Relation between measured Isc/Voc and power produced by MPPT controller

eugenesan · August 2017

I need help understanding the relation between measured Isc/Voc and power produced by MPPT controller.

For some time now I am trying to figure why is my solar system performs poorly (in my opinion).
I have 2x100w Mono/Flexible (HQST/Renogy) panels connected to a 20A MPPT controller (EPSolar).
The absolute maximum I am able to get from the MPPT controller is ~130W while at the same time I am measuring Isc/Voc equivalent of 192W.
Both manufacturers of the panel and the controller defend their creations claiming everything is performing as expected.

I wonder, is it me failing to understand the principles of solar system operation or one of the components is not performing as it should?

P.S.
1. I did several test under full sun at optimal summer position
2. I tried both parallel and series panels connections
3. My panels are installed on flat roof (no tilt)
4. I use appropriate wiring and connectors and there is no measurable voltage drop

BB. · August 2017

With the unit charging (bulk voltage charging, i.e., the battery bank is less than ~80% state of charge), what is the voltage at the Vpanel input for your charge controller.

In general, getting ~77% from a solar array (in wattage), or ~154 Watt maximum is what I would expect--And less than that if the battery is relatively charged (i.e., if the battery is at Vabsorb setpoint--Something like 14.4 to 14.8 volts), the controller is in "voltage regulation mode" and you will see less than maximum solar array power (watts) at that point.

I am not a huge fan of flexible panels--They tend to not last as long as glass faced panels as they crack/haze overtime. Getting 5-7years from flexible panels seems to be pretty good (in my limited experience)--Even getting better than 2+ years is not bad for difficult installations (such as a boat or RV).

Note that Solar Panel Voc/Ioc is "not relevant" in normal operation. Voc (voltage open circuit) is the voltage on the panel when no current is flowing, and Ioc is current into a dead short. Vmp*Imp is relevant for "cold panels" and, roughly 80%*Pmp is what to expect for "hot panels" (normal operation in sunny weather in a warm to hot day).

-Bill

Raj174 · August 2017

Mono panels tend to get a little hotter, also if the panels are flat on the roof with no air space behind them, then even higher temps will occur. Higher temperature means less power.

eugenesan · August 2017

@BB.
Thank you for your prompt response.

I did test the system with discharged batteries and/or load which exceeds potential solar output but still the output of the charger is never above 130W. For example if Voc/Isc are 35.7V/5.5V than Vp/Ip would somewhere around 29V/4.3A.

So, just to ensure I understand, measured Voc/Isc (minute before connecting to the controller) is not representative of the actual panel performance. Meaning the voltage on panel drops below optimal Vip under high current. Is that correct?
Also can above be an indication of incorrect positioning (tilt/bend/direction) or it is a sign of defected/degraded panels?

Now regarding the battery charging state, I always started the test with the batteries bank being under 12.8V and controller would start slowly raising the voltage in 13.0-13.6 range. The main reason I am doing this "investigation" is that I am afraid the current state of the system is degrading my battery bank since it never reaches 14.3 (end of manufacturer recommended bulk state) and the charging current during the bulk stage is rarely over the recommended minimum of 8A.

mike_s · August 2017

The power produced by an MPPT will always be less Isc X Voc. Isc is the maximum current, zero voltage point (=0 watts), Voc is the maximum voltage, zero current point (again, 0 watts). Never the two shall meet. Vmp is always less than Voc. Imp is always less than Isc.

jonr · August 2017

And AFAIK, how much less depends on various things. You could measure panel V and A in operation in an attempt to determine where your missing watts are going.

mcgivor · August 2017

eugenesan said:

@BB.
Thank you for your prompt response.

I did test the system with discharged batteries and/or load which exceeds potential solar output but still the output of the charger is never above 130W. For example if Voc/Isc are 35.7V/5.5V than Vp/Ip would somewhere around 29V/4.3A.

So, just to ensure I understand, measured Voc/Isc (minute before connecting to the controller) is not representative of the actual panel performance. Meaning the voltage on panel drops below optimal Vip under high current. Is that correct?
Also can above be an indication of incorrect positioning (tilt/bend/direction) or it is a sign of defected/degraded panels?

