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

time2roll
Nomad
Apr 19, 2020
Nothing is constant in solar unless the sky is perfectly clear and you can stop the sun.

Your controller will prevent overcharging by reducing voltage as the battery becomes full.
pianotuna
Nomad III
Apr 19, 2020
jdc1 wrote:
It really depends on your charge controller. Some are "smart chargers", slowing the charge rate as the battery nears the full stage.

JDC1,

Pretty much all solar charge controllers at some point in charging use PWM, so they are all "smart" chargers.
StirCrazy
Moderator
Apr 20, 2020
pianotuna wrote:
jdc1 wrote:
It really depends on your charge controller. Some are "smart chargers", slowing the charge rate as the battery nears the full stage.

JDC1,

Pretty much all solar charge controllers at some point in charging use PWM, so they are all "smart" chargers.
''Except for MPPT controllers which act as a DC charger.

Steve
pianotuna
Nomad III
Apr 20, 2020
StirCrazy wrote:
pianotuna wrote:
jdc1 wrote:
It really depends on your charge controller. Some are "smart chargers", slowing the charge rate as the battery nears the full stage.

JDC1,

Pretty much all solar charge controllers at some point in charging use PWM, so they are all "smart" chargers.
''Except for MPPT controllers which act as a DC charger.

Steve

No MPPT also have a PWM function.
red31
Explorer
Apr 20, 2020
DarkSkySeeker wrote:
Is the amperage from a solar panel constant as the battery it is charging charges up?

ignoring changes in sun, temp etc the answer is yes. At a given set of conditions the IV curve of a panel shows the amps nearly constant over a wide range of voltage. A PWM controller lets the panel connect directly to the batt and all the power goes to charging UNTIL a set pt voltage of the batt is reached and then the controller begins to regulate power to the batt not letting the battery exceed the st pt voltage. Less and less power is needed as the battery tops off.

If sun, temp etc conditions stayed the same over time then the charging amps would be the same, say a 100w panel 'makes' 5A, then the 12v battery would get 5A (60w) and later would get 5A @ 14v (70w).(watts = v * a)

A mppt controller forces the panel to operate @ the mppt (16-18v for a '12v' panel), the power would be constant if conditions stayed the same, more amps into a 12v batt than a 14v battery (A=W/V). At set pt voltage, the controller limits power to maintain set pt.

During regulation (at set pt voltage) excess power is available for other uses or is never produced.

changes in conditions changes solar output power
below is the effect of light variations
DarkSkySeeker
Explorer
Apr 20, 2020
If the controller gradually limits current to the battery as the battery charges up, where does the power from the solar panel go? I assume the panel continues to gather solar energy and sends the power it generates to the controller. What does the controller do with the power it no longer sends to the battery?
pianotuna
Nomad III
Apr 20, 2020
Unless there is a diversion load, that extra energy is simply wasted. When that happens to me, I turn on the inverter and turn on the water heater for a few minutes.
BFL13
Explorer II
Apr 20, 2020
The controller does not gradually limit the current to the battery. The battery is the one doing the limiting as amps taper.

One way the controller does limit amps to the battery is suddenly, not gradually, if it drops voltage from 14.x to 13.x as part of its programmed charging profile. The battery will then accept fewer amps at that lower voltage at the same SOC.

Another way is at first during Bulk, the controller (if an MPPT type) will "clip the amps" to ensure it does not over-heat by going over its output amps rating.
time2roll
Nomad
Apr 20, 2020
DarkSkySeeker wrote:
If the controller gradually limits current to the battery as the battery charges up, where does the power from the solar panel go? I assume the panel continues to gather solar energy and sends the power it generates to the controller. What does the controller do with the power it no longer sends to the battery?
Does not go anywhere.

Same as a 15 amp outlet at home. If you plug in a lamp that draws 1 amp where does the other 14 amps go?

The controller limits voltage not amps. The controller provides a fixed voltage and the electrochemistry of the battery decides how many amps will flow.
DarkSkySeeker
Explorer
Apr 20, 2020
And the power output of the panel drops down? Eventually meaning a 100 watt panel no longer puts out any power?