pianotuna wrote:
Hi BFL13,
If the solar is to be autonomous and not rely on (gasp) a generator, then it behooves the designer to have the possibility to equalize. If you accept (Mexico Wanderer does) that 5 amps per 100 amp-hours of storage is the bare minimum to allow that to happen, then a well designed system, where no other charging source is to be used should meet that 5 amp window. This translates to about 60 watts per 100 amp-hours.
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I don't understand this jump from amps to panel watts for equalizing. Mex also agrees that amps will taper if voltage is held at 15.5v so even the amps is changing. the question is really how to get the batts, however many, to 15v in the available daylight while also drawing amps to run the rig.
It depends on what the voltage was at the start of the day and how long is the day etc too. Where does the 60wper 100AH of capacity come from? IE what is the scenario? Isc of an 80w 12v panel is 5 amps, but you get the IV curve drop off at 15v battery. You can still equalize since at that voltage it takes few amps to keep the batts bubbling.
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A small battery bank may show a greater depth of discharge--so more solar is needed per 100 amp-hours. For a larger bank of say 500 amp-hours the three hundred watts of solar may be enough to return all the energy used.
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ok that is relating solar re-supply to AH usage so any fixed usage amount will be more percentage of a smaller bank than of a bigger bank.
What you want is how much will a given solar set-up do in a day. eg my 130w panel in may at 49N got 56AH lying flat, 70AH tilted aimed South , and 90AH tracking. But that was with a load running all day.
Where battery bank size comes in would be that a smaller bank gets to higher SOC quicker (when amps taper) so you would get fewer AH/day. Now you have to be clever and do your usage when the batts are fuller to get some AH from the "extra" solar in the PM. If you run that stuff in the AM , the batts will not get their amps to get to that SOC by dark.
All this cannot be addressed by some panel wattage vs Battery capacity AFAIK. I use the same AH per day whether on two batts or six, and the same amount of solar replaces it either way. All it means is that with more batts I am doing shallower cycles. Same size array.
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If an inverter is used then the amount of energy used may skyrocket. I can and do run my air conditioner from the battery bank. That draws about 140 amps. To replace that energy use with solar may be too much of a challenge, even if I go to MPPT and very high output panels.
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Inverter use is just part of the daily AH draw, however much you use it. Does not affect the need to replace that usage, just the amount needed.
BTW if it is hot enough out that you need air conditioning, kiss the "high output" goodbye when using 24v panels and MPPT. Stick with 12v panels and PWM in hot temps so you maintain the rated output you designed the set-up for. As posted elsewhere I am losing amps with MPPT with temps in the high 80s below where air conditioning is needed. (with panel tilted up in a breeze too--still hits 51C underneath aiming the IR gun at the white backing from a foot away-- Hate to think what mena would get at his place in the Anvil of the Sun with MPPT. Being clever, he has chosen PWM when last heard from on that.