Now regarding the battery charging state, I always started the test with the batteries bank being under 12.8V and controller would start slowly raising the voltage in 13.0-13.6 range. The main reason I am doing this "investigation" is that I am afraid the current state of the system is degrading my battery bank since it never reaches 14.3 (end of manufacturer recommended bulk state) and the charging current during the bulk stage is rarely over the recommended minimum of 8A.

The one piece of the puzzle that is missing is the Ah capacity of your battery bank, the reason your voltage never gets to 14.3V, could be related to the capacity being greater than the arrays ability to charge it, just some thoughts.

Estragon · August 2017

> @eugenesan said:
> @BB.
> Thank you for your prompt response.
>
> I did test the system with discharged batteries and/or load which exceeds potential solar output but still the output of the charger is never above 130W. For example if Voc/Isc are 35.7V/5.5V than Vp/Ip would somewhere around 29V/4.3A.
>
> So, just to ensure I understand, measured Voc/Isc (minute before connecting to the controller) is not representative of the actual panel performance. Meaning the voltage on panel drops below optimal Vip under high current. Is that correct?
> Also can above be an indication of incorrect positioning (tilt/bend/direction) or it is a sign of defected/degraded panels?
>
> Now regarding the battery charging state, I always started the test with the batteries bank being under 12.8V and controller would start slowly raising the voltage in 13.0-13.6 range. The main reason I am doing this "investigation" is that I am afraid the current state of the system is degrading my battery bank since it never reaches 14.3 (end of manufacturer recommended bulk state) and the charging current during the bulk stage is rarely over the recommended minimum of 8A.

The rating of 100w per panel is likely the STC (standard test condition) rating. This would be for clean panels at 25°C, and at 90° to the sun with 1000w/sq.meter insolation. These conditions often aren't what happens in the field. Your panels may also have a NOCT rating that adjusts for the output loss resulting from the panels getting warm in the sun, and/or a temperature coefficient spec. You could use these to estimate potential with your actual panel temps.

Panel tilt can have some effect, but with flat panels around solar noon at this time of year it ahouldn't be a huge factor.

At 12.8v with no charging source for several hours, a lead acid battery is about fully charged. At much over 12.4ish, current drops off as the internal resistance in the battery rises and gets closer to fully charged. What is the type (AGM or flooded) and capacity of your battery?

eugenesan · August 2017

@mcgivor , @Estragon
My bank is composed of two 155AH SLR (AGM) batteries. Recently I've added an additional ~20AH load to the system and after a couple of overcast days my system could not keep up anymore. Before that it would reach float charge every single day.
According to the charge controller it's able to generate 50AH a day which is marginal for my needs. Unfortunately I was too optimistic about system's performance.

The question I have now is should I try to squeeze more power form existing panels (by "air-gapping" and tilting) and potentially adding additional identical panel OR should I assume the panels are "bad" and replace them with new "hard" panels (of course the second option is less preferable due to involved expanses)?

mcgivor · August 2017

In order to maintain a battery in daily use a charge current of 10% of the battery capacity Ah rating should be applied, so for your 310Ah the charge current should be ~31A, keep in mind if there are loads during charging these will subtract from the charging current, don't forget the inverter tare load if applicable.

With the panels you have, 200W, the most you will actually get is ~150W which equates to ~10.5A, so without loads during charging your panels are only able to supply 30% of what is required, so no amount of air gapping or tilt can make up the deficit because the calculation above accounts for that already. The addition of PV to meet the requirements will help during the cloudy days, by my estimate around 500W minimum of PV are required, more if a load is present during charging and assuming the batteries are sized according to the night time loads, something not discussed.
Can you give more details about loads, inverter if applicable.

eugenesan · August 2017

@mcgivor
Are mentioned 10% required due to the self discharge of the batteries?
Are my SLR/AGM batteries are subject to the same rule? The manufacturer (VTanks) mentioned only once in a month charge during storage.

