Xantrex C40 PWM Controller: Is diversion load required?

2twisty
2twisty Solar Expert Posts: 199 ✭✭✭
I am helping a local set up a solar system. She's already purchased some hardware, and I'm concerned about the apparent need for a diversion load.

What she has:

8 SunPower SPR-435-NE panels
2 Xantrex C40 PWM Charge controllers
Outback VFX3648 Inverter.
No bateries (yet)

These panels are strange to me. They are 128-cell, and have a VOC of 85.6. We will have to wire 4x1 into each C40, since the C40 can't handle over 125V. The manual talks of diversion load. I'm not familiar with the C40 -- will a diversion load be required to prevent damage to the batteries? I was under the impression that diversion loads were only needed for wind systems.



I don't have a diversion load connected to my solar system ( Classic 150 MPPT), and my batteries are fine -- when they are done charging, it limits the current into the batteries and keeps them at float.

I'm trying to gather arguments to convince her to drop the 2 C40s in favor of a Classic 150. I am meeting with her tomorrow to take measurements for wire lengths, etc and would like to present my case to her.

Can you help?

Comments

  • Brlux
    Brlux Solar Expert Posts: 73 ✭✭✭
    You really should fully read the C40 manual. In short it can be configured as a typical charge controller placed between the panels and battery or as a diversion load. If using as a diversion load you will need a suitable load for it to dump power in to.

    Personal I don't think it would be a bad setup with all 4 panels in parallel in to one C40. If you really needed to save on wire due to a long run or something you realy would need a Classic 200 to handle 2 of those panels in series.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    2twisty wrote: »
    I am helping a local set up a solar system. She's already purchased some hardware, and I'm concerned about the apparent need for a diversion load.

    In general, you do not need a diversion load+controller for solar electric panels. Diversion loads are generally used for Wind/Water Turbines which run (more or less) independent of the loads (turbines generate more power than needed for loads+battery charging, so you need to "dump" excess power to a resistor load bank or similar). If you "turned off" a wind or water turbine, they can over speed and self destruct.

    Solar electric panels are perfectly OK to "turn off" when the battery is full.
    What she has:

    8 SunPower SPR-435-NE panels
    2 Xantrex C40 PWM Charge controllers
    Outback VFX3648 Inverter.
    No batteries (yet)

    As always, I like to understand the loads (or at least estimate the power output for the user) so that the system does not end up "surprising" them (going "dark' when they need the power most).
    These panels are strange to me. They are 128-cell, and have a VOC of 85.6. We will have to wire 4x1 into each C40, since the C40 can't handle over 125V. The manual talks of diversion load. I'm not familiar with the C40 -- will a diversion load be required to prevent damage to the batteries? I was under the impression that diversion loads were only needed for wind systems.

    The C40 has several different "Modes". 1) standard series controller for solar panels. 2) dump load--turns on when the battery is "Full" and need to dump excess current to the resistor bank and 3) (as I recall)--A load controller--Turns off the load when the battery voltage is low (not "load").

    It can do only one mode at a time.
    I don't have a diversion load connected to my solar system ( Classic 150 MPPT), and my batteries are fine -- when they are done charging, it limits the current into the batteries and keeps them at float.

    Yep, don't need a diversion/dump/shunt controller + load bank.
    I'm trying to gather arguments to convince her to drop the 2 C40s in favor of a Classic 150. I am meeting with her tomorrow to take measurements for wire lengths, etc and would like to present my case to her.

    With those panels(?):

    http://us.sunpower.com/sites/sunpowe...-datasheet.pdf

    Have a Vmp~72.9 volts... In theory, they are a "perfect match" for the PWM C40 and a 48 volt battery bank.... The Sunpower panels (as I remember) are not usually available for sale in the retail market--If you can get them (and more later, if needed, or as spares/replacements) they should be very nice. Check and make sure that SunPower will honor the warranty (although, it may not matter that much if they got them "gray market" for a good price).

    The one big issue with them is that their high Vmp voltage makes it pretty much impossible to put two panel in series and use a standard Outback/Midnite/etc. MPPT controller (the standard 140-150 volt input MPPT controller can work with around Vmp~100 volt maximum (standard test condition) for Vmp-array. If she has a large that is some distance between the array and the charge controller/battery shed--Then you can use (for example) 3x Vmp~30 volt panels in sereis for Vmp-array~90 volts.

    Do the paper design (distance from array to charge controller) and see if the size of the wiring will be acceptable.

