Charging lithium batteries with solar panels

Question

Let's say I have a battery pack made from 3 lithium-ion cells connected in a series (making a battery with around 12 volts).
That battery pack has a PCB with balancing function and also protects the battery from over charging, over discharging and short circuit by interrupting the input/output to/from the battery when needed.

If I hook up to the battery a 12 volt rated solar panel (which usually has a very variable voltage, as all solar panels) that doesn't exceed the recommended charging rate of the battery, in my opinion, it should charge just fine because when the battery gets full the PCB interrupts the charging because of the overcharging protection, but i'm no expert so the question is: in theory does this work as fine as I think it does?

If not, would putting some kind of converter to turn the variable output from the solar panels into a proper constant voltage to supply to the batteries, charge them properly (considering the overchargin protection that would be in place)?

Answer 1

The in-battery protection circuitry is usually intended to act as a gross fault protector and it is strongly recommended that it not be relied on as a means of charging control. As a means of gross short circuit protection it may be suitable as long as the values they choose for max Iout are acceptable to you.

For charging, use of one of the large number of LiIon charger ICs is recommended.
A major factor is that the over voltage circuit does not remove the applied voltage when the CC charging current falls to a low value. This means that the battery is "floated" indefinitely with the risk (I'm told) of plating out metallic Lithium.

A PV panel (solar panel) that is nominally 12V rated and intended for charging lead acid batteries, will have a loaded Vout of about 18V and an O/C or light load Vout of over 20V. The maximum voltage that you need AT the battery pack is 4.2V/cell or 12.6V in your case. PV panel available Iout values are a reasonable approximation to being linearly related to isolation (sunlight level).

However, Vout is not related to light level in the same way. A PV panel will produce over 90% of its full power voltage for light levels of a few % of maximum and above - say at 10%+ to be safe. If you want the panel to charge the battery to fully-charged even on a low sun day, if necessary, then you need a panel that is full load rated at at least 12.6V/90% = >= 14V. As above, as an SLA targeted 12V panel makes about 18V at full-sun full-load, such a panel will provide more than enough voltage under all practical light conditions.

You will get substantially longer cycle life from a LiIon cell if you terminate discharge at a slightly higher voltage than allowable absolute maximum. With LiIon , below about 3V under medium loads you have used the large majority of the stored energy.In-battery low voltage cutoff circuitry will probably allow discharge to about 2.6V/cell, which is lower than is wise for good battery lifetime.