My theoretical calculations for the system were as follows:
* Total usable capacity of 155Ah (to never discharge below 50%)
* Total daily load is: Inverter connected equipment - 45 Ah, Charge Controller connected equipment 20Ah
* 200W solar array with 15% in losses - 70Ah

I've assembled the system during the winter and didn't have the right conditions to test it properly.
Now after analyzing your responses and after adding additional 20Ah of load I've realized I was far too optimistic.

Also, seems like the temperature factor might be my main suspect for the poor panels' performance. My measurements shows 58C on the surface of the panels and 62C on the roof surface. Short research showed that every 20C could degrade performance by up to 40%. I am not sure if the information is directly applies to the panels I use, but 100+% above STC probably has it's implications.

mcgivor · August 2017

The percentage has nothing to do with the self discharge, it is a recommended figure to provide sufficient PV capacity to support the battery. For every 100Ah of battery capacity, 10A or more, of charge current would be required . So in the planning stage of a system for a 310Ah 12V nominal battery bank, the PV array should have the ability to provide 31A thereabouts, so rough calculation 12V×31A=372W, panels generally produce 75% of rated output, so 372W×1.25=465W, this is the wattage of panels required, round it off to 500W.
In addition geographic location has to be taken into account, many overestimate the hours of useful sunlight, use this link to find the hours at your location, http://solarelectricityhandbook.com/solar-irradiance.html pay particular attention to winter months as this will be a determining factor. Please note all the above is a rough calculation there are many more contributing factors not included, the purpose is to point out that perhaps you have over estimated the amount the 2 ×100W panels are capable of supporting. Sometimes it's better to start from the beginning again with the loads, battery capacity needed to support them, charging system to support the battery, geographic location etcetera, etcetera. Most have made the mistakes, me included, with the help offered from various members, a successful system can be developed.

mvas · August 2017

eugenesan said:

@mcgivor
Are mentioned 10% required due to the self discharge of the batteries?
Are my SLR/AGM batteries are subject to the same rule?
The manufacturer (VTanks) mentioned only once in a month charge during storage.

My theoretical calculations for the system were as follows:
* Total usable capacity of 155Ah (to never discharge below 50%)
* Total daily load is: Inverter connected equipment - 45 Ah, Charge Controller connected equipment 20Ah
* 200W solar array with 15% in losses - 70Ah

I've assembled the system during the winter and didn't have the right conditions to test it properly.
Now after analyzing your responses and after adding additional 20Ah of load I've realized I was far too optimistic.

Also, seems like the temperature factor might be my main suspect for the poor panels' performance. My measurements shows 58C on the surface of the panels and 62C on the roof surface. Short research showed that every 20C could degrade performance by up to 40%. I am not sure if the information is directly applies to the panels I use, but 100+% above STC probably has it's implications.

10% of the C/20 Rating is a minimum charging rate for AGM Batteries
But, you do not have 10 Hours of Full Sun everyday to slowly recharge your battery bank.
You only have 5 Hours of "Full Sun Equivalent" each day, not 10 hours.
Your target for charging amps should be 15% of the C/20 rating.
46 Amps charging rate = 15% x 310 AH Battery Bank
Your Solar Panels are significantly under-rated for your battery bank size.
You need 800 Watts ( 46 Amps x 14 Volts / 80% ) of Solar Panels - just to fully recharge the battery bank.

115 AH = 2.5 Hours in Bulk Mode, up to 85% SOC
46 AH = 2.0 Hours in Absorb Mode, up to 99% SOC
1 AH = 0.5 Hour in Float Mode, up to 100% SOC
=======================================
162 AH = 5.0 Hours "Full Sun Equivalent"

And you MUST put 10% more AH back in to the battery bank, compared to the 150 AH you draw out.
Yes, the sun is in the sky for 10 - 12 hours but it is only 5 HOURS of Full Sun Equivalent daily.

And if you have ANY daytime loads while recharging (Sunrise to Sunset)
then your solar panels need to be that much larger.

Also, I thought VMaxTank Batteries were to be Bulk Charged until 14.5 Volts, not 14.3 Volts (per your msg #4)
Maybe ( 14.3v ? ) because your battery bank is hot?

