Hope i am not about to destroy anything!

sunnyskiessunnyskies Registered Users Posts: 14 ✭✭
So after a few years of struggling with my first attempt at solar at our cabin in northern MN (no grid within 5 miles) i learned that 1) i didn't have a clue 2) any time without a generator running is priceless! The first attempt consisted of 2 15 watt 12 volt panels that advertised on the box you can run this and this and this. 4 105 ah expensive Concorde agm's and a home built charge controller 2 stage bulk and float. well i am sure you know the rest of the story, the batts sulfated to almost no capacity and we limped along with a home built batt charger using the same 2 stage control circuit as the charge controller and a Honda 2000i. Hardly ideal. After our serval fridge tried to kill us one night (burner malfunction) we decided to upgrade large enough to run a small fridge( 3.1 cube top freezer 2 door Edgestar actual draw 82w at start up settling down to 62w running.) The new system consists of 2 300 watt Renasola vmp36.6 imp8.2 in series on a home built tracking mount( Redrock module.) feeding a Morningstar Prostar 25 charging 4 Fullriver 115ah agm's wired series parallel for 24 volts. The batts feed a Aims 1500 pure sine 24 volt inverter (.7 amp dc at idle)  So i installed the system during our 2 week vacation and turned it on. Everything seems fine but no remote meter(rm-1) well i get a batt symbol and the ah are increasing. no min/ max volts no array volts ect Morningstar is sending a new unit. so vacations over, i left the system running with the fridge as the only load and locked the door. When i got home started researching meter problem and stumbled across this forum and after a few hours of browsing realized that once again i really don't know for sure if i am about to ruin another set of batts or not. any help determining if this system will run 24/7-365 would be greatly appreciated


  • ThomThom Solar Expert Posts: 190 ✭✭✭
    Thanks for the great read. 

    Off grid since 1984. 430w of panel, 300w suresine , 4 gc batteries 12v system, Rogue mpt3024 charge controller , air breeze windmill, Mikita 2400w generator
  • BB.BB. Super Moderators, Administrators Posts: 29,823 admin
    SunnySkies, let us see if we can get some basic math done here and see if it matches your needs.

    2x 300 Watt panels 36.6 Vmp.

    If your controller is a PWM type, then connecting the two panels in parallel to a 24 volt battery bank is fine. Placing the two panels in series with an MPPT charge controller will also work well.

    You have 4x 110 AH @ 12 volt batteries (AGM) in a 24 volt battery bank. In general, 1-3 days of storage to 50% max discharge (for longer battery life). 2 days and 50% discharge seems to work well (very nice for a part time/seasonal cabin, almost a necessity for a full time off grid home). Such a bank would supply (no-sun) power of:
    • 220 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge = 1,122 WH per day (for 2 days of no-sun).
    Your refrigerator, if it very efficient, can draw around 1,000 WH per day (typical Energy Star 120 VAC refrigerator). It could be higher (1.2 to 2.0 kWH per day for a full size Energy Star Fridge).

    Obviously, if you use only 1 day of storage, the system would supply ~2,244 WH @ 120 volts from the battery bank.

    So, knowing your loads by using a Kill-a-Watt type meter (measure your AC loads), will be helpful here.


    Plug your load(s) in for 24 hours (or more) and figure out your average WH/kWH per day energy usage (1,000 WH = 1.0 kWH).

    Next. Two calculations for sizing the solar array. First is based on the size (AH * Voltage) rating of the battery bank. Larger batteries need more charging current for proper operation. Typically 5% minimum rate of charge for seasonal/weekend system minimum. 10%-13% works well for a full time off grid system. These are just "rules of thumbs" for a quick and dirty back of the envelope design, but they do generally give a system that will work well with average cabins/homes.
    • 220 AH * 28.8 volts charging * 1/0.77 panel and controller deratings * 0.05 rate of charge = 411 Watt array minimum rate of charge
    • 220 AH * 28.8 volts charging * 1/0.77 panel and controller deratings * 0.10 rate of charge = 823 Watt array nominal rate of charge
    • 220 AH * 28.8 volts charging * 1/0.77 panel and controller deratings * 0.13 rate of charge = 1,070 Watt array "cost effective" maximum rate of charge
    Then there is sizing the array based on your daily loads. Assuming 2 axis tracking, Green Bush Mn (Fargo ND weather station):
    MonthSolar Radiation
    ( kWh / m2 / day )

    If this is anywhere near accurate, that is a crazy amount of sun/energy (2 axis tracking, clear weather). You appear to have trees in the area--So any shading at all usually kills 50% or more of your array output--So your millage my vary.

    But, the basic 300 array to supply energy for this month (July at 8.95 hours) and 5 hours (February through October):
    • 600 Watt array * 0.59 AGM + AC off grid system eff * 8.95 hours of sun (July) = 3,168 WH per day (average July day)
    • 600 Watt array * 0.59 AGM + AC off grid system eff * 5.00 hours of sun (non-winter months minimum) =1,770 WH per day
    And to "size the system" based on energy needs, I typically suggest for 3.3 kWH per day for a cabin with a refrigerator, well pump, fan, lighting, laptop computer, even a clothes washing machine (lots of conservation, but a "near normal" electrical life for off grid existence):
    • 3,300 WH per day * 1/0.59 AGM+AC off grid system eff * 1/5.0 hours of sun minimum = 1,119 Watt array (based on 3.3 kWH per day load)
    Of course, your battery bank is on the small size to support that large of load... But for this time of year, your 600 Watt 2-axis tracking array seems to be up to the job very nicely. And even towards winter, it stands a chance of keeping a fridge running with a bit of help from a genset during poor weather.

