Running a 1/2 hp hydraulic power unit with solar panels

J,
Just to clarify, when you add batteries in series, the voltage adds. 6 volt + 6 volt = 12 volt battery bus.

When you add batteries in parallel, their capacities add. For example, take the 2x series string above (12 volt @ 200 AH) beccomes 200 AH + 200 AH = 400 AH for the bank.

So a 2s x 2p battery bank (made from 4x 6 volt @ 200 AH "golf cart" batteries) would be 12 volts @ 400 AH.

The reasoning behind the battery bank design and solar array sizing... Flooded Cell Lead Acid batteries, while they have lots of surge current capability (i.e., a relatively small car battery can output ~50-100 Amps to start a car for a few seconds or minutes at a time). But to pull larger amounts of current for longer periods of time, you need a larger battery bank. Generally, a C/8 (from 100% to 0% state of chage in 8 hour period), is about the maximum you would like to reliably pull from a FLA battery bank. You might be able to get a C/5 hour discharge rate (2x more current), but this might be a little iffy if when the battery bank gets over a couple years old in cold weather with a partial charge.

So, using ~746 Watts per HP, 80% efficiency, and 10.5 minimum voltage (assuming you are loading to 1/2 HP on motor):

• 746 Watts per HP * 1/2 HP * 1/10.5 volts loaded battery = 35.5 amps (worse case "nominal" estimate)

And if you want to discharge the FLA battery bank at 35.5 amps at an 8 hour rate (I am assuming you are using a 12 VDC 1/2 motor--If you are using a 120 VAC motor and inverter, some different fudge factors):

• 35.5 Amps * 8 hour rate = 284 AH @ 12 volt battery bank "nominal"

Every 2x series 6v@200AH string makes a 12v@200AH ... So 1x string is a bit on the small side, and 2x string is on the big side (200 vs 400 AH and target is ~284 AH based on our first pass guesses).

Big (relatively) 12 VDC loads for 10 minutes--Need large(r) bank because of your high load current--But it is only 10 minutes, so the actual AH or WH per day is not that large.

In general, if you "had" to run 10 minutes every day regardless of weather/season, then you do want a larger solar array (i.e., 40 Watt panel assumes you use 100% of December's daily harvest). So, I would be suggesting 2x (50% of daily harvest) for your "base loads" (loads that need to be supported every day--And if you have a string of very cloudy days, the next choice is more panels/more storage or using a genset for the few days of bad weather you may have).

And this is where we have an issue with this design. Very high discharge current (1/2 HP pump motor) gives us a relatively large bank size to manage to supply the current. And the fact it is 10 minutes every day or two--Only a small amount of AH capacity/storage is being used:

• 35.5 Amps * 10 minutes per day / 60 minutes per hour = 5.9 AH @ 12 volts of battery stored energy

Using our normal 2 days of storage and 50% discharge (i.e., if you were using LED lighting for 5 hours per night):

• 5.9 AH / 5 hours per night = 1.2 Amp load (for 5 hours per night)

So, if this was a small load divided across 5 hours (2 days x 1/0.50 max discharge) or ~24 AH @ 12 volts for a "non motor" load.

Anyway, here we are stuck with a relatively large battery bank. For FLA batteries, a minimum of 5% rate of charge (based on the 20 Hour AH capacity of the battery bank) is good for a lightly used system (which you describe). The "optimum" rate of charge is 10% for FLA deep cycle batteries. And that is where we get the large solar array (vs your actual load).

So--This really depends on what you are going to use the system for, and how much pain it would be if it stopped working for you one day (poor solar harvest, needed to cycle the pump 10x over several days because of conditions or somebody servicing the pump/hydraulics).

If you wanted to keep the costs of the system down, and if the system ends up not supplying your needs reliably does not hurt you badly (i.e., you go out in a few days with more battery/genset/more solar panels to support the extra loads that were not planned for, etc.)...

I would start with a 2x "golf cart battery" system (assuming my current estimates are correct)... 2x 6 volt @ 200 AH batteries in series gives you a 12 volt @ 200 AH battery bank. If you are normally going to cycle the system 2-4 days a week, then a solar array would be:

• 14.5 volts charging * 200 AH * 1/0.77 panel+controller deratings * 0.05 rate of charge = 188 Watt array (minimum suggested).

Note that solar numbers are pretty much +/- 10% (i.e., 188 watt array vs 207 Watt array) is pretty much "dead on".

FLA batteries have (relatively) high self discharge (especially as the batteries get older/towards end of life) and less than optimum charge acceptance (not a lot of hours of sun during winter)... So the 5% rate of charge is a minimum I would suggest... You could get away with less array, but under charging/over discharging a battery bank is a common cause for failure.

If you picked an AGM (can take higher 10%+ charge current) or GEL (can only take a maximum of 5% rate of charge) battery, these support higher surge current and have lower self discharge. You could get away with a smaller battery bank (say C/2 hour discharge rate):

• 35.5 amps * 2 hour discharge rate = 71 AH @ 12 volts capacity

And if you use 5% rate of charge:

• 71 Amps * 14.4 volts charging * 1/0.77 panel+controller derate * 0.05 rate of charge = 66 Watt minimum panel size

Here is a nice (higher quality) 12 volt @ 85 AH AGM battery for $255:

https://www.solar-electric.com/fullriver-dc85-12-agm-sealed-battery-12v-85ah.html

(above links are to our host NAWS Northern Arizona Wind & Sun--I am just giving you a starting point for searching--I am not saying this is the only/best solution, and if you purchase elsewhere, that is fine with us).

So, now, you can use a smaller array--But that also gives you less "head room" in case you need more power (or other things you may like to run at this site).

If you are using a 120 VAC 1/2 motor, we need to account for the inverter's efficiency and some other issues.

Also, it depends on if this site is visited every few days and you manually run the pump, or you only want to drop by once a month and forget about it otherwise.

Solar systems really need supervision, they do not do well if not inspected/maintained regularly. For example, if you want to automate the site and only go by once a month, a larger battery bank and solar array just allows for things to somewhat "go wrong" but the system still works and does not "murder" your battery bank.

For example, a 200 or 400 AH battery bank can go many days or week+ without sun/proper charging (or extra cycling, especially with the larger solar array). A 71 AH battery bank will work for 2-4 days without much sun and then kill the battery (over discharge) unless you drop by and checkup every day or 3...

Some folks like to "remote monitor" the site or add secondary systems to prevent the system from hurting itself/notify you of problems. More complex, more money, and more wild and wonderful ways things can go wrong.

It all depends on your needs and expectations.

-Bill