off before the cell is drained too far. With a panel, a Schottky diode, two Zeners, a comparator and a MOSFET you can build one, and see exactly what each part is for.
The plan
- Blocking diode: a Schottky from the panel's + to the cell's +, so the cell can't drain back through the panel at night.
- Voltage limit: a 3.9 V Zener across the cell, cathode on the +. The panel already limits the current, so it's the charger's constant-current stage; the Zener is the constant-voltage stage, taking the panel's current once the cell is full.
- The load: a lamp through a resistor, switched on its low side by an N-channel MOSFET.
- The cutoff: a comparator with a 2.0 V Zener reference on −, and R1 = 10 kΩ over R2 = 20 kΩ on +, so it trips at 2.0 × 30 ÷ 20 = 3.0 V.
Why the Zener needs no series resistor
A Zener straight across a battery cooks, because nothing limits its current. Here the panel is a current source: it can't push more than about 100 mA, so the Zener just takes whatever the cell doesn't. In full sun, with the cell full and the lamp off, that's nearly all of it:
Choose a Zener rated well above that.
Live experiment
Test the power bank through a day
Sun until 3 s, then unplugged into the night until 10 s. Pick the voltage limit and the cutoff's R2, and find the pair that passes all four checks: lamp on in the sun, cell full but not overcharged, lamp on after sunset, and off at 3.0 V.
“Project: Solar power bank” is the project that ends Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Test it like a day
Run it in the sun until 3 s, then unplug it into the night. A good build lights the lamp in the sun, holds the cell near 4.1 V and no higher than 4.25 V by sunset, keeps the lamp going on the cell after unplugging, and cuts it off near 3.0 V. Leave out the voltage Zener and the cell climbs to about 4.5 V: overcharged. Use R2 = 10 kΩ and the cutoff trips at 4.0 V, just after sunset. Use 47 kΩ and it wouldn't trip until about 2.4 V, too deep.
How long it lasts
Runtime is charge ÷ current. The simulator's cell is a 0.068 F capacitor standing in for a lithium cell: from 4.1 V down to 3.0 V at about 15 mA it runs for about 5 s. A real 2000 mAh cell at 15 mA would run for over 100 hours. Twice the current, half the time.
For a real build
The Zener clamp is a teaching circuit. A lithium cell wants 4.2 V held to about 1%, and a Zener's voltage drifts with current and temperature. Real power banks use a charger chip such as the TP4056 for CC-CV charging, a protection chip for over-charge, over-discharge and short circuits, and a boost converter to give 5 V at the USB port.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Project: Solar power bank” is the project that ends Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: 3 s: half as long. Twice the current empties the same charge in half the time.
- Why not “The same”? The charge is the same, but it's drawn twice as fast, so it lasts half as long.
- Why not “12 s”? 12 s would be half the current, 10 mA. More current means less time, not more.
- Why not “6 s”? 6 s is how long it lasts at 20 mA. Twice the current empties it in half the time.
Show the answer
Answer: The + side, the top of the load. A zener regulates when it's wired backwards for a diode: cathode to the higher voltage. The other way round it's just a diode and the load sees 0.7 V.
- Why not “Either end”? A Zener is polarised. Only the cathode on the + side makes it hold its rated voltage.
- Why not “The battery's −”? Cathode to − is the forward direction: the Zener would conduct like a plain diode at about 0.7 V.
- Why not “The battery's +, before the resistor”? Before the resistor, the Zener sits straight across the battery with nothing to limit its current.
Show the answer
Answer: The zener tries to hold 5.1 V against 12 and takes amps: it cooks. Nothing limits the current. The zener clamps at 5.1 V and the battery pushes everything it has through it: far past its rating.
- Why not “A perfect 5.1 V”? Nothing limits the current, so the Zener can't hold 5.1 V safely against a 12 V battery.
- Why not “Nothing happens”? A Zener across 12 V conducts hard. Something happens fast, and it's heat.
- Why not “12 V on the load”? The battery and the Zener fight over the voltage, and the Zener takes amps doing it. It overheats long before anything settles.
“Project: Solar power bank” is the project that ends Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 20 before it first.
Questions people ask
Can you charge a lithium battery straight from a solar panel?
Not safely on its own. You need something that stops at 4.2 V, a diode or controller that prevents drain at night, and ideally a proper CC-CV charger chip. Small solar chargers usually pair the panel with a TP4056-style charger and a protection board.
What size solar panel does a power bank need?
Divide the energy you need by the hours of good sun, then allow for losses. Filling a 10 Wh pack in 5 hours needs about 2 W of real output, so choose a panel rated around 3 to 5 W, since panels rarely deliver their full rating.
Why does a power bank need a low-voltage cutoff?
Lithium cells can be damaged if they're drained below about 2.5 to 3 V. The cutoff disconnects the load first, keeping a little charge in reserve, so the cell stays healthy and can be recharged safely.
Related topics
- Where does a solar panel give its most power?Shorted it gives amps, open it gives volts, and only the knee in between gives watts.
- How a low-voltage cutoff protects a lithium cellA comparator watches the cell against a steady reference and switches the load off at 3.0 V.
- How to charge a lithium-ion batteryConstant current, then constant voltage at 4.2 V: the two-stage curve every lithium charger follows.
- How to calculate battery life from mAhRuntime = capacity ÷ current: mAh, watt-hours, C-rate, and why real batteries give a little less.