Assembling a small system from used components

manzanitamanzanita Registered Users Posts: 37 ✭✭✭
Hi. I'm trying to assemble a small off-grid system starting with what I already have and hoping that I'll be able to expand later.

I have one old BP275ul panel (21.4voc, 4.75isc, ~75watt) and two Shell SP75 panels (21.7voc, 4.8isc, 75w@STC). The inverter is an Exceltech 12v 1100w full sine.

I was thinking about buying the Tristar 45 mppt controller, their remote meter and the battery temp sensor. And a pair of Trojan T-105s.

The system is smaller than I like. I'll have to restrict usage to keep the battery charged, hence the remote meter.

With a 45amp controller, I can add more panels as long as I pay attention to the array voc and amps. I chose MPPT so that I can put my panels in series and save a little on wire costs. When expanding, I realize that I can't put a low amperage panel in series with a high amperage panel.

I'm guessing that, even with just two T-105s in a single string, I should have at least another 75w pv panel to be more confident about charging the battery. More PV would be even better, but we're back to the money issue again.

My favorite use is my computer & satellite modem (200-300 watts?). The battery won't last long at 16-25 amps output. I'm guessing that I'll mostly be using the computer when the battery is full and the sun is shining. (I'm hoping that my computer uses less power than stated. I'll see. )

Insolation values: Dec 2.2 kWh/m²/day, Jun 7.7 kWh/m²/day

What concerns come to mind as you look at my plan?

It seems to me that the charge controller would be the best part of the system. Just about everything else needs to be expanded or upgraded.

I see that buying 75w panels is probably not the best way to buy panels anymore. Wiring a 75w panel is just as much work as wiring a more modern 100-200w panel.


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Assembling a small system from used components

    Welcome to the forum.

    One thing is that when you're checking panel specs what matters isn't the Voc and Isc but the Vmp and Imp. Voc is important for not exceeding input maximums on charge controllers.

    You're going to have three 75 Watt panels for a total of 225 Watts. You can probably expect:
    225W @ 77% efficiency / 12 Volts = 14 Amps peak charge current.
    That would be 6% of the batteries' 225 Amp hour capacity; just above minimum but below Trojan's 10% recommendation (not including any power going to concurrent loads).

    Whenever possible, check your loads with a Kill-A-Watt to get real world numbers; too often the data on power supplies is a bunch of hogwash. Sometimes it's low, sometimes high. Although the batteries are capable of supplying up to 1kW hour of power, the panels can really only harvest about 450 (+/-) per day. That's not much operating time for a 300 Watt load. Don't forget that inverter uses power itself. In fact the Morningstar 300 Watt inverter would be better here as it consumes much less than the Exeltech 1100. But if you plan to expand you may want to run more loads simultaneously.

    A 45 Amp controller won't be very efficient on this set-up either, but your plan to add more panels makes it a better choice than buying a smaller unit and then having to replace it.

    As it is, doubling the panels would be good - get the recharge rate up so you can easily make use of 25% of the battery capacity. In future you could go to 450 Amp hours with that controller, giving you about 1.3 kW hours @ 25% DOD - providing you increase the array size to around 700 Watts. Ironically at that point an MPPT charger may offer you some advantage.

    Which brings us to the conclusion you've already drawn: small panels are expensive per Watt and just as much work to install as larger ones. Sometimes there's no point in trying to match up old and new, particularly low Wattage with higher Wattage. So you end up discarding (selling) the small panels anyway.

    Trying to expand a system is a pain. It gets you no matter how well in advance you plan. I can add more Sharp 175's to my system because I planned on it. I did not plan on Sharp discontinuing production of under 200 Watt panels. Fortunately I can work around that, but it is annoying to have to.
  • manzanitamanzanita Registered Users Posts: 37 ✭✭✭
    Re: Assembling a small system from used components

    Thanks for the welcome. I love your I Robot quote. So appropriate for this forum. :D

    I was hoping that I'd be able to do better than 14 amps. But considering that the BP Solar panel has browned cells, even the 14 might be optimistic. I don't know what I was thinking, the panels list 4.4 and 4.45 Imp. Expecting closer to 20 amps was unrealistic of me.

    Still, I badly need to get something going. Using cheap generators to create 120vac and cars to generate 12vdc (for the inverter) is more expensive than expanding the solar system. If I had more money, I'd do a good battery-inverter-generator setup. But a small solar system is cheaper, if more limited, just not as cheap as I'd like.

    Thanks for your input and recommendations. It helps clarify the choices I'm considering.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Assembling a small system from used components

    No, solar is not as cheap as we'd like. What is these days? Still, solar prices are coming down and there's few things else that can be said about!

    There are problems trying to recharge deep cycle batteries from automotive chargers or alternators. They don't charge at the right Voltages or times. Take a few minutes to read through the deep cycle battery FAQ's here: That will familiarize you with what you want to be happening with the T105's.

    Do you have a particular use for this solar project in mind? Or is it a learning experience?
  • manzanitamanzanita Registered Users Posts: 37 ✭✭✭
    Re: Assembling a small system from used components

    You have a good point, there aren't many other things that are getting cheaper.

    Thanks for the link.

