Need advice about backup (prepper) solar charge controller to convert Grid Tie panels to batties

cdherman
cdherman Solar Expert Posts: 32 ✭✭
I am not usually paranoid, not usually worried about the end, not a gun hoarder etc.  But current world events have me wondering.  I have 39 Kyocera 210w panels on grid tie system using a Fronius inverter.  They are now over 12 years old, with zero maintenance.  I paid a lot at the time, and they will never really pay themselves off (compared to what my $25,000 would be worth today, had I invested it in the stock market).  But that's not the point.

Question(s):

-- I read that I should not purchase a solar MPPT charge controller unless I know what batteries I will be using.  But in a SHTF scenario, there will be a lot of battery scrounging.  Even a rather in-efficient system is better than nothing.  I was thinking that I should look for a charge controller that has maximal flexibility in terms of settings.  I'm willing to spend good money here, budget is somewhat flexible. 

-- I obviously have far greater capacity in terms of panels than any practical battery bank, so I have no illusions that all the panels would be used.  But I'd like to have some flexibility.  If I get a larger, say 75 or 100A charge controller, is there a downside if I end up using only 2 or three panels?  Or is there a downside to using too large a controller with too few panels and batteries?

-- I have three strings of 13 panels each.  They are no where close to 210w anymore.  But I do hit 7000w once in a while which equates to 180w.  One string would 2340w, or 195a.  Am I correct that a battery pack for 195A of 12v input would be pretty large and expensive?  As would be the controller?

Sorry for the silly questions.  I show up here and ask them about every 10 years.  I know a fair bit about grid tie, but little about off-grid.

Thanks in advance....

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    The battery question is usually around Li Ion with BMS vs others (lead acid, AGM, etc.).

    With Lithium Ion, having a charge controller (or inverter+charge controller) that integrates with the BMS can be very nice.

    But for a SHTF situation, Li with BMS are not going to found sitting in a local store (Li Ion are great because they can sit 1/2 charged for years and should last upwards of 10 years (?) if stored in a cool dry place and check the voltage every year (recharge if under 20% back to 50%-80% SoC).

    Lead Acid batteries--Those are the ones you will probably find. Deep Cycle Gol cart in a 2 series * 1/2/3 Parallel strings is a "useful bank" for a 12 volt @ 200/400/600 AH battery bank. With a 300 Watt AC inverter you can do most 12vdc and 120vac projects. If you want to run a fridge, you want a 24 volt bank @ ~600 AH as a starting point for the standard home Energy Start simple fridge.

    Say you aim @ 12 volts @ 400 AH (4x GC batteries). For full time off grid, minimum of 10% rate of charge (5% for emergency use/float bank, 13% for full time off grid will used system). A 10% array would be:
    • 400 AH * 10% = 40 Amp charger
    • 400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watt array "nominal" off grid system
    Lets say you will use 25% of battery capacity per day for 2 days, and 50% planned max discharge (for longer battery life with FLA GC batteries). The math would be:
    • 12 volts * 400 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,272 WH per day
    Say near Kansas City, fixed array facing south:

    http://www.solarelectricityhandbook.com/solar-irradiance.html

    Kansas City
    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 51° angle from vertical:
    (For best year-round performance)

    JanFebMarAprMayJun
    3.51
     
    3.71
     
    4.61
     
    4.86
     
    5.09
     
    5.37
     
    JulAugSepOctNovDec
    5.59
     
    5.34
     
    5.25
     
    4.61
     
    3.49
     
    3.24
     

    For "break even harvest" in December (bad weather, use less power, or use genset):
    • 1,272 WH per day * 1/0.52 off grid system eff * 1/3.24 hours of sun per day = 755 Watt array "December break even"
    1,000 WH per day is a nice cabin size system with LED lighting, DC/RV Water pump, cell phone charging, some light usage of a small lap.

    If you want a full size fridge, then ~3,300 WH per day for a "near normal" electrical existence (a bit of microwave usage, induction hot plate, solar friendly well pump, clothes washer a couple times a week, etc.)... All assuming using wood/propane/etc. for cooking, hot water, heating and a 1,200 to 1,800 Watt AC inverter.

    Some of the RV folks out there are using induction "hot plate" for cooking small meals on a 1,000+ WH per day systems.

    I pray that the SHTF season does not come--But being prepared and not needing it vs not being prepared and needing it--Prepping (food, water, a bit of power, cooking fuel--I.e., Camping like existence) is not a bad idea.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cdherman
    cdherman Solar Expert Posts: 32 ✭✭
    Thank you Bill for your long reply.  I like your last sentence in particular.  I have a 2000w 12v Pure Sine wave inverter.  I have been playing with the idea of a couple 12v LiFepo4 batts too.  Tempting as they seem to offer long life times and capacities.  But where I would go (remote western KS) in a SHTF scenario is also populated with numerous large 12v batteries in large agricultural equipment.  They are starter batteries, but large and numerous.  I would steal a row of panels off my Kansas City array and "get out of Dodge" except more like the reverse......

