@Hedges is more or less correct.
Let's take an example to make it easier to see how it works.
I have some experience with solarland panels, so here is an example panel data sheet from the web site. It is just an example, nearly all solar panels will more or less behave similarly.
https://solarlandusa.com/content/Product spec sheet/SLP200S-12U 200031201A.pdf I also attached it for convenience.
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Look at the voltage / current curve on the data sheet. What is shows is that over quite a range of temperature and output voltage, the current will remain more or less the same. ~ 12 amps on a very bright sunny day. On a very overcast day, it might drop to 2 amps and the voltage will also drop some.
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So now imagine that you have this solar panel directly feeding your 50% discharged AGM battery through fuses for safety reasons. The solar panel output at 12 amps is a small fraction of what the battery can accept in terms of bulk charging.
For fun and as an example of this, I built a small test fixture so that it is easier to follow:
Sometimes it is easier to show people things in photos than to describe them. So I made a little test fixture from left over parts, nothing fancy. In this case, some people asked about methods of dealing with pre-charging the capacitors on the input stage of inverters, and I had suggested that...
diysolarforum.com
The battery will pull down the panel voltage to match what it is at while it charges. Literally all of the power coming off of that panel ( minus small losses in wires and fuses) will be pumped into the battery for charging.
You can even use this as an emergency method to charge a starter battery in the middle of no where.
The limitation is that at some point the battery will reach full charge, and you will need to manually disconnect the solar panel from it.
What the flexcharge does is that it monitors this voltage and turns on an off an internal relay to control the charging. They use a relay to get very low impedance and RF noise in a small package.
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So the efficiency is very high ( it is a hard connection ). What is lost is small delta in voltage being turned into useful power that is from roughly 15 to 14 volts that the panel could in theory produce, but in reality, there are wire losses, etc so it is not all that significant for this example.