    I was actually thinking that the automotive charger could assist the panels for the bulk part of the charge. But it is unlikely that a small 15 amp charger would like the low resistance of two T-105s. I would think that the Tristar charge controller would have a much better charging algorithm.

    No special use. Just supplying a travel trailer sitting on a remote property that gets a lot of use. The computer & satellite modem are the biggest consumers, they will be the problem. I'm pretty certain that the meter will be telling me that I shouldn't be running them. The other uses are much smaller, charge a cell phone, power a small radio and power a CFL light. And a nicad battery charger now and then. Other uses that would consume electricity aren't even on the table and won't be connected to the system.

    As you can see, I'm not going to be able to run the computer much unless the batteries are full and the sun is shining. I really do need more panels.

    It would be a treat to flip a switch at night and have a light come on rather than needing to grab a flashlight. I should probably install a remote on/off switch for the inverter as I imagine it will not help the system to have it idle all night.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Assembling a small system from used components

    I run a computer & satellite modem myself. The biggest improvement there is to ditch the desktop! Saved over 100 Watts switching to a laptop computer.

    The thing about the office consumption isn't the total Watts (under 100 now) but the total Watt hours from running it 10 hours a day. Yep: you hit that 1 kW hour mark pretty fast. That is a matter of battery capacity and panels to recharge.

    Yes you can bulk up batteries with an auto charger. Trouble is they're designed to go to about 13.4-14 Volts and taper off current the closer they get to that point. Some are worse than others. You might want to get an Iota charger: It will do proper 3-stage charging of deep cycles and is cheaper than buying a lot of solar panels for occasional use. You could run it off the generator and allow the panels to keep batteries up when you're not around.
  • manzanitamanzanita Registered Users Posts: 37 ✭✭✭
    Re: Assembling a small system from used components

    I hadn't thought about the laptop angle. Thanks for that and the iota link. I do need a good non-solar charger. It would probably do a better job charging other batteries as well.

    Yes, the watt-HOURS are where I get into trouble. I really do need to expand the system. Starting out in Nov-Dec with a system that is too small, the system isn't going to be able to spare anything. The Iota charger is a really good idea.

    I should get something like your eu2000, but that's another $1000.

    How practical would it be to buy a 25 amp PWM controller to charge the T105s via a vehicle's engine? My alternator is rated way above 25a.

    You're right about solar prices. The price per watt is lower than what I expected. I was looking at the NAWS offerings trying to decide what would be good choice if I was buying one panel at a time over a long period. I had a good laugh comparing a 240w module to my 75w panels.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Assembling a small system from used components

    Using your vehicle as a portable generator will do in a pinch, but it's going to be lousy on gas. Alternators don't put out their full rated power until you rev the engine - and the battery is low. You don't need a PWM controller for this so much as a battery isolator which can switch the alternator's output from one battery to the other.

    Maybe you could find a used Honda or Yamaha locally. Just watch out for the other inverter-generators; some of them have MSW output (non-sine wave) and some of them are just plain junk. There's all sorts of "Honda clones" out there which are poor quality. Some of them even have supposedly reputable name brands on them.

    You could also buy an inexpensive conventional generator like this: Not so good from a noise standpoint but able to do the job for a little money and a little gasoline. Note this type of generator is most efficient when loaded above 50% capacity, which doesn't always fit well with charging batteries (like 12 Volts * 25 Amps = 300 Watts or 20% of a 1500 Watt generator).
  • manzanitamanzanita Registered Users Posts: 37 ✭✭✭
    Re: Assembling a small system from used components

    Thanks for the comments about generators, I've been reading what users have been posting in the forum about generators. I've worn out several cheap generators in my off grid adventures.

    I've been thinking about adding more PV panels and am seeing the obstacles to mixing panels on one charge controller. The cost of a small panel is close to the cost of a larger panel, thus making it rather pointless to buy any more small panels. So it looks like a better idea to put the three 75w panels on a separate small controller, perhaps like the Morningstar SS-20L 20 Amp PWM Solar Charge Controller w/LVD which would mean putting the three panels in parallel. I was hoping to put them in series to ensure that the summer voltage was higher, but for less than 225 watts of output it may not be worth a more expensive controller. In any case, I wouldn't be adding anything else to that string & controller.

    I'm still trying to figure out what panels and mppt controller I'd like best to avoid future mismatches in panels acquired over the next several years. I wonder if more suppliers will stop producing panels under 200 watts?

    I had chosen an mppt 45amp controller so that it could handle a string of L-16 batteries when the T105 batteries are replaced. But that's down the road, at this point my system is too small even for the T105 batteries.

    (responding to something off topic and off forum - here's a short Wikipedia article about the history of Spences Bridge, BC.)
  • BB.BB. Super Moderators, Administrators Posts: 33,327 admin
    Re: Assembling a small system from used components

    These days, it is difficult to plan ahead on expanding your solar array... Like electronics in general, the production life of solar panels looks to be on the order of a couple years (+/-)--Let alone the life of the manufacturer :cry:.

    At this point, plan your needs and finances... And build what makes sense.

    Down the road a couple years, you may be able to mix and new and old panels--or you may have to buy a second charge controller for the new array. It is a toss-up whether you will be able to mix/match down the road.

    And many people found out that they can still buy the old panels a year or two later, but at 2x the $$/watt figure of the "new and improved" panels.

    -Bill :confused:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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