Chronically under-charging your battery bank, like your are currently doing, will kill them in one year or less.
If recharged correctly, wiith the heat and the 50% DOD every night, you will be lucky to get 3 years.

eugenesan · August 2017

@mcgivor , @mvas
Thank you for such an informative responses.

First, I would like to understand why are you recommending such a high charging rate while the manufacturer only mention a minimal (8A per battery) and maximum (20A per battery) rates. Is it due to assumption I am going to use the full capacity of the bank and have to have the means to recharge it daily?
What if I will use only 20% of the bank's capacity and will have proportionally sized solar array and have alternative source of charging to cover possible discharge of over 20%.

In last two days I was limiting the load to avoid discharge of over 20% and the system manages to successfully reach float charge stage by early afternoon.
Isn't that a sign my above assumptions are correct or I am just fooling myself?

mcgivor · August 2017

Thanks @mvas my calculations were conservative, as a demonstration of forming a base line, your contribution only exemplifies the importance of maintaining a sufficient charging regime.

Horsefly · August 2017

@mvas - I have to admit I don't understand everything you've said. That may mean the OP won't either. What does the 10% of C/20 rating have to do with 10 hours of full sun?

I try to think about what you said relative to my own install, and can't translate it. My battery bank is also 310Ah (@ 24V), and also made up of Vmax Charge Tank batteries. It sounds like the OPs bank is 12V, but we may well have the same batteries. I think you are right that his PV may be undersized, but I don't think I get your math. Where does the 115Ah in bulk come from? Same question for the 46Ah in absorption.

I've only built my one system, but did enough research and math that I thought I understood it all. I'm sure you know more about this than I do, so I really am just trying to understand it better. If you could explain a bit more, I (and maybe the OP) would appreciate it.

Estragon · August 2017

The 10% of capacity is a rough rule of thumb, and does include some assumptions.

For example, a weekend cabin application might be able to get by on 1/2 that, because any deficit in charging will be made up during the week when there are no/minimal loads. Location is also a factor. A cool high altitude location might even get better than STC power from panels, whereas a hazy hot coastal location would do considerably worse.

If you can get to float most days, that's a sign the loads and panels are about balanced. There are always trade-offs like how often you can or want to run a generator, for example, that are considerations in sizing pv.

mcgivor · August 2017

Think about the statement, 8A minium, 20A maximum, per battery, in parallel that is 16A minium, 40A maximum, you can exceed the maximum for short periods of time with no ill effects,as soon as the voltage climbs the current will reduce. Focus on the maximum, because there are only so many hours of productive sunlight per day.

pdh · August 2017

> What does the 10% of C/20 rating have to do with 10 hours of full sun?

It was to explain a potential fallacy. Some people may think, OK I'm charging at a 10% rate, and I have 10 hours of sun every day (because the sun seems to shine pretty brightly for at least that long on clear summer days). So, 10% x 10 hours means I can do a 100% recharge every day. Hence if I have enough panels to yield 10% of the C/20 rate, I'm good.

The fallacy is that you don't in fact get 10 hours of good sun every day, even on clear summer days -- 5 hours is more like it. (You'll get less in the winter, and on cloudy days.) Also the charging process is not 100% efficient. As a result, you need enough panels to yield more than a 10% charging rate.

Horsefly · August 2017

I guess I still don't get it. If @pdh is correct that this was the intent, it's assuming a bunch of things that I would hope no one assumes. Even if the idea is that 10% of C/20 for 10 hours equals 100% recharge, the underlying assumption is 100% DoD. No one here ever suggests 100% DoD, so why would someone say you need 10hrs at 10% of C/20?

mvas · August 2017

eugenesan said:

@mcgivor , @mvas
Thank you for such an informative responses.

First, I would like to understand why are you recommending such a high charging rate while the manufacturer only mention a minimal (8A per battery) and maximum (20A per battery) rates. Is it due to assumption I am going to use the full capacity of the bank and have to have the means to recharge it daily?
What if I will use only 20% of the bank's capacity and will have proportionally sized solar array and have alternative source of charging to cover possible discharge of over 20%.

