Author Topic: Electrical question for the smart folks  (Read 496 times)

Offline klickitat jim

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Electrical question for the smart folks
« on: March 11, 2015, 10:28:01 PM »
If I was running some appliance (lets say 800 watts) off of four 6 volt batteries with ~400 amp hours, through a 1500 watt inverter, for 8 hours.... how many watts of solar would I need to recharge the batteries through a 30 amp charge controller on a normal sunny day?

It cant be as simple as 800 watts used for 8 hrs takes 400 watts charge for 16 hrs?

Offline yso191

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Re: Electrical question for the smart folks
« Reply #1 on: March 12, 2015, 02:40:34 AM »
I can't answer your question Jim, but these guys just down the hill from you in Yakima could I'd bet:

http://www.costlessenergy.com/
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Offline klickitat jim

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Re: Electrical question for the smart folks
« Reply #2 on: March 12, 2015, 03:40:23 AM »
Thanks Steve

Offline wingnut

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Re: Electrical question for the smart folks
« Reply #3 on: March 12, 2015, 12:35:22 PM »
Yeah... a bit of "it depends" goes into the equation.

I don't have a solar array, but I did work on the solar car team back in college.... so I can present the theory... but I am sure someone else out there can speak from experience.

-The theory-

The 30 Amp charger will tell you the maximum amps you are putting into your batteries, but the actual power will be less than that.  (The 30 Amp charger should have a larger capacity than your solar array... or you would smoke your charger).

The actual solar power will vary during the day... it may be near 30amps at peak times, but typically much lower than that. 

To calculate, you would need to know the average amp output of the solar array during the day, and multiply that times the number of hours to get an  idea of the amp hours your are putting back into your batteries. 

Also your charger is not 100% efficient, so for a more accurate equation, you would need to multiply the solar array amps times the effiicency of the charger (been a while, but 80% is a good rule of thumb based on automotive alternators) to figure out how much is going to the batteries.  That should get you pretty close.

I would think the solar array has an watt or amp indicator on it.  (watts are volts times amps)  with that you should be able to start estimating the re-charge time.


Good luck!
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Offline klickitat jim

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Re: Electrical question for the smart folks
« Reply #4 on: March 12, 2015, 07:39:06 PM »
Yeah... a bit of "it depends" goes into the equation.

I don't have a solar array, but I did work on the solar car team back in college.... so I can present the theory... but I am sure someone else out there can speak from experience.

-The theory-

The 30 Amp charger will tell you the maximum amps you are putting into your batteries, but the actual power will be less than that.  (The 30 Amp charger should have a larger capacity than your solar array... or you would smoke your charger).

The actual solar power will vary during the day... it may be near 30amps at peak times, but typically much lower than that. 

To calculate, you would need to know the average amp output of the solar array during the day, and multiply that times the number of hours to get an  idea of the amp hours your are putting back into your batteries. 

Also your charger is not 100% efficient, so for a more accurate equation, you would need to multiply the solar array amps times the effiicency of the charger (been a while, but 80% is a good rule of thumb based on automotive alternators) to figure out how much is going to the batteries.  That should get you pretty close.

I would think the solar array has an watt or amp indicator on it.  (watts are volts times amps)  with that you should be able to start estimating the re-charge time.


Good luck!
Thanks wingnut. I'm starting to get the idea.

Offline wingnut

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Re: Electrical question for the smart folks
« Reply #5 on: March 13, 2015, 12:43:39 PM »
Check this site out....

www.batterystuff.com/kb/tools/solar-calculator.html


looks like it does all the "slide rule stuff" for you... and you get to just play with the conditions of your system.   (looks kind of like playing with Palmer's water spread sheet, or beersmith)

One thing to note... In your previous post, you had noted you had 6volt batteries.... and my spidey sense is going off.

Long story short, you likely have a 24V system... not 6V. I point this out because any amperage gauges you have are likely based on the 24V(assuming) not 6V.   

6 Volt batteries are common in a power bank system, but usually what happens is that the batteries are hooked up in such a way (series) so that you get 12, 18 or 24V systems.  (picture 4 batteries connecteced like this:  -+ -+ -+ -+     that gives you a 24V system. 6+6+6+6)  Likely you have a LOT of banks of batteries in parallel ... so lots of banks of 4 batteries in parallel, to give you the storage capacity you need, and larger amp capacity.

hopefully the site will allow for an easy estimation of time.   If you get some funny results, I am happy to provide a reality check. 


 

-- Wingnut - Cheers!

Offline a10t2

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Re: Electrical question for the smart folks
« Reply #6 on: March 13, 2015, 02:53:24 PM »
What he said, except a solid-state charger is going to be more like 99% efficient.

Also, whatever you're trying to do, it doesn't sound practical. Assuming you can get 1 A at 6 VDC in direct sunlight, you'd still need 400 hours of direct sunlight to charge the batteries. And 6 W is a big solar panel - roughly the size of a sheet of paper. To charge the batteries in an 8-hour day you'd need an array of panels totaling at least 300 W. Which would probably be on the order of 5 ft on a side.
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Offline erockrph

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Re: Electrical question for the smart folks
« Reply #7 on: March 13, 2015, 05:32:02 PM »
What he said, except a solid-state charger is going to be more like 99% efficient.

Also, whatever you're trying to do, it doesn't sound practical. Assuming you can get 1 A at 6 VDC in direct sunlight, you'd still need 400 hours of direct sunlight to charge the batteries. And 6 W is a big solar panel - roughly the size of a sheet of paper. To charge the batteries in an 8-hour day you'd need an array of panels totaling at least 300 W. Which would probably be on the order of 5 ft on a side.
I'll defer to the experts on the technical side, but I will definitely agree with Sean's assessment on the practicality side.

My driveway is lit by a 20W LED array flood lamp run off a 12V battery "on loan" from my trolling motor. That is charged off a 25W solar panel (it's about 16"x20" in size). In the winter, I am lucky to get about 5-6 hours of light out of it on a good night. In the summer, I might get closer to 7-8 hours.
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