Is the amperage from a solar panel constant as the battery it is charging charges up?I.e., as your battery reaches full charge, the current drops to zero either via the charge controller or the equalization of the panel and battery, right?So this simple math is false? 120 watt panel, 12 volt system, 10 amps, 10 hours to charge a 100 Amp Hour battery?

Yes, false. Charging to 80-85% goes quickly with the battery accepting everything or almost all sent to it. During the absorption phase the battery accepts less and less.

A 120w panel does not output 10ampsOut put it more like 7 ampsFull Working voltage of a 12v panel is around 17v,Output from the controller is 14.x to properly charge the battery12v won't charge a battery, a battery at 12v is only 50% charged/capacity

It really depends on your charge controller. Some are "smart chargers", slowing the charge rate as the battery nears the full stage.

There are still experienced RVers with dumb controllers?

DarkSkySeeker wrote:Is the amperage from a solar panel constant as the battery it is charging charges up?I.e., as your battery reaches full charge, the current drops to zero either via the charge controller or the equalization of the panel and battery, right?So this simple math is false? 120 watt panel, 12 volt system, 10 amps, 10 hours to charge a 100 Amp Hour battery?False in many ways :(The current to battery rises from first light to high noon (when the panel output is as high as it can get with the sun that high--which is not necessarily the most the panel could do if the sun were higher at that time of year in that place-- and then it goes back down till dark as the sun's angle gets less direct again.So no constant amps as the battery charges up.With PWM, the "panel voltage" is the battery voltage--they do not eventually equalize. Current output is iaw the IV curves for that panel--see the specs for any panel for its IV curves.As the battery nears full charge, its amps acceptance rate tapers as usual, so whatever the solar output is by then, amps will taper. Now you might see 'extra amps' that can be used to run loads without stealing any from battery charging.To learn what AH haul a panel can provide, there are tables on line for locations and times of year that give "insolation" figures. Use those with the panel specs.

Solar Charging and Amp Hours | Good Sam Community

red31
Explorer
Apr 20, 2020
yes, cuz it is not connected to anything!

excess power is not made, the panel is disconnected from the controller.
The on/off connection to the panel is done at high freq, the OFF time gets longer and longer to limit power to the controller/battery.

add a big load and then the battery is getting very little but the controller allows the extra to run the load.
time2roll
Nomad
Apr 20, 2020
DarkSkySeeker wrote:
And the power output of the panel drops down? Eventually meaning a 100 watt panel no longer puts out any power?
Power (watts) = amps x volts..... Zero amps, zero power.
DarkSkySeeker
Explorer
Apr 20, 2020
Thank you for this help. In my mind, I thought only the battery side of the controller was modulated. Now I understand the charge controller allows current flow from the panel in the same way.
full_mosey
Explorer
Apr 21, 2020
DarkSkySeeker wrote:
Thank you for this help. In my mind, I thought only the battery side of the controller was modulated. Now I understand the charge controller allows current flow from the panel in the same way.

Amps are DRAWN by the consuming device. That is why your 1 Amp incandescent bulb does not explode when connected the same battery that can send 200A to the starter motor. You explode the bulb by applying too much Voltage.

HTH;
John

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