Parallel vs series  with MPPT controller - Page 1
Forum section: electronics / general

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 03, 2018, 07:34:55 pm
content: With a MPPT solar charge controller, is it better to try to get the highest voltage (without blowing it) as possible, or is it best to keep in parallel? My train of thought is that if they are in series, when it's overcast or if there are shadows the resulting voltage may be high enough so that the MPPT controller can then bring it down to a level where it can charge the batteries. If they are in parallel then the voltage may be too low to charge the batteries when it's overcast.So is it best to try to get close enough to the MPPT controller's max voltage, instead of keeping the strings of panels lower voltage? I'm thinking of buying 2-4 100w panels for my shed and assuming each panel is going to be 12v nominal I could put them all in parallel but I could also put them in series, or in groups of 2 in series. I'm leaning towards groups of 2 in series. I did not buy the panels yet so of course it's ratings may change my idea. My controller has a max open circuit voltage of 60v, so putting the 4 in parallel may be cutting it close (assuming 48v nominal, but it will be much higher in full sun) so leaning towards groups of 2.Am I on the right track? Or should I just go parallel? For now I have two 60w modules in parallel and thinking I might rewire them in series just to see.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2018, 09:16:51 pm
content: Depends on how efficient the MPPT converter is at varying voltages.

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 03, 2018, 09:46:34 pm
content: This is the controller I got:https://www.amazon.ca/gp/product/B01BXRIRFS/It does claim 98% efficiency but I'll take that with a grain of salt as with most things on Amazon. I also imagine there is a curve there it won't be the same for all voltage ranges.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 03, 2018, 10:02:16 pm
content: Looks like you'll have to try it and see.

author: Seekonk
author_rank: Super Contributor
timestamp: August 04, 2018, 12:16:56 am
content: thinking of buying 2-4 100w panels for my shedI tell everyone not to do that. With a MPPT you have the opportunity to buy grid tie panels which are much cheaper. Contact a few local installers and you can get +280 for well under $200 each out the door. I've done this many times with different contractors that are hungry or have left over panels from a pallet. Panels are a commodity and they buy whatever is cheaper that week. Buy all you can at the same time as you may never get another one to match physically. Nobody likes a onesie buyer. If you have shading and run 12V, parallel may work better. It depends on the type of shading. Any shading and you have pretty much lost it unless you are over 100V. It would be nice if shade just knocked out one section of a panel. That almost never happens.

author: mtdoc
author_rank: Super Contributor
timestamp: August 04, 2018, 12:37:54 am
content: You generally want to arrange your PV array so that string Vmp are at least 20V or so above your battery bank absorb voltage (or constant voltage charge voltage). You want some headroom for the MPPT controller to work with in less than full sun, partial shading, etc. On the other hand if your string voltages get too far above the charge voltage you will loose efficiency. So for example, with a 48V nominal LA battery bank, a string voltage of 80-100 is considered optimal. Divide that by 2 or 4 when dealing with 24V or 12V LA battery banks.The high quality charge controller manufacturers will provide power graphs and even string sizing tools. For example see attachment below from Midnite Solar.Also be aware that you want to leave enough headroom so that the controllers max open circuit voltage is never in danger of being violated. On a clear, cold day panel voltages can approach Voc.

author: Red Squirrel
author_rank: Super Contributor
timestamp: August 04, 2018, 12:53:21 am
content: Quote from: Seekonk on August 04, 2018, 12:16:56 amThe issue is the shipping cost. I found a few sites that do sell 280w panels in Canada but they want over $200 just to ship one. The panels run a bit over $200. I can get 100w panels at the local RV dealer so they should be able to drop them off at my house with a pickup free of charge or super cheap, hopefully. Otherwise I would go with 280w panels as they are cheaper per watt. Still thinking of doing the whole house but the issue is obtaining the panels. For a bigger system the 280w ones would make more sense.As for grid tie that's not really an option here, and I rather make this an off grid setup anyway, as I want to use it as a pilot to test the viability of doing whole house, or a cabin in the woods in the future or what not. Winter is tricky (which is most of the year here) as the days are so short, but I'm going to try it anyway. In the shed it will not be running much. Basically just the inverter and maybe some outside lights. Have not done all the electrical yet. While I could do 12v stuff I find a 120v system is simpler as anything I want to install/run is going to run off it. I may also switch to a 24v system in the future with 4 golf cart batteries. Right now I have a single RV 12v battery but it may not be enough for winter if I do plan to run stuff in the shed. Backup NAS etc.