    An electric refrigerator is the load that usually pushes a "small off grid system" to a "medium size" off grid system. I.e., a 300 Watt AC inverter takes ~6 watts just to turn on (no loads) and can run your lights, laptop computer, cell charger, and a DC water pump pretty nicely. When you add a refrigerator, you have high starting surge (need ~1,200 to 1,500 watt inverter minimum and a large enough battery bank to support the starting surge). Your system is sort of "in between".

    In any case, the other issue is that things go wrong when you are away. Tracker fails, refrigerator draws too much power for some reason, smoke from a forest fire drops solar production, charge controller goes bad, etc... Having loads "on" when the cabin is not being used can be an issue (i.e., your $$$ battery bank gets killed by loads and no charging). But adding complexity to the system to make it "more reliable" can cause its own issues (more complexity, more "wild and wonderful" ways a system can fail).

    Assuming that all is working well, and you have mostly clear line of sight to the sun this time of year, your system should be OK unless bad weather or other issue(s) happens.

    -Bill "in my humble opinion from several thousand miles away" B.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • sunnyskiessunnyskies Registered Users Posts: 14 ✭✭
    Thanks greatly for all your calculations Bill you obviously know your stuff. so my just go buy more of everything approach actually worked out?? looks like a rate of charge at 7ish% should keep the batts happy. the fridge draws 60 watts ac at about 1/3 duty cycle so 480 w day at 75f as measured by my fluke scopemeter running a trend for 24 hrs. will get a kill a watt meter for the other loads (easy hookup compared to the fluke) my tracker is only single axis so deduct 15% off solar rad? it looks like your calculations take into account inverter inefficiencies something i had not considered. the inverter shuts down at 19 volts i suppose thats to low to be of any use in saving the batts in the event of no charge current would one trip down to 19 volts kill the batts or would it take more? any one make a lvd that will shut down an inverter b4 the batts are ruined? sidenote went out this afternoon to mow and check system... cold beverages in the fridge :) controller in float at 1:00 pm :) Thanks again bill!!! 
  • BB.BB. Super Moderators, Administrators Posts: 29,823 admin
    edited July 2017 #5
    In general, resting voltage of 24.36 volts is around 50%... Under load, you can get down to ~23.0 volts under load (~25C/75F room temperature batteries--Depending on light vs heavy load, etc.).

    As always, read the manual for your batteries (not sure if yours or not):


    AGM (and other sealed batteries like GEL and VRLA) do not usually like being charged at "higher voltages" for absorb--They can run hot, vent, and ruin the catalysts in the cells.

    In general, the low voltage cutoff of AC inverters is there to protect the inverter, not your battery bank. Inverters are (mostly) constant power devices... If you run an inverter at 1,000 watts on a 24 volt battery bank:
    • 1,000 Watts * 1/0.85 AC inverter eff * 1/28.8 volts charging = 40.8 amps from battery bank
    • 1,000 Watts * 1/0.85 AC inverter eff * 1/26.0 volts "resting" = 45.2 amps from battery bank
    • 1,000 Watts * 1/0.85 AC inverter eff * 1/21.0 volts battery cutoff = 56.0 amps from battery bank
    The AC inverter low battery cutoff prevents the inverter from drawing too much current and overheating... Secondary is most AC inverters will start "browning out" their AC output voltage as the battery voltage falls.

    In general, resting voltage below ~24.0 volts, you want to cut the loads and recharge the next day (and/or start your genset).

    Under (relatively heavy loads), under ~23.0 volts, you should cut the loads and/or start the genset.

    The batteries will work down towards 20% state of charge--But it is hard on the batteries (less cycle life) and can take several days or more to recharge with a 5-10% rate of charge solar power system.

    For a seasonal cabin, your batteries will last probably somewhere around 5-7 years (under good conditions). If you are only cycling 100 days per year--The batteries will probably "age out" before the "cycle out"--So being a little harder on your batteries is not probably going to cost you money--And keep the battery bank on the smaller/less expensive side.

    Relatively charged lead acid batteries like cold weather during storage, and if charged, will not freeze except in very cold conditions (75% SoC freeze at -53F. 25% state of charge batteries will freeze at 9F.

    Concorde has a very nice manual and all of this stuff is well covered.


    PS: Check the specifications for your inverter. It would not be uncommon for it to draw upwards of ~20 Watts just being "turned on" (Tare Load) (0.7 amps * 24 volts = 17 Watts, 24x7 if inverter is "left on"--Some inverters have "sleep mode" and can shut down when there is no AC load). That is approaching the load of your refrigerator (1/3 duty cycle of 60 watts = 20 watt average load).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2littleharbor2 Solar Expert Posts: 1,451 ✭✭✭✭
    One reason inverters have such  low LVD is, as the battery voltage decreases, if an inverter has a particularly high load and surge placed on it , it will momentarily pull the battery voltage down pretty far if needed. This low voltage disconnect isn't necessarily there to save the batteries. It is that low so the inverter won't trip off under a heavy load while the battery bank isn't fully charged.  If your 24 volt battery is sitting at 19 volts it's pretty much toast.

    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, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • sunnyskiessunnyskies Registered Users Posts: 14 ✭✭
    Thanks again guys! gonna run it 24/7 and see what happens. sounds like a decent batt charger would be a excellent addition. also in reading my prostar controller manual i see that the load control feature has a adjustable set point for low voltage disconnect could i use a small relay with its coil connected to the load output and its contacts in parallel with the inverter on off switch to shut down the inverter at the setpoint of my choosing??
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