Charge Controllers vs Array Input

• 2 x 100 W @ 5.8 Amp (11.6 amps for 2 parallel panels) panels @ 12 AWG @ 25' -- Assume Battery charging at 14.5 volts:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=5.211&voltage=14.5&phase=dc&noofconductor=1&distance=25&distanceunit=feet&amperes=10.8&x=67&y=18

Result

Voltage drop: 0.92
Voltage drop percentage: 6.35%
Voltage at the end: 13.58

• 14.5 volts battery + ~2.0 volts for controller/wiring drop = 16.5 volts "ideal" Vmp-array @ temperature
• For panel/Array Vmp--Say 15C "warm day" (Ontario Canada?), 100 Watt panels, Vmp~17.25 volts, Imp~5.8 amps

Guessing at -0.45% drop per C over 25C cell temperature with 20C rise:

• 17.25 Vmp * -0.0045(15Cambient-25Cstd+20Crise) = -0.78 volt drop @ 15C ambient
• 17.25 Vmp - 0.78 volt drop = 16.48
• Panel "ideal" Vmp-15C ~ 16.48 volts and Vmp-ideal ~16.48 volts -- A "perfect match"

Vmp-array at 15C is almost identical to the required minimum voltage needed to charge your battery bank on a "cool/clear" day.

A quick look from the math side--I am not sure why your array is "under performing expectations" @ 20 amps measured *(?). (4x 5.8 Imp = 23.2 amps)--To be honest, that is pretty close to what you would expect from the array on a very sunny day, near solar noon, and panels tilted towards sun.

However, when you get to 35C hot summer days:

• 17.25 Vmp * -0.0045(35Cambient-25Cstd+20Crise) = -2.33 volt drop @ 35C ambient
• 17.25 Vmp - 2.33 volt drop = 14.92 Vmp-array@35C

So--On a hot summer day--Your array temperature drop and wiring drop puts your system about (16.48-14.92= ) ~1.56 volt off the Vmp@35 peak voltage--Note great.

One quick test--With the battery bank "discharged" and/or put a "heavy load" on the battery bank to get the controller to "bulk charge" state (maximum current through controller)--You could use a DC Current Clamp DMM to measure each string (current for each string should match pretty closely) or disconnect (or flip combiner box breaker) one string at a time (or unscrew on panel connection at a time) and see that each string is carrying roughly the same current... Any single string that is 20-50% less than the rest--May be a wiring or panel failure.

Adding another panel.. Do you have the Vmp/Imp of that panel? The Vmp should be around 17.5 volts ideally. A 195 Watt panel may have Vmp~30 volts--That will "work" with the rest of the "17.25 volt" array--But will have less "real power":

• 195 Watt panel / 30 Vmp (if GT type panel) = 6.5 amps Imp
• 6.5 Imp * 17.5 volts typical Vmp (12 volt panel) = 114 Watt "equivalent" wattage on C40 w/ 12 volt battery bank (but "great" Vmp-panel@high temperatures--This will not "lose" current flow due to hot weather).

The other choice--MPPT controller, and wire 100 volt panels in 2s x 2p for Vmp-array ~ 34.5 volts... See if your 195 Watt panel has a Vmp~35-38 volts or so... That would make the system perform well in cold and hot weather. And get another ~81 Watts from the 195 Watt panel... But--Everything has to "match" Vmp-string voltages... you really don't want a Vmp=30 panel in parallel with Vmp=34.5 volt 2s panels for optimum MPPT performance.

Sometimes, it is difficult to justify trying to "salvage" the old hardware (100 Watt panels and PWM controller in this case)--But just use the old panels elsewhere (sell to somebody else) and buy a "matched" set of large format panels for your "new array and MPPT controller".

-Bill

With a PWM charge controller, the reason for "thermal losses" is different. It is because of "low" Vmp-hot is not high enough voltage and any wiring voltage drop just makes the problem worse. 17.1 volt Vmp for the 100 Watt panels is already pretty low--Add voltage drop from the 25 foot wire run just makes the drop worse. With PWM controllers, you are "matching" the solar array Vmp/Imp with the battery bank Vbatt and Ibatt values... Sort of like a 1 speed bicycle. When all "matches" it is the most efficient system.

With MPPT, it is like having a 10 speed bike. You just set the array voltage "much higher" than battery voltage. And the MPPT's internal switching DC to DC Buck Mode converter "shifts gears" to optimally match available input power to output power. Not taking losses into account:

• Power = Voltage * Current
• Vmp * Imp = Vbatt * Ibatt (the "10 speed transmission" effect--Optimum pedal speed/torque for rider--Optimum "RPM" and "Torque" to the battery bank)
• And because Vmp "falls" as panels get hot, there is less power available on the input, so less power available to the battery bank.

Vmp is a rounded "curve", not a sharp peak. Your system can be "off the peak" and the output does not "fall off cliff". This thread has some curves for solar panel Vmp (and Imp/Pmp/etc.) based on voltage and temperature:

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

You can see a Vmp~35 volt panel has a rough +/- of 5 volts (equivalent to Vmp~17.5 volt of +/- 2.5 volts) with pretty good harvest. But when panel hits +10 volts over Vmp (or +5 volts over Vmp for 17.5 volt panel), the power harvest hits zero Watt (no current flow). Note that Vmp curves are referenced to 25C cell temperature. There is an adjustment curve showing Voc (and close enough for Vmp use) based on temperatures over/under 25C).

https://forum.solar-electric.com/discussion/5458/two-strings-in-parallel-with-unequal-string-voltages

Regarding east facing array... Lead Acid Batteries need "hours on charge" to fully recharge (especially if you deep cycle below ~75% state of charge). And with short winters, "virtual tracking" can add those sorely missing hours.

