Ghurd, you listed this formula: solar amps equal to battery AH / 20 or 25.
My new solar amps based on the changes above are 26 amps. Does that mean that my ideal battery bank has only 650 AH (26 X 25)? If I use 90AH/day I'll be at 28% draw down in two days without sun, and 69% draw down in five days without sun, which is pretty common here in NH in the winter. Originally I was planning on being able to go five days with only a 20% drawdown (2200 AH). Am I reading that formula correctly?
Starting with rated Watts - say 4 125W panels, 17.3V, 7.23A wired in series...
If everything was perfectly efficient and real life equaled rated life...
Using an MPPT controller I should get Vmodule/Vbattery*Imodule amps into my 12-volt battery bank. 69.2V/13.5V*7.23A = 37A. I used 13.5V as sort of an average between 12 and 14.7.
Now, I've got to factor in the inefficiences:
Real life panel Wattage - 80% of rated Controller efficiency - 93%
So my 37A becomes 37*.8*.93 = 27.5A.
That's about 20% higher than I'd get if the four panels were in parallel, assuming the same 80% "real life" reduction, so that jives with some of the earlier comments.
Are those reasonable percentages, and are there other inefficiencies I haven't considered?
Thanks for lending your experience to this newbie - Walt[ Parent ]
Full sun with MPPT will give you around 193watts / 14volts = 13.7 amps Not as you calculated. (assuming everything works right, stated right etc.) Several times improvement over direct connection to battery, well worth doing. Else can the panel be rewired for 12v output (or cut in half) would help, still not achieving MPPT potential .
Adding a tracker can be very beneficial, several nice designs out there DIY not costly.
!!!Important discussion on Large batteries!!!!
Solar power is expensive so are LARGE batteries. Lead Acid batteries are 91% efficient charging when state of charge (SOC) is below 80%. SOC Above 80% efficiency drops to zero as 100% SOC is reached. Battery is fully charged. By 90% SOC charge efficiency maybe less than 50%. By 95% SOC charge efficiency maybe less than 25%. If you expect to get 5 days only discharging 20%, then your working in the 90% SOC most of the time. Several factors affect charging efficiency, SOC, battery temperature!, Battery voltage and charger. The above numbers can be improved... another discussion. (not by a huge amount)
Lead acid batteries are made to be used, using only the upper few percent the batteries may last for many many years, you also spent a LOT of money for them. To keep the large battery charged you will need a solar panel 2-4x larger! And the battery is 4-5x larger than needed. Over all not a good deal to over size the battery. Charging a Lead acid battery fully is also stressful, getting above 95% requires a finishing charge raising the temperature stressing the battery!
If you NEED the battery to last forever because you believe the world is going to end (or some other like reason) they by all means over size everything with the understanding in cost today. (Storing a dry battery can last many years... (my choice)).
Reducing the battery so by the 5th day ends SOC=20%, will stress the battery, if occurs frequently like a couple dozen times a year consider using a larger battery. If not then I would not size it any bigger.
Just a thought, by starting out with the smaller battery, save the money you would have spent, you have enough to buy 4 more sets! by the time all are warren out, I bet it will be longer than the time the over sized battery set would have lasted. And you saved 2-4x on the solar panels or generator run time.
Many people with over sized batteries must run a generator frequently to keep it charged, solar panel array is not large enough to keep their loads and battery fully charged. They do not understand why this is happening. Now You know.
Hope this helped!
Have fun, Scott. [ Parent ]
Very good point!! Not many suppliers of Ni-Fe batteries, may cost more than the over sized battery. Remember to keep more electrolyte around to replace it every 10 years.
One slight down side to Ni-Fe battery is the charging efficiency is 80%, not changing over life nor state of charge (anybody know differently?). Another source suggests 70%. Or should I say the efficiency is an advantage if you want to keep the battery topped off?
Have fun, Scott.[ Parent ]
www.beutilityfree.com/nife.html is a source that imports them from China.
Most of the electrolyte is KOH with a dash of LiOH, so it wouldn't be too tough to store a lifetime supply as dry powder.[ Parent ]
By this statement, I see that you are assuming a 96.5% controller efficiency:
"Full sun with MPPT will give you around 193watts / 14volts = 13.7 amps Not as you calculated." (Also note that I changed to four 125W panels halfway through the post).
But what about the full sun part. The panels will only produce their peak power in full sun, perfect conditions, which is not all the time. I thought I had already factored in the rarity of perfect conditions by using only three hours of insolation, but should I knock down the rated panel wattage by some percent also?
Thanks - Walt[ Parent ]
Welcome, hope it was help full.
Solar for a given location is rated in full sun hours. I live in Michigan, which is rated at 4.5 full sun hours a day. AZ is 9. The rating is based on average over the year and weather, fixed panels. Look around, charts for this.
The reality we have daylight for 16 hr in summer, 8-9 hr in winter.
Morning sun is weak, yet solar panels generate power during this time charging the batteries, current starts out low, reaching near peak current by 11am, stays at peak for couple hours then declines.
Tracking helps by keeping the full sun reaching the panel, can add a couple hours of near peak power a day, further north the more helpful tracking and MPPT is. (and south of equator same thing). In Michigan tracking adds about 30% power collected a day, equal to power added by MPPT (based on many assumptions, standard panels being the first one).
When I do calculations I show all of it, reduces confusion of what I'm doing, helps other to learn by seeing the example!
The 20~25% gain would be about max. I would figure closer to 10% average.
A couple things will work in your favor. You won't go 5 days with "No Sun". It may not be bright, but you will get some charging everyday. MPPT will put more amps into a lower battery than a fuller (is that a word?) battery, given the same 'sun'.
The 650AH seems reasonable to me. And that is 3 pairs of 6V golf cart batteries from Sam's Club / Costco, though I would probably go with 4 pairs. Might look into 2 pairs of L16 or L16-HC if the wallet can handle it. Interstate is less expensive than Trojan. It will make a lot more power in summer than the 3 hour insolation.
G-[ Parent ]