    Also, the C40 controllers are getting a bit long in the tooth, and the new features of the Classic (and other brands) MPPT controller can be pretty nice (as you well know).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    you will not be able to use arrays of 2 as that is ~ 146 volts from 2 panels, the CL150 could be sitting just a bit below max \voltage. It is not good for equipment life to run it at the max full time... there also is a possibility of an overvoltage shut down, edge of cloud event, a lot of the time with full sun and /or if you are in a cool area or have cold clear winters ... better to look at a 200 than the 150... poor choice of panels.....
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    PV panels do not need a dump load (as others have said)

    But one thing I will add, is that the C40 is not the best controller. It will work, but you will miss a lot of possible harvest while the batteries are in Bulk. The "power" above 58V - 73V will be wasted nearly all the time, and even more loss when batteries are low. 6 amps @ 15V = 90W loss per panel
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Brlux
    Brlux Solar Expert Posts: 73 ✭✭✭
    If it were me and I already had this equipment I would focus my resources on getting good batteries and setting the C40 to the proper bulk, float settings recommended by the battery manufacture. If I did not already have a temperature compensation probe for the C40 I would get one of those. The C40 is an older charge controller but it is a good reliable piece of equipment and seemed to be the standard for 48V off grid installations before all these nicer MPPT charge controllers started coming around a little over a decade ago. The fact that the bulk and float voltages are user selectable gives it an advantage over many other inexpensive PWM controllers. I would not put out the expense for the C40 DVM as it is very archaic with respect to today's standards, only reads amps in 1A increments and I think at 48V only reads volts in 0.4V increments. It would be better to invest in a battery monitoring system like a Trimetric.

    It will be little effort or added expense to upgrade to a nicer MPPT down the line. You would have to still run all 4 panels in parallel (unless you went with a classic 200) and you would have the same voltage drop/wire requirements coming from your panels to your charge controller.

    For this setup with these 4 panels in parallel the C40 likely will bulk around 24A while a MPPT could get up to about 30A
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    After posting this, I read the manual more completely and came to all these conclusions on my own. THANK YOU for confirming it.

    Sadly, I was not involved in the initial design of this system so that we could start with the loads and design the system properly. I came in when this lady had already purchased (and can't easily return) the panels, charge controllers and inverter. So, I am constrained to work with what she already has.


    The other added efficiency loss in in her panel mounts. Her neighbor built some fixed-angle frames (17*...WTF!) without any knowledge of solar. He built them strong enough to park a truck on them. WAAAAY overbuilt. At least they are aimed between 177* and 185*.. Yeah, they're not great, but I am constrained to work with those frames, as well.


    I've educated her about the efficiency losses with a PWM controller, as well as the fact that her 17* frames will only produce max power about this time of of the year and that she will get poor power production in the winter. I generally recommend building frames that are adjustable 4x per year.

    Also, due to budgetary constraints, she is unable to build the bank of L16s that I recommended. We are forced to go with Trojan T125 240Ah GC2s. I've explained that she will have to be EXTREMELY conservative of power when the panels are not producing.

    Yes, I know that this means that she will have surprises and that she will likely kill these batteries quickly. However, the financial constraints are keeping us from doing this right. :(

    Thanks for the input. We will do the best we can with what we have.
  • Brlux
    Brlux Solar Expert Posts: 73 ✭✭✭
    Out of curiosity why the T125 as opposed to the T105 and more specifically the T105RE?

    I am guessing she will also have a generator? Perhaps something can be done to give a more conservative warning about over discharge at least initially while she is learning to work with the system. It will mean she may need to run the generator on occasion but it will be far far less than before she had her solar set up. Can you adjust the VFX inverter to give a low battery alert sooner?

    I have been reading through your blog and it was an interesting read, thanks. Out of curiosity how much are you able to get the Trojan Golf Cart batteries for out there?
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    We opted for the T125 because of price vs. capacity. We are getting 8 of them including the core charges for $1360. I think that's a pretty good deal!

    She doesn't have a MATE with her VFX3648, but I will take mine over to do the programming. I plan to set the LVD more conservatively so that the power will cut off before she does damage to the batteries. I'll be starting a thread soon to ask about that.

    My blog is LOOOOONG overdue for an update -- I kinda lost interest, since most of my blog readers are also Facebook friends, and they get the daily, periodic, informal updates that come on FB. It just seemed redundant to type up a formal blog post to recap all the FB updates....

    She does have a generator that I plan to connect to the input on the VFX. So, they will at least be able to run the generator on cloudy days or to recharge. Sadly, she doesn't have a trimetric or a smarter charge controller that can give her an estimate of SOC. I was campaigning hard for her to pick up a Classic 150 or 200 (depending on how we wired the panels) for the added efficiency and the fact that the WBJr can give a reasonably accurate SOC estimation.

