OFFGRID Over Voltage problem

Welcome tot he forum Carlos.

PWM controllers are "on/off" type of charge controllers. They really do not have the ability to limit current. So, to high of current solar panels can damage the charge controller. So, these controllers are really too small for your application--You would need a higher rated controller for that amperage of solar array.

In any case, I have quite a few questions about your setup.

First, what is the voltage of your battery bank? 12 volts or 24 volts.

Next, a PWM charge controller really needs to match the Solar array Vmp-array with the battery bank voltage. For a 12 volt battery bank, Vmp-array should about 18 volts. And for a 24 volt battery bank, Vmp-array should be about 36 volts.

If you have a Vmp~30 volt array with a 12 volt battery bank, you will only get about 1/2 of the array wattage charging the battery bank. If you are charging a 24 volt battery bank, Vmp~30 volts is too low--The solar array will not generate enough voltage to properly recharge the battery bank. Vmp of solar panels is temperature dependent. For a hot panel, Vmp falls as the sun warms them up. On a very hot day, Vmp-array can fall towards 24 volts for a Vmp 30 volt panel. And to properly charge a 24 volt lead acid battery, you need around 29 volts minimum to do that.

Next, you talk about a 60 amp load... If you are using the load terminals on the charge controller--That is too high. At best, the terminals are probably rated for 30 amps--And some controllers, they are rated for 8 amps maximum. 60 amps is way too much.

And finally, the battery bank. 55 AH at 24 volts (two batteries in series) or 110 AH @ 12 volts (two batteries in parallel) is a very small battery bank for a 60 amp load and for 48 amps of charging current. If these are flooded cell batteries, you would be hard pressed to support a 60 amp load--I would expect the battery voltage to collapse. If these are AGM batteries, they may support the load--But they will not last very long (you will take them flat in less than two hours or so @ 12 volts, or less than one hour @ 24 volts)--Plus it is very hard on the batteries (they will not last very many cycles).

Plus, having too high of charging current available for a small battery bank--The solar panels can take the 12 volt battery bank to >> 15 volts charging. Solar charge controllers are not really designed to "regulate voltage"--The battery bank really regulates voltage short term. A charge controller does regulated long term voltage to manage the rate of charge/state of charge of the battery bank. If this is a 24 volt battery bank with your solar panels--Even though this is too high of charging current--These panels are not really capable of over voltaging the battery bank (they may over voltage the battery bank on cool days/short periods of time--but there are a lot of details and "what ifs" that I really cannot answer with the information available).

To help you with your system--We really need to understand your loads (amps, amp*hours per day, battery bank voltage, etc.)... That will let us define the optimal battery bank. Then we can look at the size/type/configuration of solar array and charge controller needed to meet your needs.

In general, it appears you are expecting a lot from a smaller off grid solar power system. For a reliable/long term operational off grid power system--I would be suggesting a much larger battery bank, and a different charge controller (more expensive MPPT type for use with your present solar panels). I don't think you can do what you want/need with the present setup.

-Bill

OK... So how many Watt*Hours or Amp*Hours per day do you plan on using... You can get (or sometimes borrow from a local library) a Kill-a-Watt type power/energy meter.

The Kill-a-Watt meter and this relatively cheap (and "good enough" for our needs) DC/AC Current Clamp DMM from Sears will give you some very nice tools to understand your loads and how it all plays together (plus let you debug stuff when needed).

I hesitate to tell you how to "fix" your system without more information on loads. If your AC inverter is your loading needs... A rule of thumb system design would look like this (a reasonably reliable system that you could run full time off grid power/loads with).

First the battery bank... For a 24 volt battery system, you want about 200 AH per 1,000 Watt of AC inverter rating minimum (you could have a 2x or larger battery bank if your peak loads are low but you use a smaller amount of power over 12-24 hours per day):

• 1,200 Watt AC inverter * 200 AH (@ 24 volt battery bank) / 1,000 Watts of AC inverter = 240 AH @ 24 volt battery bank minimum

Also, that rule of thumb works out pretty well for estimating the largest solar array you should connect to a battery bank...

• 6*250 Watt array * 200 AH / 1,000 watt of solar panels = 300 AH @ 24 volt battery bank minimum based on your solar array

To charge such a battery bank, you would need 5% to 13% rate of charge. 5% for seasonal/weekend power... Recommend 10%+ for full time off grid power (i.e., off grid cabin/loads). In your case, I would suggest 300 AH @ 24 volt minimum battery bank based on your array size:

• 300 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 564 Watt array minimum
• 300 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,130 Watt array nominal
• 300 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,469 Watt array "cost effective" maximum

You can rewrite the math and, for example if you pick 10% rate of charge, your 1,500 Watt array would support a battery bank of:

• 1,500 Watt array * 1/29.0 volts charging * 0.77 panel+controller derating * 1/0.10 rate of charge = 398 AH @ 24 volt battery bank (nice nominal design)

So--for your present system, a 240 AH to 398 AH battery bank would be a really nice size.

