Confusion About Charging and Controller Power

[DenverGuy]

I have a 100Ah (1200 watt hours) 12-volt battery and a 100W 12-volt panel, and here is what I came up with:
At full sunlight it would be delivering around 5-6 amps or (50-60 watts) to the battery.

I know I shouldn't go below 12.2 volts - 50% of the capacity.

Where I run into problems is with my understanding of the charging time and storage capacity relationship.
I hope this makes sense... How can I calculate the charging time from 50%? If I am generating 5A (60 watts) from the panel, (omitting losses), how can I calculate charging time?

Also, Will talks about how important it is to not exceed the rated output of a charge controller, but aren't the input values important, too? I would think it would be a problem to have 5 panels and 35 amps going into a 30A controller, right?

 

[MisterSandals]

At full sunlight it would be delivering around 5-6 amps or (50-60 watts) to the battery.

Assuming you are charging to 14V:
100W / 14V = 7.1A

12.8V nominal battery x 100Ah = 1280Wh capacity.

Charging 50% of your battery (100% of usable capacity) = 640Wh

640Wh / 100W panel = 6.4h

When you have the units and Ohms law, it shakes out pretty easily.

 

[MisterSandals]

I would think it would be a problem to have 5 panels and 35 amps going into a 30A controller, right?

You would not put 5 panels in parallel for most SCCs, especially MPPT.

What is the Vmp and Imp of your panel? I can show you the math.
If you have a specific SCC, with max input volts and charging amps, it’s straightforward in determining best configurations.

And you should NOT be fixated on “12V” panels with a 12V battery. It’s the job of an MPPT SCC to worry about converting array volts and amps to charging volts and amps.

If you are already considering more panels, start looking for big cheap panels. Craigslist is a good source. If you post your city or zip code, maybe someone can point you to deals in your area.

 

[DenverGuy]

Assuming you are charging to 14V:
100W / 14V = 7.1A

12.8V nominal battery x 100Ah = 1280Wh capacity.

Charging 50% of your battery (100% of usable capacity) = 640Wh

640Wh / 100W panel = 6.4h

When you have the units and Ohms law, it shakes out pretty easily.

Well, I never get 7.1A out of the panel. It's always 5-6 on the high end.
Where did the 6.4 hours come from? The panel only produces 50-60W.
Is there a specific formula for predicting charge time according to the power being produced?

 

[MisterSandals]

Well, I never get 7.1A out of the panel. It's always 5-6 on the high end.
Where did the 6.4 hours come from? The panel only produces 50-60W.
Is there a specific formula for predicting charge time according to the power being produced?

I am just using panel specs (100W) which is for optimal solar conditions (overhead).
I am not sure if your 60W is what the panel is being measured at or if that is what your SCC is pulling (will pull less as charging a full battery slows).

640Wh / 60W = 10.66h

 

[DenverGuy]

I am just using panel specs (100W) which is for optimal solar conditions (overhead).
I am not sure if your 60W is what the panel is being measured at or if that is what your SCC is pulling (will pull less as charging a full battery slows).

640Wh / 60W = 10.66h

No panel is 100% efficient. If my panel is producing 5 amps at 12 volts, I am assuming that that's 60 watts. The formula makes sense now. It's odd how they rate the batteries in amp hours, but charging is figures in watt and hours.

 

[timselectric]

It can be converted to ah's.
But, wh's is universal to all voltages.

 

[MisterSandals]

If my panel is producing 5 amps at 12 volts,

A 12V panel does not produce 12V. Look at the Vmp on your panel label, that should be the expected voltage.
So you aren’t measuring your volts or amps? Where are you getting your numbers?

And, it’s not uncommon for a panel to produce 100% of rated specs under excellent solar conditions and panel orientation.

 

[DenverGuy]

A 12V panel does not produce 12V. Look at the Vmp on your panel label, that should be the expected voltage.
So you aren’t measuring your volts or amps? Where are you getting your numbers?

And, it’s not uncommon for a panel to produce 100% of rated specs under excellent solar conditions and panel orientation.

I'll have to look at the new panels when they arrive tomorrow. The charge controller is what is giving me the voltage and amperage.