Direct connecting solar panel to 18650 lithium cell...

[Externet]

Hi.
If a 1 Watt little solar panel outputs 4.0V at full sunlight; direct feeding a lithium cell will charge it to that maximum 4.0V at a maximum capable rate of 0.25A into a discharged cell, reaching perhaps ~85% state of charge.

If the solar panel is 4.5V instead; it will apply overvoltage to the cell exceeding limitations.

If the solar panel is 5.0V instead, same as above but a 0.7V drop diode can be inserted in series to lower voltage to 4.3V maximum.

If the solar panel is 6.0V or more, a direct connection is not recommended.
But : If the solar panel is an even smaller, weak one, of lower current capability, voltage will collapse to the point of not harming the cell loading it? Unless left too long connected allowing reaching over ~4.3V ?

Extreme example: Can this also harm the cell ?
6V source(+)-----------------100KΩ-------------(+)18650(-)--------------------(-)

 

[BobK]

You always need a diode, or the battery will discharge through the panel when it is dark.

Bob

 

[Externet]

About the diode, I disagree.
A dark solar cell behaves as a diode. A dark solar panel behaves as a series of diodes.
For a diode to conduct, the voltage applied to it has to be over its Vf.

(A)---------------|>|--------------(K)
Vf=0.7

For a series of diodes (panel) to conduct, the voltage applied has to be over the sum of Vf
Example on 30 cell panel connected to lead acid 12V battery :

(A)------|>|---|>|---|>|---|>|---|>|---...---|>|---|>|---|>|---|>|---|>|--- (K)
30 diodes X 0.7Vf = 20Vf

(+)----------------------------------13.8V battery-----------------------------(-)

Above is a representation of a 'nominal' 12V solar panel and a fully charged '12V' battery.

Connecting (+) to (A) and (-) to (K) as in above : current will not flow as the battery is less than 20V.

It is the representative case of why the lack of a blocking diode in series with the solar panel does nothing to prevent current flow when dark. Simply the battery voltage does not overcome the panel Vf. The properties of the panel itself do the prevention.

In all cases of having a battery connected directly to solar panel, the sum of 0.5V generation per cell is less than the 0.7Vf per-diode presents, implying the battery will never overcome the panel Vf.

 

[(*steve*)]

I recommend you place a shunt regulator across the lithium cell.

Here is an example of a circuit that was modified for that purpose (set it to 4.2V and your battery will be safe).

Diodes have a lower voltage drop at low currents, so the battery will continue to charge (at a very low rate) even with them [used to reduce the voltage].

 

[Externet]

Thanks, *steve*.
What is worse to select; a 4.2Vz zener or two series 2.1Vf green LEDs in parallel to the lithium cell, for a high resistance charging voltage ?

6V source(+)-----------------1KΩ-------------(+)18650(-)--------------------(-)

 

[(*steve*)]

Two 2.1V zeners would be far worse. Low voltage zeners have very soft knees. Using these would lead to your batteries being discharged.

A single 4.2V zener would be better, but I would study both the characteristics of the knee and the tolerance of the zeners. If the tolerance is 5%, a reduction of 5% is of little concern, but any value over 4.2V is potentially a serious problem. Depending on the current your panel can provide, the voltage will rise a little (knee characteristics again). Also, is the zener will be subject to temperature fluctuations (including self heating), Cheech the change in voltage with temperature.

Assuming you don't overcharge the battery, the knee characteristics of a zener will mean that the battery will be partially discharged when the solar panel no longer supplies current. In this respect a zener will discharge the battery at an ever decreasing rate.

My circuit will discharge the battery at a constant rate. It is designed for a higher power dissipation, but was successfully used in exactly the situation you are contemplating.

Find a datasheet for the zener you intend to use and so some calculations and maybe some actual measurements. My greatest concern would be overcharging the battery. You may need to hand select a zener for it's actual knee voltage.

This is a bit of a non-answer, sorry.

 

[Externet]

Once I tried (empirically because I had them) 3.9V/1W zeners in parallel to lithium cells and behaved decently, not allowing to rise over 4.1V

 

[(*steve*)]

How far would the cell discharge if the zener was left connected? (I suspect they would discharge to 3.6V or so)

 

[Externet]

Am not sure if remember details that long ago. It may had been just while charging wired to the cell holder, then cell removed. Will find those zeners and measure current when paralleled to a cell not being charged.
About the 3.6V figure, there is something that bothers me, unrelated to the zeners... Cells properly charged individually all to 4.2V on the same charger and removed, weeks after some stay at 4.2V , some stabilize at 3.7V Do not know if chemistry differs, brand, age...
If the zener paralleling test is done with a '4.2V stable cell' or with a '3.7V stable cell' , could produce inconclusive evaluations.

 

[Externet]

Found it. It is a circuit I made when chargers were not available, not even cells were on the market, and charging voltage was "never more than 4.1V", cells were only available salvaging from defunct equipment - 1N5335B 3.9V 5W


Connected a zener to a cell with 3.95V ; drained 95mA. Significant. Seems that is why did the schematic to pull it out of circuit when not charging... Worked decently for years.