    Are there charge controllers that are adaptable enough to allow use of LiFepo4 or Lead acid batteries both?

    If I attach a large charge controller to a large string, as described above, and the batteries are full, do the more modern controllers have a way of shunting the excess watts without damage?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited October 2022 #4
    Fixed typo below (0.030 is "correct") -BB

    Flooded Cell Lead acid batteries are pretty forgiving as long as you program the correct voltages, do not over discharge, fully charge  at least once a week, don't let them sit below ~75% State of Charge for days/weeks/months as sulfation will kill capacity, and monitor electrolyte levels and use distilled water or filtered rain water to top off--Should be OK.

    Note that EQ (equalization charging) is controlled over charging of cells at 100% to bring up weak cells that are less than 100%--Typically once a month or if Specific Gravity is >0.050  >0.030 SG units between low and high cell(s). EQ is hard on FLA batteries, so only do when needed and for a limited time (and don't overheat batteries).

    Lithium Ion are a different animal. Chemistry matters (lots of different batteries out there).

    There are bare cells--You bolt up your own bank. And usually add a BMS (battery monitor/management system) to protect against over/under voltage cells, operating below ~50F. Some BMS can active balance cells, and some have over/under voltage cutoff transistors. And some BMS have digital communications back to a charge controller/cell phone/etc.

    Some folks choose to balances the cells then run "unprotected" by BMS. Li Ion cells go out of balancAnd the pretty slowly (months?) and monitoring with a volt meter can work. Over voltage can be set by proper charge controller charging values. You can have an under voltage meter/alarm too.

    And there are the All in One Lithium Ion batteries. Integrated active BMS/shutdown, etc. Can work well even wih a "dumb" charge controller. Advertises as FLA drop in replacements. Read the manual(s).

    Generally do not recommend paralleling a FLA with a Li Ion into the same battery bank. Always better to (if possible) to have banks with same brand/age/type/capacity batteries.

    Of course--SHTF--You do what you have to do.

    There are two major classes of charge controllers. Series and Shunt.

    A series controller simply turns on and off the charging source as needed. Solar panels work fine (and for battery bank, probably the best) with series type charge controllers. 

    Subset of series charge controllers are PWM (pulse width modulated--Cheapest) and MPPT (Maximum power point tracking--More expensive, and usually more capable to mix and match solar panels+battery bank voltages).

    The other major class are shunt controllers. This are for energy sources you cannot just turn on and off... Wind turbines, Water Turbines are a couple of examples. Standard HAWT (horizontal axis wind turbines) and Water turbines can over-speed and self destruct if they are not generating power into a load. So the choice is to use a Shunt/Diversion controller... When the battery bank is full, the controller turns on and "dumps" excess current to a load bank, or other optional load (electric water heater is a common option).

    Not really great for getting the most out of your battery bank--But it is done when needed.

    With electronics there are a whole bunch of variations out there... The above addresses the basics, but you may find a bunch of other options too.

    For example some MPPT series charge controllers can be programmed with a "waste not" function. As battery approaches full charge, will send out a signal that can be used turn on "optional loads" (water pump, washing machine, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    Look into the "all in one" inverter/chargers. The Midnite MN 5048 DIY for example Has a max dc input voltage of 500 volts allowing you to keep your high voltage grid tie PV array wired as is and switch over from grid to off grid easilly. It will take grid power to maintain batteries or a generator. They are very flexable as to how you use them. They are priced very reasonably for what you get.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • cdherman
    cdherman Solar Expert Posts: 32 ✭✭
    Thank you both!  I will start reading about the MN 5048.  I am actually quite handy, just not well informed.  I tend to always end up outside of the box! 
  • cdherman
    cdherman Solar Expert Posts: 32 ✭✭
    Well, MN 5048 sure does look interesting.  But a little overkill for my situation where I would only use in a SHTF situation.  I don't need that much capacity as I would not have a very large battery bank.  But the smaller unit from MidNite, the 3024 can only handle 100v DC from the panels, which would mean I could not directly plug a string into the solar controller.  And my panels are 15 feel up on a large Morton shed.......

    I might start trolling for a used 5048......  Anyone have an alternative suggestion, that could handle up to say 350v or so, that would allow me to keep the string intact........???
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,039 ✭✭✭✭✭
    cdherman said:
    Well, MN 5048 sure does look interesting.  But a little overkill for my situation where I would only use in a SHTF situation.  I don't need that much capacity as I would not have a very large battery bank.  But the smaller unit from MidNite, the 3024 can only handle 100v DC from the panels, which would mean I could not directly plug a string into the solar controller.  And my panels are 15 feel up on a large Morton shed.......

    I might start trolling for a used 5048......  Anyone have an alternative suggestion, that could handle up to say 350v or so, that would allow me to keep the string intact........???
    There are many brands of AIO, (All In One) inverter/chargers out there now. Check the specs on others for one that's a better fit. The thing with the Midnite brand is their customer service is real, live and in the USA. Many of the others, not so much.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.