    I plan to have her pick up a cheap DVM that they can measure voltage to estimate SOC. I'll teach her how to use a hydrometer, bot I don't expect that she will see the point of logging SG readings until she kills some batteries. People rarely (myself included) believe the warnings that we get from experts. But there's nothing like a slap in the pocket book to teach you.

    I admit, I don't log SG either. I've got 2 strings of 8 batteries with 3 cells each. That's a LOT of SG readings to keep track of. I check the water levels, and spot-check SG, but rarely do I pull samples from all the cells.
  • Brlux
    Brlux Solar Expert Posts: 73 ✭✭✭
    It seemed like the T-105s have a longer service life than the T-125s. The T125 have a slightly higher capacity because the plates go lower in the battery, but the trade off is as the battery gets older the deteriorated plate material that collects in the bottom will short out cells sooner. The T-105RE batteries are superposed to have a something like 25% longer service life than the T--105 but it is a bit to soon for that to be proven.

    I have been pricing them recently as I just got 4 new batteries a few months ago (12V 2s2P). A local brick and mortar solar dealer down the road from me could do 8 of the T105RE batteries out the door for $1360, he charged no core fee when going in to a new solar installations and AZ has no sales tax on solar stuff if you get it form a dealer.

    I wound up getting 4 regular T105s from a golf cart dealer for $104 each with tax because I did have old cores to trade in. His core charge was something crazy like $35 a battery.

    It seems like 8 of those panels even with the C40 might be a bit of a high charge rate for 8 golf cart batteries. You are pushing 21% charge rate, it might be better to go with 6 panels if you only have 1 string of 8 golf cart batteries.
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    Hmm. You have a point about the charge rate. I can't limit it with those C40s like I can with my Classic. I will talk to her about that. Somehow, I suspect that she will NOT want to lose 800W of panels.. But I've already told her that she needs to get a second string as soon as possible. So, maybe I drop 2 panels and tell her I can hook them up when her other batteries get here.

  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    Dumb question: could you suck out the plate dust to prevent the short-out that you're referring to? Put a long thin straw down inside and get it out of there?

    Just thinking outside -- erm, well, INSIDE the box.
  • Brlux
    Brlux Solar Expert Posts: 73 ✭✭✭
    I don't know, in theory it sounds like it would work, but sucking sulfuric acid sounds like a bad thing for the sucker. You would probably have to suck out most of the acid, filter it and then return it to the battery. It also might be difficult to get a straw down to the bottom of the chamber without damaging the plates or insulators?

    It always seems like a much preferable choice to add batteries at the same time. You really don't want to get one string and then add a second in parallel a year or two down the line. Best to do what you can to add both at the same time.



    Disclaimer: This is dangerous and I could have blown up a battery in my face.
    I have some old T-105s some of which I used to trade in for the new ones. Well they were dead and sitting in the junk pile for a year or so around 2V each, One day I got the bright idea to see what could be done to get some life in to them. I figured they were already garbage and couldn't get any worse. I used a big 220V stick welder buzz box that has DC outputs to try to zap them and bring some life back. I think the open circuit voltage was about 80V and at first only a few hundred miliamps would flow through each 6V battery individually. But after a few minutes it started to pick up. By the time I stopped thay were well over 100A and were really boiling. The acid looked like chocolate milk due to string up all the junk at the bottom of the cell. When unhooking the welder the spark did ignite gas venting out of one of the caps vent hole melted that vent cap. that really scared me seeing how close I came to being covered in exploded battery acid. I hoked 4 batteries up in a 2s2p 12V configuration and set them outside my garage. I wired 3 70W parallel panels directly to them without a charge controller and then wired an inexpensive modified sine inverter. That setup was used for garage lighting and operated 23W CF bulb 24/7 for several months. I think the batteries recovered to about 15-20Ah each. Any time the sun was not shining the battery bank quickly drooped down in the low 11V range. I had to remove the low voltage alarm buzzer from the inverter because I would always wake up to it buzzing <11V. It would shut off once the bank reached 10V. Over the course of several months they kept degrading, likely due to no charge controller or just because they were garbage, but I did keep them watered and they would really go through the water.

    Sorry that was a long winded story stired up over string up the junk at the bottom of a battery.
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    Hmm.. SO what about drilling a small hole at the bottom of each cell and pumping the electrolyte through a filter? Of course, make sure that you always put the same electrolyte into the same cell. If you put the hole in the right place and get the electrolyte flowing through the battery fast enough, it would conceivably flush out the gunk and un-short the plates...

    Of course, just replacing the batteries is the best choice, but in the interest of tinkering......