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Lets say that I have a CC/CV charger based on buck topology and it is charging a 12V lead-acid battery through a solar panel. The lead acid-battery is being charged in CV (Constant Voltage) mode and requires 14.6V to be maintained across it until the charging current drops below a preset current threshold. However, the solar panel is no longer able to provide the voltage required for the buck converter to maintain 14.6V at its output and instead is providing a maximum constant voltage of 14.2V to the battery. What would happen in this scenario? And what would happen if the voltage goes much lower?

My understanding of this scenario is that if 14.2V is given to the battery during CV charging instead of 14.6V, it would just take longer to "top it off" and for the charging current in that situation to drop to the current threshold where it is considered completely charged.

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My reasoning is that (imagining a simple battery model with a resistor in series with an ideal voltage source) if the charging voltage across the battery is less than the required charging voltage then there is a smaller potential difference across the resistor and therefore a smaller current will flow into the battery. And assuming that the total amount of charge that needs to be pumped into the battery during CV mode is constant and irrespective of the charging voltage used, then with a smaller current it will take longer to deliver that same quantity of charge at 14.2V then at 14.6V.

And if the voltage goes lower than the "ideal voltage source" in the simple battery model, then the battery will start discharging into the buck converter and solar panel. Though, I have no clue what that voltage threshold would be for the battery to discharge into the buck converter but I'm guessing it would probably be lower than its floating voltage.

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    \$\begingroup\$ "my second query". Careful there: someone will close your question for violation of "Lacks focus" rule. Ask only one question. \$\endgroup\$ Commented Mar 19, 2023 at 14:15
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    \$\begingroup\$ @DavideAndrea My question is on how charging of a battery is impacted if a lower constant voltage is applied instead of the required constant voltage during CV charging, for example, due to a drop in voltage of the solar panel during CV charging. My "queries" are presenting different scenarios of lower constant voltage and what would be the impact, which is part of my question. However, the wording I've chosen is misleading and gives the impression of multiple questions being presented and a lack of focus. I have edited my question to fix this. \$\endgroup\$
    – xrosaber
    Commented Mar 19, 2023 at 17:53

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The charger can only charge the battery to the voltage that is input to the charger.

If the battery is required to reach some constant voltage to be full, then the charger that can only output a lower voltage can never fully charge the battery.

So it does not matter how long you charge, the output voltage to battery can't climb any higher than the input voltage.

There must also be some discharge protection, as if the voltage is lower at the charger input, it may start discharging that battery, but obviously a good charger would not discharge the battery.

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    \$\begingroup\$ @xrosaber - I upvoted the above answer, but you still might be OK, as some lead acid batteries require a slightly lower CV to fully charge than 14.6; best to check with the manufacturer to see if the 14.2 is enough to fully charge the specific battery that you have. I have lead acid batteries that fully charge at 14.4v. Otherwise, if your battery does indeed require 14.6v, it will be always be slightly undercharged which is not healthy for lead acid. \$\endgroup\$
    – Filek
    Commented Mar 19, 2023 at 22:31

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