I've been testing a GPS / GSM tracking unit with a solar cell to charge the Li-Ion battery, instead of from the usual 12/24 V supply.
The solar cell voltage is only a bit more than the battery voltage, so I haven't bothered with MPPT, and all I have used is the normal charging IC, the L6924D (
https://www.st.com/resource/en/datasheet/l6924d.pdf). I've increased the maximum charging time to around 12 hours.
Obviously the charge rate will often be very low. From the voltage graphs, there are no clear points where charging starts or stops each day as the sun gets round to the solar cell. The fastest charge is approximately C/10 but it could be anywhere from zero up to that.
The charging IC is happy to work with a current-limited supply which is what the solar cell is, and to limit the voltage. The IC has a constant voltage phase, at the maximum voltage, and it stops charging when the current taken by the battery falls below a preset level. When run off the solar cell, the charging doesn't seem to stop and the voltage is held at the maximum voltage for a couple of hours until the sunlight leaves the solar cell.
Will that harm the battery? Does limiting at 4.1 V rather than 4.2 V make it easier on the battery?