With simple Solar Electric Handbook, you can see how much overall harvest you lose with different panel orientations. For example, W/E/S facing 45 degree tilt array in Ottawa Canada:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Ottawa
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 45° angle WEST:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.10	2.93	3.58	3.79	3.80	3.94
Jul	Aug	Sep	Oct	Nov	Dec
4.03	3.86	3.38	2.55	1.84	1.67

Measured in kWh/m2/day onto a solar panel set at a 45° angle EAST:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.10	2.93	3.58	3.79	3.80	3.94
Jul	Aug	Sep	Oct	Nov	Dec
4.03	3.86	3.38	2.55	1.84	1.67

Measured in kWh/m2/day onto a solar panel set at a 45° angle SOUTH:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.63	3.66	4.48	4.74	4.75	4.92
Jul	Aug	Sep	Oct	Nov	Dec
5.04	4.83	4.23	3.19	2.30	2.09

You can see that your E/W vs S solar is losing around 1/2 hour (1.67 vs 2.09 hours of sun) of harvest in Winter, and almost 1 hour of harvest (3.94 vs 4.94 hours) in Summer.

You can model the time on sun and overall harvest with the PV Watts program. It has an Hour by Hour spread sheet output option for 365 days a year--If you want to really model the various options in detail:
https://pvwatts.nrel.gov/

In Winter, a larger array of (2.09/1.67= ) 1.25x would make up for the "non-south" facing loss of harvest (with the advantage of more time on charge from morning to evening).

You did not give us the Vmp/Imp of the 195 Watt panel--But guessing it is closer to 17.5 or 18.0 volts.. Which does help you with the PWM controller (higher Vmp peak voltage).

If you where going to transition to an MPPT type charge controller--Could you get a second 195 Watt panel... You could then put 2s x 2p of the 100 Watt panels and 2s x 1p of the 195 Watt panels into an array--And have high enough Vmp-array for a very happy MPPT controller.

Add a link to David Angelini's Post about why mixing panels and array directions with a single MPPT controller can be an issue (article is written towards comparing central GT inverters and micro-GT inverters)--But does discuss MPPT issues:

https://forum.solar-electric.com/discussion/comment/393434#Comment_393434

Dave Angelini said:

The arrays harvest need good static mpv and dynamic. There are plenty of cases where one controller fails to do this right. The article below was from 2010 when we were field testing the first 600v mppt. The shade tolerant feature was pretty amazing and it worked well in moving clouds.

https://us.v-cdn.net/6024911/uploads/editor/37/fpqzmxmhly43.pdf

With MPPT controller and an East/West array--Having one MPPT controller trying to track the two different arrays could give you poor harvest due to the variable Vmp-peaks with the different array temperatures and shading... Not all MPPT controllers are equal, and you may find that you need an "east" and "west" MPPT controller for optimum harvest with the two arrays (two array = two MPPT controllers).

-Bill

Just to double check the charging current.

• 650 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 612 Watt array minimum
• 650 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 1,224 Watt array nominal
• 650 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 1,591 Watt array "typical" cost effective maximum

Your 400+195=595 Watt array is just at the minimum I would be be suggesting for charging. OK for Weekend/Sunny weather charging, but if you are closer to full time off grid usage, 10%+ rate of charge is better. Usually 10% minimum rate of charge is the requirement for FLA deep cycle battery manufacturers.

Nominally, for a daily use system, 2 days of storage and 50% discharge (for longer battery life) is a good place to start--1 day of storage+50% dischage for RV/weekend systems... 3 days+50% for longer "bad weather" ride through.

• 650 AH * 1/2 days * 0.50 max discharge = 162.5 AH per day (or over night) battery usage:
• 650 AH * 12 volts * 0.85 * 1/2 days * 0.50 discharge = 1,658 Watt*Hours per day nominal usage

With a well running 595 Watt array, you would need an array of:

• 1,658 WH per day * 1/0.52 off grid system eff * 1/595 Watt array = 5.4 Hours of sun per day "break even"

That is a lot of sun.. For a weekend system, you have the rest of the week to get back to full charge. For a daily use system (or weeks at a time during "cabin season"), you probably would need a backup genset to get the battery bank full at least once a week (again, depending on your daily loads).

And adding a refrigerator is usually the load that takes an off grid system from "small" to "medium" size.

Just adding batteries to a "too small" solar array for the loads, many times, ends up undercharging/over discharging the battery bank. The #1 cause of early battery failures.

Just some food for thought.

-Bill

Chest Fridge OR Freezer (must order as F/R and is small capacity) around

Voltage	10-31 VDC
Average Energy Use - DCR50 Refrigerator	114 Watt-hrs/day at 32°C 9.6 Amp-hrs/day at 12V, 32°C
Average Energy Use - DCF50 Freezer	280 Watt-hrs/day at 32°C 24.5 Amp-hrs/day at 12V, 32°C
Gross Capacity	50 L 1.8 ft3

-Bill