Battery and Charge Controller Choices for 840 Watt PV Array

larryzlarryz Registered Users Posts: 5
Hello,

I live on-grid but want to take a few appliances off-grid (refrigerator, security equipment, LED lighting, well pump, etc.). My Kill-A-Watt pegs them at about 2 KWH/yr.

I've only got 4) ES-A0210-fa3 PV's and will be pole mounting them for single-axis tracking. Vmp for the panels is 18.3 Vdc so 4 in series would be 73.2 Vdc and 840 watts.

I like the Magnum MS 4448 PAE inverter but have not purchased it yet. I'm thinking it can power my existing 220 Vac well pump.

I'd like to put in a 48 Vdc battery bank, either 1 or 2 strings of eight 6 Vdc batteries in an outbuilding that is unheated here in the freezing northern winters. Any favorite battery choices for me? I'd like to stay around $2k but would go as high as $3k on the batteries if needed. We have a local US Battery distributor but their warranty is just 1 year. The Surrettes have a long warranty but I've read posts that imply they ship subpar batteries and the buyer has to fight to get them to honor their warranty. Shipping is expensive, but I live within a couple of hours from where Crown batteries are made. Anybody use and like them?

I haven't selected an MPPT charge controller yet. The four PV panels have Imp = 11.48 amps and Isc = 12.11 amps, and they'd be wired in series. I guess I need a charge controller that would handle the 73.2 Vdc Vmp and 22.8 x 4 = 91.2 Vdc Voc. Any suggestions?

This is my first off-grid inverter attempt. Any advice is appreciated.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    Welcome to the forum.

    Since the question is bound to be asked we might as well ask it right away and get past that point:
    Why do you want to take this equipment off grid? Grid power is way cheaper than solar, so you need a real good reason for doing this.

    Next, I think you may have you K-A-W numbers confused. A refrigerator alone can use 2kW hours per day. They also tend to have high start surges, as do well pumps. Well pumps can be murder on power, and run anywhere from 1/3HP sucking 800+ Watts on up to 2 HP monsters. Makes a difference what pump it is and what it eats for power. In short, refrigerator and pump are the two worst things you could pick to try and power off grid. Well air conditioning may top them both. No, I tell a lie; some people try electric heating/cooking.

    Since some of the system has already been selected, that puts limits on the rest of it. In this case 840 Watts of panel will support 100 Amp hours of battery on a 48 Volt system. Perhaps a bit more with the tracker. That's only about 2.4 kW hours of power. Roughly the same as I run our cabin on, which does include a well pump (small) and a refrigerator (very economic but full-size one).

    As you can see, that limits your batteries - but it will not cost anyplace near $3,000 for them. Crown batteries are perfectly good; never heard even one complaint about them or Trojans. Freezing should not be a concern if they're kept charged. I leave mine all Cariboo Winter with the controller hooked up and the inverter off.

    When selecting a charge controller the things you have to match up are the array's potential maximum output expressed as Amps @ system Voltage. In this case 840/48 = 17.5. It is unlikely you will see more current than that from the array on a 48 Volt system, so any MPPT controller capable of 20 Amps or more @ 48 Volts should handle that aspect. The other thing is array Voc vs. maximum input Voltage. If you put your panels all in series you would have 91.2 Voc by your figures. That means you'd need the Morningstar MPPT 45 at least, or any of the larger controllers of this type from them or Outback, Xantrex, or MidNite.

    Nothing wrong with Magnum inverters either. They have PF corrected chargers too. Will it run the well pump? Possibly; depends on what that pump is.
  • larryzlarryz Registered Users Posts: 5
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    Thank you for the reply Cariboocoot. We have a lot of power failures here, I wanted to be able to pump well water, have a few lights and keep the refrigeration I guess. And I wanted to do as much as I could afford to do, just because its the right thing to do. I think it was Gandhi that said "Live Simply, So That Others May Simply Live".

