Never connect a panel straight to a lithium cell. Use a charge controller made for that battery.
What the message means
A panel gives a voltage that varies with light. A lithium cell needs a controlled charge, and a panel voltage below the battery voltage gives no charge.
Fix it in this order
- Check the panel open-circuit voltage is above the battery voltage with margin.
- Use a solar charge controller or a lithium charger module with a suitable input.
- Add a blocking diode, or use a controller that stops reverse flow at night.
- Measure the panel current in full sun and compare it with your load.
- Point the panel at the sun and keep it clean.
- Protect the battery with a BMS.
Why it happens
A lithium-ion cell needs constant current followed by constant voltage up to 4.2 V, which a bare panel cannot provide. The panel’s voltage swings with light, and when it falls below the battery voltage the current stops or even reverses. A charge controller or charger module handles both problems. The TP4056 datasheet lists an input range of 4 to 8 V, and 8 V is also its absolute maximum, so a panel whose open-circuit voltage is near or above that needs a regulator in front. Check the panel’s open-circuit voltage against the module you choose.
A Schottky diode in series stops current flowing back into the panel at night, at the cost of a voltage drop of a few tenths of a volt, which matters on a small panel. Many controllers include this protection already, so check before adding a second one.
If you see this, do this
| You see | Do this |
|---|---|
| No charge in cloudy light | Panel voltage too low. |
| Battery drains at night | Reverse current. Add a diode. |
Related
Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.
Why this happens
A solar panel behaves like a current source that is limited by light, with a voltage that rises toward its open-circuit value as the current falls. A lithium-ion cell wants a charger that holds a constant current first, then a constant voltage of 4.2 V near the end. A panel connected directly cannot do that; the voltage follows the light and the cell voltage, and a cell can be overcharged or damaged.
A cell also takes current only when the panel voltage is above the cell voltage plus the losses in any diode or charger. On a cloudy day the panel may produce voltage but too little to charge. At night, without a blocking diode or a controller, the cell can feed current back into the panel.
How to confirm the cause
- Measure the panel’s open-circuit voltage in sun. It is usually higher than the rating on the label.
- Measure the short-circuit current with the meter on its amps range, in full sun, for a moment only. Compare it with the charger’s current limit.
- Measure the voltage at the charger input while the panel is connected. If it collapses toward the battery voltage, the panel cannot supply the demand.
- Check the charger module’s LED and its input voltage range. The TP4056 input range is 4 to 8 V.
- Check the cell’s voltage. A deeply discharged cell may need a slow start.
Worked example
Estimating the charging time for a 2 W, 6 V panel and a 2000 mAh cell in ideal full sun:
Panel current at best = P / V = 2 / 6 = 0.333 A (333 mA)
Ideal time = 2000 mAh / 333 mA = 6.0 h
In practice charging is slower: light varies,
the panel runs off its best point, and the last
stage of charging tapers the current.
A load drawing 150 mA at the same time leaves
333 - 150 = 183 mA for charging.Common mistakes
- Connecting a panel straight to a lithium cell without a charger.
- Using a panel whose open-circuit voltage is higher than the charger input limit.
- Charging a lithium cell below 0 degrees C.
- Skipping the cell protection board.
- Testing indoors behind window glass and judging the panel by what it gives there.
If this did not fix it
- Reversed polarity at the charger input.
- Partial shade on one cell of the panel, which drops the whole panel output.
- A charger that is fine but a cell that is worn out and no longer holds charge.