Now, what could such a battery bank support. Say we choose 398 AH @ 24 volts (4x 6 volt @ ~200 AH "golf cart batteries in series, and two parallel strings for a total of 8 batteries). Generally, 2 days of no sun storage and 50% maximum discharge on an AC inverter system:

• 398 AH * 24 volt battery bank * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge (for longer battery life) = 2,030 Watt*Hours per day for 2 days of "no sun"

And you also have a 1,500 Watt solar array... You sun may vary between 2 hours of sun (average) per day in winter to over 6 hours per day in summer. You can use a tool like this to figure out "how many hours per day" by month for your system. Assuming you live in a relatively sunny region with 4 hours of minimum sun for 9 months of the year:

• 1,500 Watt solar array * 0.52 off grid system eff * 4.0 hours of sun per day = 3,120 WH per day in "nominal sun"

Of course, actual solar radiation varies with weather (and seasons)--Your "base loads" should probably be around 65-75% of your "predicted output power"... You don't want to be on the "edge" of loads vs available power--You end up having to "manage" the system and loads every day--Some people love that, others hate it.

Then there is picking a solar charge controller... Because you have Vmp~30 volt solar panels, you need a MPPT type charge controller (maximum power point tracking) to charge your battery bank efficiently. The minimum size controller should be:

• 1,500 Watt array * 1/29 volts charging * 0.77 panel+controller derating = 39.8 amp minimum recommend MPPT controller (price/performance minimum)

MorningStar makes a nice TS MPPT 45 amp charge controller. Midnite and Outback make larger (60-90 amp) charge controllers too... Any of those would be good choices (read the manuals/ask questions--These are very complex pieces of hardware--And not cheap). There are other, less expensive charge controllers out there--Some good, many not so good... Do your research.

If you get flooded cell batteries, another needed tool is a hydrometer to measure specific gravity for each battery cell. The "gold standard" for estimating battery capacity/health. This one is pretty nice. Be sure to rinse with a couple squirts of distilled water before putting away (drying electrolyte gets sticky/gummy).

I will stop here--There is a lot of math and choices to be made--And your loads is the information we need to make these system design choices. I have made lots of guesses--This will help you narrow your choices, but the details of your needs are all that matter--My guesses are just guesses.

Hope this helps.

-Bill

GEL batteries are not a good choice (in general) for off grid solar power systems... They can take about C/20 or 5% rate of charge. It can take several days to recharge a GEL battery because of the slow rate of charge. High rates of charge (C/10 to C/5+) will damage GEL batteries (there seem to be some European GEL batteries which are rated for higher charging current--But no US batteries that I recall).

Next--If these three "circuits" are all in the same location--I would design one large array+battery bank to support your loads. Fragmenting into multiple small systems is a huge pain in the behind (3x more parts to maintain/debug/fail--You cannot easily move "excess" energy between systems, etc.).

And yes, good quality MPPT controllers can support 140 to 150 VDC Vpanel input -- or even higher for some vendor/models.

To properly recharge your 24 volt battery bank, you need to put two 30 volt Vmp panels in series and use a MPPT controller with a minimum rated Vpanel input of ~100 VDC minimum. A good MPPT controller will limit its output current to its rated value. These typically have a "switch mode/buck converter" in them--Has some very nice properties for use with solar panels and battery banks.

Even though you want to run your loads during the day--Solar charge controllers are typically "relatively slow" when adjusting charging voltage. Basically, the battery bank is what regulates the system DC voltage between ~11.5 to 15.0 volts (12 volt battery bus). If you supply too much charging current to a small battery bank, it is possible that the solar array will raise the battery voltage >> your 13.2 to 14.4 volt nominal battery voltage--You may fault the AC inverter (most fault around 15 to 16 volts on the DC bus for 12 volt inverters) before the solar charge controller can shut down/back off on the charging current (actual solar charger voltage response is variable on brand/model--I don't have that information).

For most people/applications, we really want to use significant power at night/during bad weather... So a typical "balanced" system design has a large enough battery bank to manage several days without sun--Which also is a large enough battery bank to not "over voltage" relative to the array size.

-Bill

Depending on the size of battery bank, golf cart batteries can be very cost efficient.

-Bill

What is the the brand/model of Inverter-Charger?

What is the DC input voltage--I assume a 24 VDC battery bus--But should be sure that it does not require a 12 volt bus.

-Bill