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Why can I recharge lead acid batteries with solar but not lithium ion? Answered

I just got a 5 watt solar panel and started experimenting with it today.  I want to use it to charge a single 10 watt hour/2.5 amp hour 18650 lithium ion cell.  The solar panels have an open voltage of 22 volts.  I connected the solar panel to an anyvolt3 (boost/buck power regulator), set the anyvolt3 to put out 4.2 volts, then connected  the anyvolt to the lithium cell with a blocking diode.  I accounted for the voltage drop of the blocking diode and the panels charged the lithium cell at .1 watts.  Pathetic.

Then I tried charging a 20 amp hour Werker sealed lead acid 12 volt battery.  I set the anyvolt to 14.4 volts.  The panels charged the lead acid battery at 4.5 watts.  Thats pretty good. 

Now why can I charge the lead acid but not the lithium.  The lithium is what I really want to charge because i want to use the solar panel to charge lithium cells when I go on long bike tours.  I don't want to take any heavy lead acid batteries on my bike trip.



If I have it right you set your anyvolt3 to give you 4.2 volts at the battery to charge a 4.2 volt battery.

Reset it for 5.2 volts

You are under voltage.

Are you sure you charge 4.2 volt lithium cells at 5.2 volts. You have done this yourself?

Yea solar is worse than others.

As strange as it sounds your meter lies.

If you read your booklet that comes with your meter it has a tolerance and other factors that tell you it is out by a certain amount.

And every charger I have check and know always gives you a higher voltage reading then the voltage it is giving you because a charger is a current pusher and the meter is impedance fixed.

This works out to about 1 volt on the lower settings and goes up with the higher settings.

It does the same on amps or resistance.

So if you are getting exactly what you want by your meter you are not really getting it.

It is called loading effect generally it is small and on a charger you want about 1 volt more.

If you follow a lot of the members here like kiteman (a science teacher) or stevestrouk (an engineer) and kelseymh (a PHD) they always ask if the circuit is loaded.

In short always test a circuit loaded and make sure it is giving more then you want.


Okay I will try it. I have PCB protected and unprotected cells. I will be able to try 5.2 volts tomorrow. I think it will work with the unprotected cells buy probably won't work on the protected one.

If I left the charger on for a long time wouldn't the battery charge to 5.2 volts and then be fried. Lithium ion cells are not supposed to charge beyond 4.2 volts. I can see the logic in charging them at 5.2 volts when they are depleted because the voltage while charging would still be less than 4.2 volts, but eventually the cell voltage will rise above 4.2 volts and I am pretty sure it would break.

It will if you don’t have a shut off or monitor it.

A simple comparator shut off set for between 4.2 and 4.4 should do it.

Anyvolt 3?  This:
must be the power converter you are describing? Yes?

Regarding the charging a li-ion cell, I claim that this page:
presents a recipe for doing that correctly.

The thing you're doing, i.e. charging a li-ion cell using a constant voltage, well, that's something different.

It is unfortunate that Anyvolt 3 thingamajig cannot have its duty cycle be instantaneously controlled by some other signal (from some other circuit).  Then that way  you could maybe have it do things besides regulated output voltage; i.e things like constant current while watching voltage,  then switch to constant voltage mode, while watching current, then switch itself off (zero current),  so that you could really charge that li-ion cell the right way.

Of course this would be complicated by the fact that you're using solar power, so basically it is the kind of situation where the darn thing shuts off every time the sun goes behind a cloud.

Final note , that Powerstream article said trickle charging li-ion cells was bad.  I'm not sure if that means you're going to invoke the wrath of the gods by doing it, or um, just break the battery.

Anyway, I claim that's the story. That is to say lead acid batteries do not mind being charged by a constant voltage, but the li-ion batteries want to be charged in a way that is different than this.


5 years ago

That 4.2V Li battery may have needed more starting current then your
whole solar panel can provide leaving your anyvolt3 sucking sunbeams.

The Pb battery 4.5watts are 14.4volts times a 315ma current which is
probable max limit the solar panel can deliver but not enough for your
Li cell..