# Mppt controller sizing

Registered Users Posts: 432 ✭✭
edited October 2019 #1
I'll start by saying I'M a cheapo. I buy  Local pickup used 190 watt 26.7 vmp 7.12 imp panels at .30 cents a watt.
I have a epever 40 amp 100 volt controller. Specs say I can over panel to 780 watts on 12 volts.  I Can't do parallel at 150 feet so  I have 2 panels in series 3 was over voc.
I'm seeing 19-20 amps. On a 400 ah 12 volt bank.
So Wouldn't I be able  to use 4 190s 760 watts  38-40 amps in a 2s 2p configuration?

2kw array 6 345 q cells  make sky blue 60 cc
6 230ah GC @36 volts
18 amp accusense charger. 3650 champion

Assuming you get subfreezing weather, 2 panels in series with your 100Volt max Epever seems about right.

The typical "best" harvest from your 4x 190 Watt array:
• 4 * 190 Watt panels * 0.77 panel+controller derating * 1/14.5 volts charging = 40.4 Amps @ 14.5 volt battery bus
So, that does seem like a good match (keep charge controller in well ventilated space to keep cool).

If you are only seeing ~20 amps peak charging current--What mode is the controller in (bulk/absorb/float)? What is the battery bus voltage? Assuming measurements are made at solar noon, cool clear day, with panels pointed at sun (Cosine of 10 Degrees = 0.985 or if you are off by 10 degrees, you would get 98.5% of solar energy vs being exactly pointed at sun... Cosine of 45 Degrees = 0.707 .... ).

If the controller is in bulk, either you could have one set of panel not carrying rated current (use a current clamp meter to see if current is equal or different between panel strings), or possibly you have too much voltage drop between the Charge Controller and the battery bank (dirty/loose connections, too long/small AWG cable from controller ot battery bank, etc.).

If you have a switch/breaker per string, you can try switching off one string, then the other, and see through the charge controller that the two strings are equal current or not (or through a dark tarp over one or two panels in a string and check current, etc.).

You can also measure the input voltage from the Array... Is Vmp-array around 53.4 volt volts (very cold day/panels) to 42.7 volts (very hot panels/day, depressed Vmp)? The MPPT controller should be running panels in that voltage range. If under 42 volts or over 54 volts then controller MPPT is not working right or other issues (poor electrical connections to panels/battery bank, running a Battery Desulfator, electrically noisy DC loads like universal/brushed motors, etc.).

Obviously, there can be issues with the charge controller or it is not in Bulk--But the above is a quick check to see if the array is OK.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 432 ✭✭
edited October 2019 #3
I'm thinking the low amps are from 10 awg at 150 foot.4℅ v drop. With 2 panels 380 watts  Im  seeing ~44 volts at 20 amps in bulk  53 or so when  floating.
2kw array 6 345 q cells  make sky blue 60 cc
6 230ah GC @36 volts
18 amp accusense charger. 3650 champion
10 AWG * 150 Feet * 2*7.12 amps = 14.24 amps array Imp
https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=54&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=14.24&x=69&y=32

## Result

Voltage drop: 4.27
Voltage drop percentage: 7.90%
Voltage at the end: 49.73

So, you are looking at ~4.3 volt drop at Imp-array.
• 20 amps (Battery current?) * 14.5 volts bulk (voltage?) = 290 Watts into battery (?)
• 44 volts array at Vinput Controller * 14.24 Imp-array * 0.95 charge controller eff = 595 Watts (in theory) to battery bank at solar noon
I am still wondering about the current in your two string at the array... The MPPT array voltage seems sort of reasonable for a pretty hot panel. Need to measure array/string currents.

-Bill

Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 432 ✭✭
edited October 2019 #5
The panels are probably 5 years old. But they checked out on voc and isc. IllI'do some more testing with my dc clamp.       I ran through the calculation on 2 panels in series 150 foot on 10 awg ~4℅
With 4 panels I'LL run 2 sets of expensive 8 awg wires for 2.51℅.
2kw array 6 345 q cells  make sky blue 60 cc
6 230ah GC @36 volts
18 amp accusense charger. 3650 champion
• Registered Users Posts: 432 ✭✭
edited October 2019 #6
Just looking at 8 awg prices it would probably be cheaper to buy a 150 volt  controller and keep the 10 awg  I already have and run all 4 in series for a 2℅ drop.
Rather than to buy 600 foot of 8awg. Would 2 runs of 10 awg at 4℅ drop be acceptable?
2kw array 6 345 q cells  make sky blue 60 cc
6 230ah GC @36 volts
18 amp accusense charger. 3650 champion
Second run of 10 AWG wire (actually, Imp-array/2 = 7.12 amps):

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=54&phase=dc&noofconductor=1&distance=150&distanceunit=feet&amperes=7.12&x=0&y=0

## Result

Voltage drop: 2.13
Voltage drop percentage: 3.95%
Voltage at the end: 51.87

• 20 amps (Battery current?) * 14.5 volts bulk (voltage?) = 290 Watts into battery (?)
• 44+(4.3 - 2.13 volt drop instead) volts array at Vinput Controller * 14.24 Imp-array on parallel 10 awg cables * 0.95 charge controller eff = 625 Watts (in theory) to battery bank at solar noon
• 625 Watts - 595 Watts with one run of 10 AWG cable = ~30 Watts increased power
• 30 Watts * 5 hours of sun per day nominal yearly average (made up number) * 365 days = 54,750 WH of "extra harvest" (over estimated?)
• 54.75 kWH * \$1 per kWH off grid "cost of power" = \$54.75 in "value" of extra cable run per year
\$54.75 is a very rough estimate (average hours of sun per day over a year for your location, \$1 per kWH "value" of off grid power--Could be closer to \$0.50 or \$2.00 per kWH--Depends on your costs of hardware/batteries/battery replacment).

