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the HF panels are part of a 12v kit but individually they each have a rating. most panels have labels on the back that tell you what the volts and amps.
the HF kits have the following specs on the back. I had a 10.5-30v GTI and my 2 HF kits worked well wired in parallel. My meter would show about 14-17v and 2-4 amps coming from the panels. I chose the 10.5 since i wanted the GTI to kick in as soon as possible. Having a higher Voltage GTI like 28-55 would mean that the GTI will not start producing until your panels hit the cut in voltage (probably lower than 28 someone with one would have to validate).
15w HF panel specs
Power (W) | 15 |
Open Circuit Voltage (V) | 23.57 |
Short Circuit Current (A) | 1.15 |
Maximum Power Voltage (V) | 17.5 |
Maximum Power Current (A) | 0.86 |
I'm not sure I understand the diff. between short circuit and max power.
When I use the formula...
Current equals watts divided by volts...
I = w/v
so watts being 15 per panel divided by volts 12 the answer is 1.25 amps or I panel = 1 and 1/4 amps
That to me means 3 panels under very good conditions can produce 3.75 amps. Most of the time we will get something like 1 amp per panel maybe a little over.
However if you use max volts number (17.5) and use the same formula Watts divided by Volts the answer is .85 amps per panel and for set of 3 panels 2.57amps.
My guess is that with amps being as they are your true amps per set of 3 panels, a harbor freight set of panels, will produce between 2.57 and 3.75 amps. if all is connected properly, good connections, bright sunny day, low humidity, not too hot, etc. etc.
That tells me my 12 panels, 4 sets, will produce between 10.28 and 15 amps per hour into battery bank or GTI or an inverter.
Stay with me here...for example if I had a battery bank that is rated at 200 amp hours and I used 50 amps last night (roughly 25% of battery bank) and my panels put out 12 amps per hour back into the batteries they should be recharged and full in about 4 to 5 hours and I'm ready to do it again and again.
Now to back up just a bit I have 6 batteries at 33 amp hours each wired plus to plus and negative to negative which gives me 33 X 6 = 198 amp hours total. They are rated at 1.75 amps per hour ea. That tells me I can take (since I have 6 of them) 6 x 1.75 amps per hour which equals 10.5 amps).
Now you can take more or less amps out of the battery but the more you take and the faster you take it will not be very good for your batteries. Best to stay within the 1.75 amps per hour. Suppose you will be pulling more amps per hour then get batteries that have a higher amp per hour rate...you will be glad you did.
BTW these are AGM batteries, AGM batteries are the most trouble free and safest out there, for one thing normal freezing will not harm unless a hard freeze may hurt them a bit....not so with acid type or others.
Can any of you tell me if this is correct or am I out in left field?
Dave T
Hi Dave,
Your math is correct but won't exactly fit the HF solar panel since the panel output (Watts) is not linear. To be linear, the curved line in the following graph would be a straight line from the maximum voltage to the maximum current. W/Volts = Amps is a linear equation.
This graph is approximately how the HF panel outputs its wattage.
HF rates its panel at 15 Watts, using an Operating Point of 17.5 volts and 0.857 amps (0.86 amps).
The panel Short Circuit current would be 1.15 amps per specifications posted by Raymond Diaz above.
As you can see from the graph, maximum Short Circuit current comes at close to zero volts (the only voltage available is what the internal resistance of an ammeter develops). The red box would collapse to a straight line to the right. Not many people can use the maximum 1.15 amps at close to zero volts.
Now, starting at the Operating Point of 17.5 volts, raise your voltage to the Open Circuit Voltage of 23.57 volts. As the voltage goes up the red box gets more narrow to the left until it becomes a straight line up and down. The only current available is whatever the voltmeter needs to produce a reading. Again, not many people can use the 23.57 volts with essentially no current available.
15 Watts/12 volts = 1.25 Amps is correct math, but that HF panel won't produce 1.25 Amps at 12 volts. It will only produce 1.15 amps, and that's with the panel leads shorted.
The only way to tell exactly how many amps your panel will produce at 12 volts is to measure it with an ammeter in series with the panel positive output lead and the 12 volt battery.
Hillbilly Gene
Okay my math does not work but is correct. I think you are telling me how amps work under a load and not under a load.
When calculating battery drain and recharge time are my numbers correct or what?
Thanks
Dave T
Hi David,
My reply was intended as more of an explanation of the limitations of using Ohm's Law on the HF panel rather than of how amps work under load or no load, but you seem to get the concept.
I see no fault with your reasoning or math with either of your two battery examples.
The AGM 33 amp-hour/1.75 amps gives an 18.86 hour rating. A little odd, but if that's how that battery manufacturer rates its batteries, then by all means stay within their ratings. As you say, if you need more amps per hour than that, get higher amperage rated batteries. Your battery bank will last longer.
Hillbilly Gene
I have 6ea 33ah batteries based on 20hr rate of discharge at 1.75 amps per hour per battery is...1.75 x 6 = I can draw 10.5 amps per hour total being nice to the batteries.
I think you goofed with the 18.86 hour rating or I'm loosing it.
Help me here Gene?
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