Best Way to Charge

[4-20mA]

In a stand alone system...

I've never used these or tried this, but trying to figure it out now. Using batteries to operate a very remote gate. Anyway, should I use a bigger controller to utilize more amp from parralleling the charger panels?

I *think* I need to take out the third battery AND add the second panel to get 70Ah capacity on a full charge.

100W panel at optimum power (5hrs good sun per day) yields 500Wh/day
(100W * 5hr = 500Wh)

12V * (35Ah * 3batt) = 1260wh
which means that even with two panels (1000Wh) I can't charge all three batteries


so...


2 panels (in parallel, I think, to maintain the voltage and add the current) should yield 1000Wh

Panel: 12V @ 100W -> 1000Wh/day

Batt: 12V * (35Ah * 2batt) = 840Wh


Provided that the charge controller can handle the amps that should do it. Trying to figure out the amps:
--------------------------------

- Panels In Parallel -


V * A = W

12V * X amp = 200W (both panels in parallel)


X = 17Amp available


17 * 1.25 safety factor = 20.83 amp

So would I need a 50amp controller then or a 20amp DC breaker? What say you

 

[wwhitney]

Not following your analysis process.

If the PV panels on a given day produce more energy than your gate uses, the battery SOC will increase that day (or the energy won't be generated if the battery SOC is already at 100%). If they produce less energy on a given day than your gate uses, then the battery SOC will decrease that day.

So as a first approximation, the PV panels should sized so that the average power (at the worst time of year) slightly exceeds the gate's average power usage. That means you'll need to predict the average number of gate operations per day and the gate's energy use per operation.

Of course, it's not that simple, you really need a statistical model. E.g. so you can figure out the worst case run of PV production deficits you can expect (cloudy days), and compare that with the worst case gate energy usage over the time period (could be more operations than average), and use that deficit size to size your batteries.

And it's conceivable that adding more PV would let you reduce your required battery size. E.g. if the controlling worst case were 10 cloudy days followed by 3 sunny days followed by 10 cloudy days, more PV power could give you a higher state of charge going into that second set of 10 cloudy days.

Cheers, Wayne

 

[jaggedben]

To agree with Wayne and put a fairly simple gloss on it...

Size the battery in kWh to be able to power the load for a few rainy days, no sun. Consider lowest SOC you're willing to risk, depends on battery type. The energy draw of the load is the info you left out of your post.

Size the PV panels to provide somewhat more monthly kWH than necessary, in December. Use pvwatts.nrel.gov

Size the charge controller for the max amps the PV may put out, so that you don't miss out on energy harvest (unless batteries are already full).

Don't worry too much about the battery SOC dropping over successive rainy days if it will charge back to full over successive sunny days. Better to have too much battery and not use it all the time, if keeping the load on justifies the cost.

 

[4-20mA]

Thanks for the wise words, fellas!

 

[solarmatt]

I like to use the following process for sizing standalone systems with lead acid batteries:

1. Estimate daily load in Wh or kWh
2. Size a battery for at least 3, preferably 5 days of storage if you don't have a backup power source
3. Select a charge controller with a current rating as close as possible to the maximum charge rate of the batteries
4. Max out the charge controller with solar

This process ensures you have plenty of storage and that the batteries will get recharged quickly after low sun periods.
Lithium has a higher charge rate per Wh of storage than FLA, so this is overkill for lithium.