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As I understand it, an MPPT algorithm adjusts the charging voltage until it finds the point where maximum power is delivered into the battery.
If a 12 V lead-acid battery is used, is there an upper limit for the applied charging voltage? If, for instance, the MPP is at 18 V, is it OK to apply 18 V to a 12 V battery? Is this something the battery data sheet would mention or is there a rule of thumb for this?
Your question: "If a 12 V lead-acid battery is used, is there an upper limit for the applied charging voltage?" Answer: If ANY battery is used there must be a limit to the applied charging voltage. The charger is forced to drop from MPPT mode for that, because the battery can't consume enough power.
An MPPT controller only works at the Maximum Power Point drawing Maximum Power, if it can dump whatever power it draws into whatever is downstream. If that's a battery, and it's fully charged, then the MPPT has to be told to stop being an MPPT, otherwise it will damage the battery.
As long as it is of limited duration, 15 V is OK on a lead-acid battery, in fact it helps prevent sulphation. However, for indefinite application, 13.8 V is the maximum you should use.
As I understand it, an MPPT algorithm adjusts the charging voltage
No, it doesn't. The MPPT algorithm in most set-ups changes the input impedance of a buck converter in such a way that the output current of the buck converter is maximised. This keeps a solar panel at its MPP.
The input of the buck converter is regulated, the output isn't, and the output will behave roughly like a current source; the output voltage is set by the battery, not by the MPPT algorithm, or the charger.
If the MPP is at 18 V, then that is the voltage on the solar panel side and it has nothing to do with the voltage on the battery side, which is set by the battery.
A charger will, of course, have to make sure that a battery isn't charged beyond its maximum voltage, or charged with too large a current, but that is separate from the MPPT algorithm.
It does mean, however, that the solar panel will sometimes be moved away from its MPP; it could be delivering too much current/power at its MPP for some stages of the charging algorithm, especially when the battery is (almost) fully charged.
According to this mod:
If you used a MPPT (maximum power point tracking--more expensive) type charge controller, it sort of acts like a voltage transformer. Taking high voltage/low current and down converting to low voltage/high current to support your battery/DC loads. In this (made up) example, it would look like (ignoring losses and other details):
So it seems that panel voltage is only limited by charge controller's max pv input as the controller takes care of stepping down the voltage to charge the battery at safe levels.
https://forum.solar-electric.com/discussion/352531/best-way-to-charge-12v-battery-36v-12v-mppt
