Adding to system

[loonie]

Hi, I currently have 10 90 watt panels, seriesed and paralleled to a bank of 16, L-16 batteries thru a solar boost 50 24 volt. I am also flying a xl-1 bergey. The 90 watt panels are BP with a vmp of 18.5. the new panel I want to connect are 6 BP 75 watt panels with a vmp of 17.0. I know from searching the board that I should series the like panels and then parallel them with the 90's. my question is if I parallel them to the 90's thru the solar boost will it loose efficacy with the difference in vmp (18.5 to 17.0) or would I be better to run the 6 panels thru the bergey controller which allows for 30 amps of solar but I don't believe with mppt. Thanks for any help with this.

Loonie

[dnix71]

If you can afford all that panel capacity (I won't ask how much that all cost), then it would be simpler to buy another MPPT for your expansion.

Two separate sets of panels/controllers gives you backup in case of failure and allows downtime for routine maintanence, as well.

[loonie]

Everything in my system was bought second hand over the past 9 years. The power company wanted 9 grand to put power to my home I still haven't reached that amount yet. these last 6 panels set me back  all of $50 each(less then a dollar a watt). A new mppt controller would be around $400.  I don't think the extra gained would be worth the money spent. I was just wondering if the 75 watt panels would drag down the 90 watt panels to the point that it would be more efficient to hook them up thru the bergey controller without mppt

[dnix71]

It will pull down your peak power a bit, but you'll probably still come out ahead.

How did you get used panels for $1 a watt?

I have a BZ 250HV MPPT that only cost me $135. Thier 500 watt version 12/24/48 volt 500 watt model is $280. Mine originally had an assembly flaw, but they replaced it quickly when I told them about it. It works fine with cheap Harbor Freight panels. Automatic 2 stage charging with a temp sensor and adjustable float.

http://www.millionsolarroofs.com/bzproductsmppt500hvmpptchargecontroller500w122448vdcvoc100vmaxoptim
izedfor48v.aspx

[dnix71]

$220 here: http://windandsunpower.com/store/index.php?main_page=product_info&products_id=2

[OuttaSight]

I doubt that with such a small difference in Vmp that you'll lose much.  The MPPT controller should settle on whichever peak in the combined I-V curve yields the most power.  As the 18.5V array is powerful and if it has very low volt drops in its wiring then you'll maybe see the lower voltage array stall (as the MPPT controller will settle on a voltage that is above its Vmp and output from that part of the array will collapse).  You should have one blocking diode in each series string to stop mis-match losses between parallel strings (for shading and mis-matched Vmp strings like yours). In part light conditions, it probably won't make much difference as the whole array will sit some way below its nominal Vmp anyway.

With my Excel head on... Say the MPPT settles on 34.0V and now your 10x 90Wp panels are making the same 24.32A as before but are operating at 34.0V instead of 37.0V.  So your 900Wp array is now 827Wp.  But you've gained 13.24A (450Wp) from the new 6x 75Wp array.  This is 37.56A solar amps into the MPPT controller and that would translate into 51.0A into a battery sitting at 25.0V (ignoring conversion losses).  If you put the 6x 75Wp array on the other non-MPPT controller it will give you 36.0A on the original array into the battery and 13.24A on the new non-MPPT array, making 49.24A into 25V.

The difference will get bigger for a flat battery at 24V and smaller for a nearly full battery at 28V. But most of us want more Amps into a flat battery

So... my money is on putting the new array on the MPPT controller and stomaching the losses on the 900W old array.

My arrays are on two different MPPT controllers because a) one controller couldn't take all the power and b) part of the array is 46.2Vmp and the rest is cobbled together 35Vmp assorted random second hand panels so it made much more sense to use separate controllers optimised for each Vmp

[loonie]

I put a classified add looking for used panels. It ran for 3 months, most calls wanted more then I could buy new. Then I got lucky, a guy had 6 75w panels and was asking $50 each.

[loonie]

Thanks for your reply. I tried hooking them both ways shuting of all power draws. At 25.4 volts on the batteries, I was getting 20 amps from the 90w array. When I added the 75w panels amps went to 29-30 amps. When conected to the other non mppt controler amps went to 30-31 amps. Now I can't wait to try this at lower volts. I agree It would be better to have the extra at the lower baterry voltage.