In last two days I was limiting the load to avoid discharge of over 20% and the system manages to successfully reach float charge stage by early afternoon.
Isn't that a sign my above assumptions are correct or I am just fooling myself?

Charging RATE vs Charging TIME
==========================
Yes, VMax may recommend a s-l-o-o-o-w 10% of C/20 Rating for their battery chargers.
But at that slower the RATE, it takes a longer TIME.
We are severely limited on TIME = 5 Hours of Full Sun Equivalent.
The sun comes up and the sun goes down - it does not care if our battery bank is 100% SOC or not.
We do not have the luxury ( option ) to recharge a 50% DOD Battery Bank at a slow 10% of C/20 Rate for 10 hours.
We live in the Solar World controlled by the "Sun Hours".
We MUST recharge our battery bank to 100% SOC and get into FLOAT, way before sunset.

20% DOD vs 50% DOD
==================
20% DOD vs 50% DOD is a huge difference.
If you have decreased to only 20% DOD and you reach Float before sunset then you have a "Balanced" system.
Now your Solar Panels can recharge your Battery Bank within the time allowed by the Sun.

Is you Battery Bank temperature compensated?

Estragon · August 2017

At 50%DOD and 10% of capacity charging, it will take roughly 3 hours in full sun in bulk to reach 80%, and another 2-3hours in absorb to get fully charged. In absorb, current tapers off, so waning afternoon sun may still get the job done.

At 20%DOD, bulk will be minimal, and absorb may still get done even with well under 10% of capacity charge.

BB. · August 2017

We have several (somewhat) competing issues here...

We have the battery sized for supplying loads (continuous and surge current ratings). We have the minimum charging current required for various charging modes (Absorb Current, some vendors like Trojan spec. 10% minimum rate of charge for their batteries--And others require 5% minimum rate of charge for Equalizing).

You also run into issues where "we" use 20 hour discharge capacity rating, and, for example, Rolls/Surette uses 6 or 10 (??) hour capacity ratings with different charging current numbers in some of their (older?) literature.

http://support.rollsbattery.com/support/solutions/articles/428-state-of-charge-charging-flooded-lead-acid-batteries

Where Rolls suggests absorption time is based on battery capacity divided by charger maximum Amperage rating * 0.42.

And some batteries are more efficient at charging (lower internal resistance) like AGM, and they have slower self discharge.

Most battery vendors assume a fixed power supply (i.e., grid or generator) and an industry standard charge profile (there are several ISO charging profiles out there--Voltage and Current based).

For folks on solar--you are at the whims of sun and weather. And few battery vendors will give you the "ideal" solar charging profile because(?) if there was a standard solar profile, nobody could meet it (location, seasons, sun, loads are all different for off grid systems).

So... As a quick rule of thumb, we use 5% for weekend and seasonal (i.e., summer time) residences where there is either the rest of the week to fully charge the bank and/or enough sun in summer to keep the bank charging to near 100% every day.

And for full time residences, and users that have significant power usage during the day (washer, well pumps, computer, etc.), the "extra charging current" is usually enough to get the battery bank full most days even with daytime loads. So, 10% minimum rate of charge will do the job for the "average" off grid home (without having to account specifically for day time loads).

And the 13% or so maximum solar charging--That works out for most users here to quickly charge the battery bank to where they are in "float" in the early afternoon, and adding more charging current from the solar array was a waste of money (especially when solar panels were $5-$10 per Watt). But since solar panels can be under $1 per Watt these days, adding more panels is relatively cheap insurance for bad weather and running less generator + expen$ive fuel.

Other issues why, for example, the rule of 2 days of storage and 50% maximum discharge work well for lead acid batteries--They take time to charge... If you are looking at ~2-6 hours in absorb, plus the heavy current to bulk up to ~80% or so State of charge in the morning--On average the battery bank is only discharged by ~25% per day--The typical hours of sun per day is enough hours to both bulk and absorb the battery bank.

