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My understanding is, for charging to occur, a solar charge controller needs to output at a higher voltage than the battery's voltage.

If so, why, when you test the battery connection of a solar charge controller while it is charging, a voltmeter reports the battery's voltage, and not the charging voltage? Or, how does a solar charge controller accurately report the battery's voltage, while it is charging at a, presumably, higher voltage?

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  • \$\begingroup\$ The battery voltage IS the charging voltage (which is usually stepped down from the solar panel voltage) \$\endgroup\$ Jan 15 at 13:45
  • \$\begingroup\$ Unless current times series resistance isn’t significant, you won’t see any instantaneous change in voltage. \$\endgroup\$
    – winny
    Jan 15 at 15:33

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A solar panel is a current source over most of its characteristic, and the solar charger sets a charging current for the battery (usually until a pre-set maximum voltage has been reached).

While being charged with a (constant) current, it is the battery itself that determines the voltage; the charger doesn't set a voltage. It does move to a next step in the charging algorithm when a pre-set maximum voltage is reached.

So, it is the battery setting the voltage, and this is the voltage you are measuring when it is being charged. As it is connected to the charger with near-zero-resistance wires, this will of course also be the voltage you measure at the charger output.

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  • \$\begingroup\$ But in order for current to flow there has to be a voltage, Ohm's law demands it. You should expound on that. \$\endgroup\$ Jan 15 at 18:54
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    \$\begingroup\$ @MarkRansom neither batteries nor solar panels are Ohmic devices. If you neglect the resistance of the wires, Ohm's law doesn't factor in. \$\endgroup\$
    – mbrig
    Jan 15 at 20:10
  • \$\begingroup\$ You cannot have current flow without voltage applied somewhere. That's why it's Ohm's law. \$\endgroup\$ Jan 15 at 21:00
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    \$\begingroup\$ @MarkRansom First of all, Ohm's law represents a common material property: plenty of materials violate Ohm's Law. "Law" here just means "functional relationship". Second, although you are formally right here (the copper or aluminum connections are ohmic), the resistance is so low that the voltage drop is negligible. The battery itself, for a given state of charge, has a non-ohmic voltage/current relationship, and indeed the voltage must exceed the battery's open circuit voltage to charge it, but the open circuit voltage is not the voltage you measure when the battery is charging. \$\endgroup\$
    – John Doty
    Jan 15 at 22:38
  • \$\begingroup\$ @JohnDoty All non-ohmic means is that the relationship between voltage and current is non-linear. It does not mean that you can magically create current without voltage. But at least I think we agree on that, given your last statement. \$\endgroup\$ Jan 16 at 0:37
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A solar charge controller needs to output at a higher voltage than the battery's voltage.

Depends what you think "battery's voltage" means. When the output of the battery charger is connected to the battery, then the output voltage is the battery voltage and vice versa.

In order to charge the battery, that voltage must be higher than the open circuit voltage of the battery (i.e., the voltage that you would measure between the battery's terminals, when the battery is not connected to anything else besides the meter.)

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When you connect two things with a conductor, there's no such thing as "voltage of the thing on one side, and voltage of the thing on the other side".

You're confusing the unloaded battery voltage with the voltage across the battery while charging.

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