    Anyway, I had the Kill-A-Watt meter on my fridge for 238 hours and consumed 11 KWH. Wouldn't that be 11 KWH/238 hr *24 hr/day = 1.109 KWH/day? 1.109 KWH/day x 365 day/yr = 405 KWH/yr. The security stuff has a bad power factor, and pulls 51 volt-amps AC x 8760 hr/yr = 447 KWH/yr. LED light pulls 47 volt-amps AC x 1460 hr/yr = 69 KWH/yr. Not sure about the power factor on the well pump, but I get 6 amps AC @ 244 Vac and if I guess at .75 PF that would be 6 x 244 / .75 = 1952 volt-amps when running at about 548 hr/yr = 1070 KWH/yr.

    405 + 447 + 69 + 1070 = 1991 KWH/yr. Sorry, I meant to say 2,000 KWH/yr in my initial post.

    Please, how did you arrive at the 100 AH battery size at 48 Vdc, when mated to the 840 watt array?

    I'll look at your charge controller suggestions, thanks.

    I know the well pump will have a high current for the first second or so, I don't know how to tell in advance if the MS 4448 PAE inverter will handle it. Its specs say 8500 watt surge for 5 seconds.

    You mentioned the fact that I am limited by already having the 12 volt nominal PV panels. This is really true. I actually have 6 of them. I have them left over from some battery-charging systems I was building. Even in series, the voltage seems too low for any grid-tie inverters I can find online. I'd consider the grid-tie approach if not for that. I live in a net metering state and could get approval for grid tie in a month or two after applying.

    Thanks again for replies.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    Okay: 2,000 kW hours per year I'll believe. :D

    That well pump sounds like it may be in the 1 to 2 HP range. Deep well by any chance? You can't measure the start-up surge with ordinary equipment, unfortunately; most of it just doesn't "sample" fast enough to catch things like in-rush current and start surges. The good news is that small-frame submerged pump motors don't draw as bad as the large-frame surface pump motors. The bad news is it may still exceed 4kW on start. The good news is the Magnum can probably handle that. If it doesn't and you're forced to run a generator for the pump would that be okay? Others will no doubt suggest changing the pump or making changes to it (soft start) but you're probably not interested in re-vamping the water system to accommodate that.

    Simple, "ball park" calculations for array size:
    840 Watts * 77% typical efficiency = 646 Watts / 59.6 charging Volts (for 48 Volt system) = 10 Amps, which is 10% of 100 Amp hours. That's about in the middle. There are things which can make your panels put out better and things that can make them worse. Mainly variations in temperatures and elevation, or anything that affects atmospheric conditions.

    You can also use the Icarus Formula to get "AC Watt hours out the door" so to speak:
    840 Watts * hours of equivalent good sun / 2 (for over-all system efficiency of 50% from panel rating to actual power output from inverter). The tracker will improve that.

    A couple of web sites that can add to your knowledge:
    Mac's Lab panel angles: http://www.macslab.com/optsolar.html This also includes a graph showing fixed vs. tracking so you can get some idea of the improvement.
    PV Watts: http://www.nrel.gov/rredc/pvwatts/ You can run some theoretical scenarios with that to see what you might expect to harvest.

    Grid-tie would be far cheaper, but the central inverters require array Voltages of 200+ and the microinverters have specific panels they're designed to work with. Both require permits and cooperation from the utility, and they don't supply back-up power unless you use a hybrid inverter like the Xantrex XW or the Sunny Island/Sunny Boy set-up from SMA.

    BTW, you could run all six of your panels if you did two parallel strings of three on a 24 Volt system (which might not have enough inverter capacity for the water pump) or all six in series using the MidNite Classic 200 charge controller (the 150 might work, but if the temps go cold where you are the panel Voc may exceed it's 150 Volt limit even with the hyper Voc capacity). They're very nice controllers - but over $700.

    Running all six panels = 1260 Watt array, which would support 162 Amp hours on 48 Volts (plus or minus a bit). That would be around 3.8 kW hours maximum potential.
  • larryzlarryz Registered Users Posts: 5
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    Thank you (again) for the followup reply Cariboocoot.

    My well is 80 foot deep I think. I guess I'll just try the Magnum MS 4448 PAE and hope the pump will start up.

    Thank you for the links, I have looked at them.

    I went to the nrel.gov link, then followed it to the PV Watts calculator. If I put in the 0.84 kW number, it predicts 1135 kWh/yr AC for my location. Not much, but I guess it's a very small array. It's about 12% of my annual kWh consumption.

