Solar floodlight: 24 SMD LEDs on three 3.2 V LiFePO4 cells
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How the cheap solar floodlights are built. By day a 6 V 10 W panel charges three 32700 LiFePO4 cells in parallel (3.2 V, 18 Ah). There is no charger chip, as in most of these lights: a solar panel is a current source, so it simply pushes its current (about 1.5 A in full sun) through the Schottky diode D1 into the cells, and the diode stops the cells draining back into the panel at night. The 1S LiFePO4 protection board ends the charge at 3.65 V and cuts the light off if the cells are run below 2.5 V. The panel is also the light sensor. While the sun is up its voltage turns Q2 (BC547) on, which holds the MOSFET’s gate low, so the light stays off. At dusk the panel voltage falls, Q2 lets go, R3 pulls the gate up to the battery and Q1 switches the LED board on: 24 × 2835 0.5 W LEDs in parallel, each with its own 4.7 Ω resistor: about 1 A and 470 lm at the cells’ 3.26 V plateau (more just after a full charge). A full 18 Ah lasts about 17 hours, but one sunny day (about 5 hours of full sun at 1.5 A) puts back only 7–8 Ah, about 7 hours of light: that is why the panel is the real limit of these lights, whatever wattage the box claims. There is no driver: the LEDs run straight from the cell, so the light is brightest when the cells are full and dims as they settle to their 3.2 V plateau. That is why the resistors matter: without them the LED with the lowest forward voltage takes most of the current. Try it: set the panel’s sunlight to 850 W/m² to see it charge and switch off, back to 0 for night; click the LED board and set its ballast resistor to 0.
What is on the bench
22 parts, 30 wires.
| LiFePO4 cell | 3 |
|---|---|
| Resistor | 3 |
| Small poly panel | 1 |
| Schottky diode | 1 |
| 1S LiFePO4 protection board | 1 |
| NPN transistor | 1 |
| Fuse | 1 |
| Toggle switch | 1 |
| LED board | 1 |
| N-channel MOSFET | 1 |
Put it on your page
The project runs live inside the frame: readers can press buttons, turn knobs and read the meters.
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Link
https://zencircuitlab.com/p/zclsolarflood
Published by its author under the lab's community terms: it may be opened, copied and changed by anyone. Simulation results are for learning; check a real build against the parts' datasheets.