If you discharged to 50% every day (1/2 size lead acid battery bank), there are not enough hours in a day to fully recharge the battery bank every day (8+ hours of good sun).

Another reason for the suggested 13% rate of charge for solar--Flooded Cell Lead Acid batteries do get warm when subjected to heavy charging currents (especially when SoC is >80% state of charge). For Flooded Cell Lead Acid Storage Batteries, the suggested maximum rate is around C/8 (12.5%) rate of charge unless there is a remote battery temperature sensor.

Without an RBTS, the battery can get hot (approaching 110 to 120F), which depresses the battery's charging voltage, and the controller outputs more current, battery gets hotter, then you have a meltdown.

With an RBTS, the battery's temperature is compensated for by the charge controller. Most industrial (i.e., forklift chargers, from what I have seen) have 12+ hours per day to charge and charge at C/8 or less--So they do not have the issue that off grid people have of getting as much energy into the battery bank in a limited (by sun and weather) as off grid folks do.

With genset charging (person watching, generally charging to ~80% SOC or so, then letting solar finish up), or with solar +RBTS, you can generally run 20-25% rate of charge (if you want) without damaging the flooded cell battery bank.

In my humble opinion.

-Bill

mvas · August 2017

Horsefly said:

@mvas - I have to admit I don't understand everything you've said. That may mean the OP won't either. What does the 10% of C/20 rating have to do with 10 hours of full sun?

I try to think about what you said relative to my own install, and can't translate it. My battery bank is also 310Ah (@ 24V), and also made up of Vmax Charge Tank batteries. It sounds like the OPs bank is 12V, but we may well have the same batteries. I think you are right that his PV may be undersized, but I don't think I get your math. Where does the 115Ah in bulk come from? Same question for the 46Ah in absorption.

I've only built my one system, but did enough research and math that I thought I understood it all. I'm sure you know more about this than I do, so I really am just trying to understand it better. If you could explain a bit more, I (and maybe the OP) would appreciate it.

Your Battery Bank is twice as large (kwh) as the OP ...
OP = 310 AH @ 12 Volts = 3.7 kwh
You = 310 AH @ 24 Volts = 7.4 kwh

Your PV Array is twice as large as my suggested for OP ....
OP = 800 Watts
You = 1,600 Watts

You have the EXACT same proportions, right?

To calculate Amp-Hours, you multiply Amps times Hours ...
115AH BULK = 46 Amps Bulk x 2.5 Full Sun Equiv Hours
46AH ABSORB = 23 AVG ( 46 Amps to 0 Amps ) Absorb x 2.0 Full Sun Equiv Hours

Horsefly · August 2017

I get that my system meets what you recommended for him, but my reasoning is quite counter to yours. In fact, I was using the 10% charging rate as my threshold, which it sounds like you are saying is way too low. That's why I couldn't make sense of it.

I still don't get the 10 hrs, or why 2.5 hrs for bulk, or why 2 hrs for absorb.

I guess I'll just let it go. Maybe there's nothing wrong with the way you came to the conclusion, it's just different than how I've learned.

eugenesan · August 2017

> @mvas said:
> eugenesan said:
>
>
> @mcgivor , @mvas
>
> Thank you for such an informative responses.
>
>
>
> First, I would like to understand why are you recommending such a high charging rate while the manufacturer only mention a minimal (8A per battery) and maximum (20A per battery) rates. Is it due to assumption I am going to use the full capacity of the bank and have to have the means to recharge it daily?
>
> What if I will use only 20% of the bank's capacity and will have proportionally sized solar array and have alternative source of charging to cover possible discharge of over 20%.
>
>
>
> In last two days I was limiting the load to avoid discharge of over 20% and the system manages to successfully reach float charge stage by early afternoon.
>
> Isn't that a sign my above assumptions are correct or I am just fooling myself?
>
>
>
>
> Charging RATE vs Charging TIME
> ==========================
> Yes, VMax may recommend a s-l-o-o-o-w 10% of C/20 Rating for their battery chargers.
> But at that slower the RATE, it takes a longer TIME.
> We are severely limited on TIME = 5 Hours of Full Sun Equivalent.
> The sun comes up and the sun goes down - it does not care if our battery bank is 100% SOC or not.
> We do not have the luxury ( option ) to recharge a 50% DOD Battery Bank at a slow 10% of C/20 Rate for 10 hours.
> We live in the Solar World controlled by the "Sun Hours".
> We MUST recharge our battery bank to 100% SOC and get into FLOAT, way before sunset.
>
> 20% DOD vs 50% DOD
> ==================
> 20% DOD vs 50% DOD is a huge difference.
> If you have decreased to only 20% DOD and you reach Float before sunset then you have a "Balanced" system.
> Now your Solar Panels can recharge your Battery Bank within the time allowed by the Sun.
>
> Is you Battery Bank temperature compensated?

