Good Sam Club Open Roads Forum: Tech Issues: Solar Charging and Amp Hours
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DarkSkySeeker

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Posted: 04/19/20 01:15pm Link  |  Quote  |  Print  |  Notify Moderator

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?


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Lwiddis

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Posted: 04/19/20 01:26pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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MrWizard

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Posted: 04/19/20 01:37pm Link  |  Quote  |  Print  |  Notify Moderator

A 120w panel does not output 10amps

Out put it more like 7 amps

Full Working voltage of a 12v panel is around 17v,
Output from the controller is 14.x to properly charge the battery

12v won't charge a battery, a battery at 12v is only 50% charged/capacity


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jdc1

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Posted: 04/19/20 01:49pm Link  |  Quote  |  Print  |  Notify Moderator

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

Lwiddis

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Posted: 04/19/20 01:58pm Link  |  Quote  |  Print  |  Notify Moderator

There are still experienced RVers with dumb controllers?

BFL13

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Posted: 04/19/20 02:17pm Link  |  Quote  |  Print  |  Notify Moderator

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 [emoticon]

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.


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full_mosey

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Posted: 04/19/20 02:37pm Link  |  Quote  |  Print  |  Notify Moderator

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?


I am going to try to answer a question you posed last June. Then we can work up to explaining what solar panel specs there are.

YOU wrote:

What I don't understand is how a system that generates 20-50+ amps doesn't fry the batteries.


Amps alone cannot 'fry' batteries, I wish you would explain what 'fry' means. But I digress.

In the world of DC, Watts = Volts * Amps. Now if your battery is resting at 12.5V and you apply one million Amps at 12V, then it is likely that Amps would be taken from the battery.

What! Yes, to charge a battery, you MUST apply higher Volts than the battery's standing Volts. It is the difference between the charging Volts and the battery's Volts that matter. If you want to raise a battery to a set Volts, you must also set the desired Volts with your battery charger.

The battery's specs dictate the upper limits of both Volts and Amps. The charger must obey these limits. It is your responsibility of employ a proper charger.

When you connect a solar panel directly to your battery, you can see there is no regulation mechanism to meet the battery specs. You don't want that if the panel specs exceed the battery specs.

HTH;
John

RickLight

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Posted: 04/19/20 03:16pm Link  |  Quote  |  Print  |  Notify Moderator

Even those with training can forget the rock bottom basics.

The important thing is that current (amps) is a result, not a force. (Did you know batteries have resistance?)

Resistance is a property of things and as such doesn't vary easily. (Yes, very simplified) Voltage is generated and applied to a resistance. Those fundamental aspects will make current/amperage happen.

When current flows "work" can happen. Watts is a calculation we find useful to describe overall power/work.


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MDKMDK

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Posted: 04/19/20 03:17pm Link  |  Quote  |  Print  |  Notify Moderator

DarkSkySeeker wrote:

Is the amperage from a solar panel constant as the battery it is charging charges up?

No. Solar panels aren't an exact science. Solar input affects panel output. Shade (clouds), sun to panel angles, TOD, charge controller charge algorithms, many things affect the amount of charging output that reaches the battery.

DarkSkySeeker wrote:

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?
If the charge controller sees and reacts to the increasing internal resistance of the battery to accepting more charge as it fills up, yes.

DarkSkySeeker wrote:

So this simple math is false? 120 watt panel, 12 volt system, 10 amps, 10 hours to charge a 100 Amp Hour battery?
Yes, that math is false. Real numbers are probably closer to Mr. Wizard's numbers.

You didn't ask about the differences between lithium and lead acid charging? That's where the opinions really start to diverge. [emoticon]
p.s. I know little to nothing about this stuff.


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wopachop

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Posted: 04/19/20 04:36pm Link  |  Quote  |  Print  |  Notify Moderator

DarkSkySeeker wrote:

Is the amperage from a solar panel constant as the battery it is charging charges up?
I agree with everyone that says no. But in a very simplified view that answer can be close to a yes.

Heres an example. My solar charge controller is in the hallway. While camping i pass it often. Seems like during the day every time i look its displaying the same number of charge amps.

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