need advice

kd7irm · August 2010

guys,
i have 40 solar panels, on five panel arrays with 8 panels per array, with 4 panels in series then the two series strings are wired together on each array. all but 4 out of the 40 panels are 55 watt units that are 10 plus years old. 4 panels are 35 watts and they are in a series string together. all then are wired to a Outback MX60 MPPT controller.

-The problems i am seeing are,
i can produce about 9.3kwh per day on summer clear days (seems good), but i never see a big jump in batt voltage at the end of the day. the mx60 shows about a max of 1350 watts and about 50-60 volts with about 20-30 amps at any given time during peak times of the day.
the system is 48volts and the typical load showing on the inverters is 1-4 amps during the day time hours.

-question, can i rewire these panels in any way to get better input to charge at a higher rate?

BB. · August 2010

Re: need advice

I am still a little confused about your wiring...

You have 4 panels in series?

And what is the Vmp/Imp (brand/model) of solar panels?

Also, the 50-60 volts--is that Array Voltage or battery charging voltage?

What size battery bank do you have (AmpHours)?

How much power do you use per day? What is your bank State of Charge?

It does sound like you are getting a decent amount of kWHrs from the array.

-Bill

kd7irm · August 2010

Re: need advice

bill,
yes 4 panels in series. 2, 4 panel strings per array and 5 arrays (10, 4 panel series strings on 5 mounts/arrays) into an Outback MX60. 36 photowatt ML 7010 55 watt panels and 4 Carrizo solar 35 watt panels. The 50-60 volts is the voltage that the mx60 says is coming in from the pv. the battery bank is a 48 volt system Deka avr95-25 unigy2, three 2 volt cells per battery and 8 batteries in series with a c/8 capacity of 1140 at 1.75vpc. state of charge is above 50% at an average of 50volts. i started out this morning with about 51volts and ended the day with about 50volts, all day there has been only about 1-2 amps ac load on the inverters. putting a clamp meter on the battery cable it shows about 5-6 amps dc when the inverter says 1-2 ac output.

BB. · August 2010

Re: need advice

Hmmm... Looking at this archived document (PDF download):

ML 7010 55 Watt Panel
Vmp=16.5 volts
Imp=3.3 amps
$495 each

March of 1983

4*16.5 volts = 66 volts

I am guessing in hot weather (say 100F day)--Your 4 panels in series are only going to output ~53 volts (hot panels will output ~80% of their rated Vmp voltage at 100F)... To properly charge your battery bank (and allow for controller drop) your panels should output >60 volts (~60-62 volts to equalize your battery bank; ~58 volts Bulk/Absorb for battery charging; ~54 volts just to float).

So--The simpilest thing at the moment... Take your 55 Watt panels and re-arrange them into 5 panels in series x 4 strings in parallel. That will give your array ~66 volts at 100F. Enough to fully charge your battery bank in hot weather.

And dump the 4 Carrizo solar 35 watt panels panels for now (unless you have another 2-6 panels in the garage somewhere to make a matching 66 volt series string or two).

This will, at least, get your battery bank / MX 60 charging voltage in the right voltage range to properly charge your battery bank.

Next is the question about how much power do you use vs how much solar power you capture... You may need more panels or to use your genset if you don't have enough panels.

And then, you get in the issue of matching your existing panels and the new panels. Current panels are around 17.5 volts Vmp--which can work OK in parallel with yours--But for 100 watt or larger panels, they tend not to come in 17.5 volt battery friendly Vmp ratings anymore--they tend to come in with "non-standard" higher voltages--which may not mix with your existing panels...

So, you may want to add the 100+ watt panels behind another MPPT controller in parallel with the MX 60 (it will work fine--just added cost for a new solar charge controller).

I hope I am clear--Currently you are not able to fully charge the battery bank in hot weather because the Vmp-array-hot is just too low of output voltage. Solar Charge Controllers (for the most part) can only "drop array voltage"--they cannot increase array voltage to properly charge a battery bank).

-Bill

BB. · August 2010

Re: need advice

To address the battery bank... If I understand, your bank is 1,140 AH at 48 Volts (C/8 rating)...

If you reconfigure your array, you will have 1,100 watts. The charging current should be (assuming a 0.77 derating):

1,100 watts * 1/57.6 volts charging * 0.77 derating = 14.7 amps average peak available
14.7 amps / 1,100 AH = 0.013

You are running a pretty small array for that sized battery bank... At a minimum, it would be nice to get closer to 0.05 ratio of charging current to bank AH capacity (3.7x more solar panels than you have now).

AGM's are known for their low self discharge and you don't need higher current to "stir the electrolyte"--But it would be nice to have more solar panels than you have now to get the batteries at a higher state of charge (or use a genset/grid power to get these guys fully charged once or twice a week).

-Bill

kd7irm · August 2010

Re: need advice

bill, thanks for the info, very appreciative, thank you. you cleared up a lot of things for me. i have been trying to find info on those panels and always hit a dead end.

I was really surprised when i tested them yesterday, they are over 20 years old and are still putting out close to 80% of the rated amps and volts, very pleased.

I think i will add one string of panels to your recommendation and do 5 panels in 5 strings parallel, that will help me out on the amps side of things. should get about 16.5 amps before derating and about 13.2 after and 82.5/66 volts with 1375/1100 watts,if i did all the math right.

or maybe 5 strings of 6 panels, volts would be 99/79.2 with 16.5/13.2 amps and 1650/1320 watts in this config. i checked the outback info and i can do a max of 132 volts.

We are not connected to the grid so the diesel gen set is the only high amp charge available. The panel arrays used to be sunseeker tracking mounts, but they are no longer working, wounder if i could just charge the units with new refrigerant, i think it was r22 but will have to check.

out of the above configs, what one would be the best with what i have? i am leaning more and more towards the 5 strings of 6 panels because of the increased voltage in the Arizona heat, not a lot of 77deg days here.

BB. · August 2010

Re: need advice

6 panels in series should be fine too (I did not find any Voc or temperature coefficients for your panels--but that is pretty much fixed by physics, so I just used the numbers from another generic silicon solar panel--the ratios should all be very similar).

More than 6 in series, you will run the risk of over voltaging the controller in cold weather.

Doing higher voltage allows you to run less current through your panel to controller wiring (a bit more efficient and/or less copper wire costs)--But the charge controller will be slightly less efficient... In the end, for the small changes you are looking at, there is not really going to be much difference between the two options that I see.

Probably what ever works out best/simplest for wiring up your arrays will be the best choice (minimum number of new/changed connections and additional wire runs).

-Bill

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