    I'm a bit confused on the battery bank sizing at 100 AH. Even 8x 6V Trojan T-105's in series would yield 225 AH at the 20 hour rate. Since 225 AH > 100 AH, do you feel the 840 watts and 11.48 amps (Imp = 11.48 amps) from the PV array will not be enough to charge the 225 AH batteries? Actually, I'd like to have more than 225 AH. The Magnum inverter is also a charger, but I wouldn't want to consume electricity from the grid instead of the solar array, to charge the batteries.

    I really want to utilize the PV panels. I thought that selectively removing a few household appliances from the grid might work out. Maybe I should power up an outbuilding instead ...
  • SevenSeven Solar Expert Posts: 292 ✭✭
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    The general rule of thumb for charging is 5-13% of the ah rating of the batteries. Coots math has your system producing about 10a which is 10% of a 100ah bank. Using a 225ah bank you would want 22.5a of charge available. A little more than twice what you currently have.
  • larryzlarryz Registered Users Posts: 5
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    Right, I get it. Thanks Seven.

    11.48 A (Imp) / 225 AH = 5%, which is on the low side of your range.

    If I set up a 24 volt battery bank using four 225 AH 6V batteries in series, instead of eight in series for 48 volts, then wire the four PV's in series/parallel, I'd get 11.48 x 2 = 22.96 charging amps. The charge rate is higher, but the battery bank only stores half as much energy.
  • icarusicarus Solar Expert Posts: 5,345 ✭✭✭✭
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    Once again, it should be mentioned, for occasional outages, a good genny and fresh fuel is MUCH cheaper alternative. A Honda eu 2000 is under $1000, will run for hours on a litre of fuel.

    Pump is the bugaboo if it is 240 vac.

    Tony
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    The rating on your pump, 6 amp @ 240 volts is typical of regular, off the shelf 1/2 hp submersible pumps that usually deliver 10 gal /minute, so unless you're using or wasting a lot of water, it won't be running that much, or often. Of course this depends on how many are in the family. Also, high efficiency front load washers use very little water compared to the old top load models.
    If I guessed right on the pump, they come with either of two motors, AND in 110 OR 220 volts. One is three wire + ground, which uses a capacitor starter control box mounted usually in the house. The other motor they come with has the starter built in the motor and this motor uses 2 wires + ground = 3 wires total. These pumps are harder to get started than the ones with 3 wires + ground that use the capacitor starter box. If your pump has the capacitor control box, consider yourself lucky, if not, you might consider replacing the pump for one that does have 3 wires + ground = 4 wires total and uses a starter box, and if you get the 110 volt unit, you won't need a step-up transformer. I have one of those pumps with the 110 volt motor not in use and one of these days I'm going to check it out on my Xantrex suresine 1800 inverter so I can report. I used to use it with a 1500 watt MSW at the cottage, but sold the cottage and kept the pump.:p I expect it will work just fine, as the Xantrex is a far superior inverter than the old MSW, and with an awesome surge capacity.
  • SevenSeven Solar Expert Posts: 292 ✭✭
    Re: Battery and Charge Controller Choices for 840 Watt PV Array
    larryz wrote: »
    Right, I get it. Thanks Seven.

    11.48 A (Imp) / 225 AH = 5%, which is on the low side of your range.

    If I set up a 24 volt battery bank using four 225 AH 6V batteries in series, instead of eight in series for 48 volts, then wire the four PV's in series/parallel, I'd get 11.48 x 2 = 22.96 charging amps. The charge rate is higher, but the battery bank only stores half as much energy.
    Not exactly. The number isn't the Imp.
    840w of panel times 77% efficiency equals about 647 useable watts
    if you divide that number by the charging volts, about 60 for a 48v bank, you end up with 10.8a. If you divide it by the charging volts for a 24v bank, about 29v, you end up with 22.3a of charge. That would be 10% of a 223ah bank. It would be close enough to keep them happy, in my noob opinion.
  • larryzlarryz Registered Users Posts: 5
    Re: Battery and Charge Controller Choices for 840 Watt PV Array

    I see. OK thanks for the help.
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