Thanks.
I guess now I have the basic math about the system to be able to extrapolate toward additional load/panels.

No, my bank is not temperature balanced.
The batteries are well insulated/ventilated and air-gapped from each othet to avoid heat aggregation.
I did monitor batteries' temperature during charging with 20A charger (provided by the batteries' manufacturer) and while charging with 12A worth of solar. The batteries never were above ambient temperature. I believe my charging means are not powerful enough to trigger any significant thermal response.

eugenesan · August 2017

It might be wrong but after analyzing all the information provided in thread I wonder, if the system aimed towards 20% DoD won't it lower the average (and maybe the maximum) charging current?
I mean some sources claim the bulk stage usually end somewhere around 80% and that means very short bulk stage shortly followed by boost and float.
If I am correct, in case of 20% DoD one can size it's PV to be just above minimal charging current required by the manufacturer.
Of course the above idea, assumes there is a backup charging solution for charging during bad weather or over-discharge.

Also, I wonder why is there a minimal charging current recommendation. Does charging with lower current causes permanent bank degradation?

Estragon · August 2017

Most of the rise in temperature will happen towards the end of charging when the internal resistance of the battery reduces charge efficiency to the point where much of the current is going to heat and gassing. Temp compensation isn't used so much to stop batteries from overheating (although it does that too), it's so the charging voltage is corrected for the temperature. A cold battery needs higher voltage, and vice versa. AGMs in particular are sensitive to overly high voltage as they can vent gas and be permanently damaged.

Only going to 20%DOD will definitely lower average current. Probably not the (potential) peak though, as the battery will take peak current or close to it getting up to absorb voltage. At 80% full, it just won't take long to get there, and current will taper off when it does.

mike95490 · August 2017

Sealed batteries, while they have several attractive qualities, are a less than ideal choice for starting off with.

Roughly, they cost 2x as much, and last half as long, as quality Flooded Batteries. Any mistakes, and you can easily damage the bank. and being sealed, there is no way to really check the state of charge with a hydrometer.

What needs to be done, is to get a solid number for the ACTUAL SOLAR hours (well aimed panels will generally produce 80% of nameplate for 20 minutes daily, the rest of the time is somewhat less and nowhere on earth, is there a usable 10 hours.)
You need (even for AGM) to allow for replacing 120% of what was consumed, within 48 hours. Adding up all the loads, conversion losses and recharge losses, and you roughly need solar nameplate rating to harvest 2x your loads

mvas · August 2017

@eugenesan ,

How do you know that your AGM Battery Bank is at 20% DOD when under load?
Do you have an Shunt and an Amp-Hour Meter?
How do you know when your AGM Battery Bank is recharged to 100% SOC?

mcgivor · August 2017

@eugenesan

Rewinding to your original questions, your batteries manufacturer states minimum charge current of 8 amps, you have ~11 amps, but you have 2 batteries in parallel, so assuming internal resistance of each are equal, the current flow would be equal or 5.5 amps through each battery, below the minimum requirement. To add to this the minimum guidelines are based on grid charging, where unlimited hours are available, not just a small window of opportunity per day, so you need much more charging capacity to account for this. You really don't want to be on the fine line of battery failure where you currently are, sure lowering loads to almost nothing may help, but what's the point of having a system that can't support the loads you require.

Relation between measured Isc/Voc and power produced by MPPT controller

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