Two 10 AWG cables in parallel is -3 AWG larger gauge wire (every 2x increase in wire sqr inches in surface area/cut wire area, is a -3 AWG change in wire gauge).

So, 2x 10 AWG cables in parallel (one per array string), is a larger AWG (7 AWG) than pulling the 10 and replacing it with 8 AWG.

Assuming that Imp-array does not change much with "small changes in Vmp-array due to wire resistance", assuming current is "fixed", then change in V = change in power (deltaP=DeltaV*I constant current -- ΔP=ΔV*I). So changing from 4.27 volt drop to 1.0 drop would be an increase in harvested energy of ~3.7% from your array... In reality, probably almost an impossibly small increase in harvest to measure (10%+ increase is barely measurable--5% or less, probably would never be noticed).

Is this worth it to you? Probably not by itself. If there are problems with the charge controller (new higher voltage MPPT controller, placing 4 panels in series--Would be nice. If you are going to install a larger MPPT controller -- 60 to 90 amp range, then a higher voltage array and controller would make significant improvement in harvest, even if you left the 10 AWG cable behind (cost of larger array, cost of new MPPT controller, no changes to array cabling).

You can put 3x of your present panels in series--And if you get a Midnite Classic (150 VDC max input, with "hypervolt" protection), and depending how cold it gets, you may be able to put 4x panels in series... Use the Midnite string calculator to see (based on your minimum temperatures for your home):

http://www.midnitesolar.com/sizingTool/index.php

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 432 ✭✭
edited October 2019 #8
Thanks bill for taking the time  to help me try and understand. Shouldn't With 380 watts x.77÷14.5 volts be ~20  amps?
If I understand it correctly  charging at 14.5 volts and 40 amps the most I could produce  is  580 watts into the battery.
2kw array 6 345 q cells  make sky blue 60 cc
6 230ah GC @36 volts
18 amp accusense charger. 3650 champion
Yes you are correct ~20 Amps--Optimum panel sunlight and temperature (near noon, on axis sun, cool/cold day) into a battery bank that can absorb 20 amps and be at ~14.5 volts or less, you will see this for a relatively short time (hour or so), a handful of days a year (not all that often, so not much energy loss during your harvest).

Of course, if you battery bank is significantly discharged, lower Vbatt => higher charging current (at same available power).

I am trying to give general answers on what to expect... If you have your absorb setpoint at 14.8 volts, then the power will be:
• 40 amps * 14.8 Volts = 592 Watts
Just before the battery and charge controller switch over to Absorb charging. Bulk=Current Limited (or max power from array). Absorb=Voltage limited (so battery will naturally reduce current acceptance over the next 2-6 hours, and from ~80% to >90%+ State of Charge.

But all of this is variable... What is the C/xx rate of charge (higher current charging into a "smaller" battery will reach the Absorb Setpoint vs a low charging current.

Here is a chart that shows a "generic" lead acid battery and Voltage Vs Rate of Charge (and Discharge) vs State of charge (and you can throw in temperature compensation to make things even more "spaghetti like").

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 432 ✭✭
edited October 2019 #10
Very informative download. I travel to the mountains of Virginia in summer from Myrtle beach area.
Much cooler there. It takes a couple days to get everything set up.
Solar panels  and Iota are  usually the last to get set up.
when batteries are down to 50-60 soc.
during  this time I charge from a 40 amp high setting 20 amp low setting unregulated voltage car charger.
I charge at C/10 to 14.8 volts it takes ~75 minutes  then flip the switch to  C/20 to 14.8. Another 45 minutes. which according to this chart is about 110℅ soc. Maybe I should only charge to 14.2 on c/20 for 100℅ soc.

2kw array 6 345 q cells  make sky blue 60 cc
6 230ah GC @36 volts
18 amp accusense charger. 3650 champion
What kind of battery are you charging? Flooded Cell, AGM, etc.?

If flooded cell, you also look at the amount of distilled water you are using... Typically one would expect to refill the cells every 1-6 months. 1 month or less, cut back on the charging voltage. Over 6 months, jack up the charging voltage...

Of course, the amount of water usage (and even the size of the water storage above the plates) are Brand/model/details of additives to plate material/etc.) also affect water usage.

Holding the absorb setpoint voltage for 2-6 hours (shorter if shallow cycled, longer if deep cycled) is one other suggestion.

The other is to watch the tail of the charging curve at Absorb Setpoint... For FLA batteries, when they drop below ~2% or 1% of AH rating (i.e., 400 AH, falls to 4 Amps or less)--Or newer/AGM type batteries, to 0.5% or less of capacity (400 AH, 2 amps or less) is probably the ideal method of determining full charge.

In reality, unless you really crank up the absorb/float voltages, most people are not really risking over charging their daily use lead acid batteries on a pure solar system... There simply is not enough hours of sunlight in a day to over charge with "reasonable" absorb/float settings (something like 4 hours of Bulk, and 4 hours of absorb, 8 hours of enough sunlight just does not happen for many (shading, non-summer, farther away from equator, etc.).

For systems that are summer/vacation use only, on 24x7 AC chargers (UPS/backup power systems), it is certainly easier to over charge.

In my humble opinion.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset