Undercharged Battery Bank? Low Specific Gravity

Welcome to the forum Skip,

Just to make sure, you have an accurate DC volt meter and are measuring actual battery bus voltage and it never exceeds 27.8 volts?

Assuming that is true, your battery bank is simply not receiving enough charging current and/or hours on charge.

A solar charge controller should charge maximum available current until your battery bus voltage hits ~30.8 volts, hold it there for something like 6 hours (2-6 hours, in your case, lots of time because the battery is severely under charged). And after it hold the Absorb voltage for 2-6 hours, drop back to 27.8 volts float.

We probably need to know more about your system:

• Battery bank AH and Voltage (is this a 357 AH @ 20 hour @ 6 volt battery, or a 24 volt @ 1,071 AH battery bank?)
• Charge controller Outback FM 60
• Solar array (XYZ panels, xxx Watt, YY Vmp, ZZ Imp), number of panels and configuration.
• Where roughly is the system (nearest big city)
• Array free of shadows/shading during most of the day
• What is your genset and DC battery charger type/size/configuration

This is a very big (and nice) battery bank--I fear you simply have too small of Solar array (Wattage) and/or the array is miss-configured (improper Vmp-array).

Do you have  a DC current clamp type meter so you can verify charging current to the battery bank during the day?

https://www.amazon.com/gp/product/B019CY4FB4 (something like this?)

At this point, your battery bank sounds severely undercharged and will be damaged (if not ruined) if not properly brought back to >90% state of charge quickly. The longer it stays at low charge, the faster it will sulfate.

Do not fill the cells any more (1/2 full, plates at least covered for now). If you do get vigorous charging voltage/current, the batteries will heat, gas form, and possibly puke out electrolyte during charging (gas in battery, electrolyte expanding as it gets hot, electrolyte spitting out caps/top of cell/making mess).

-Bill

Ok, assuming that is a single 6 volts @ 428 AmpHour battery in 4x series for 24 volt by 3 parallel strings for 1,284 AH battery bank. That is a very large battery bank for 24 volts.

In general, for a flooded cell lead acid battery bank, you would want 5% to 13% (to 20%) rate of charge. 5% is low, but can work for a weekend/(sunny) seasonal cabin or emergency backup. For for full time off grid/following the battery mfg. recommendations, you really need a 10%+ rate of charge (a larger battery bank needs a large solar array to properly recharge the battery bank)... And example would be:

• 1,284 Amp*Hour * 29.0 volts charging * 1/0.77 solar panel and controller derating * 0.05 rate of charge = 2,418 Watt array minimum
• 1,284 Amp*Hour * 29.0 volts charging * 1/0.77 solar panel and controller derating * 0.10 rate of charge = 4,836 Watt array nominal
• 1,284 Amp*Hour * 29.0 volts charging * 1/0.77 solar panel and controller derating * 0.13 rate of charge = 6,287 Watt array typical "cost effective" maximum
• 1,284 Amp*Hour * 29.0 volts charging * 1/0.77 solar panel and controller derating * 0.20 rate of charge = 9,672 typical absolute maximum battery charging current (FLA)

Your solar array:

• 320 Watt array * 5 panels = 1,600 Watt array

So, from a battery bank charging current point of view, you have about 2/3rds the minimum solar array you should have for proper charging.

And if you where using 60 amp MPPT charge controllers (typically would suggest >= 80 amp controllers):

• 4,836 Watt nominal array * 1/29.0 volts charging * 0.77 panel+controller derating * 1/60 amp MPPT controller = 2.1 parallel 60 amp MPPT charge controllers (plus two array circuits)

Normally, for an MPPT charge controller, you would want to see 2x of your present solar panels in series, and 2 or 3 parallel strings to get your Vmp array voltage higher. MPPT controllers need higher input voltage (~1.3x * 1/0.8 Vmp-array-hot derating = ~1.6 Vbatt charging voltage) to work optimally:

• 29.0 volts charging * 1.6 Array voltage uprate for MPPT charging on hot day = 46.4 volts minimum nominal Vmp-array-std voltage

Your array is Vmp~37.7 volts and not great for charging a 24 volt battery bank on hot days (Vmp falls as cells/panels heat up).

For AC inverter-charger, the charging current from a generator would typically be around 10-20% of battery bank AH capacity... Or:

• 1,284 AmpHour * 0.10 rate of charge = 128.4 amps typical
• 1,284 AmpHour * 0.10 rate of charge = 256.8 Amps max

Then there is the load on your system... Guessing from your first post:

• 400 Watts * 12 hours per day = 4,800 WH per day
• 150 Watts * 12 hours per night = 1800 WH per night
• Total ~ 6,600 WH per day 

If you assume 2 days of backup power (no sun) and 50% maximum battery discharge, the "optimum" battery bank would be:

• 6,600 WH per day * 1/24 volt bank * 1/0.85 AC inverter eff * 2 days storage * 1/0.50 max discharge = 1,294 AH @ 24 volt battery bank...

Your bank is sized well for your (my guess) loads. I would really be suggesting a 48 volt battery bank (a bank over ~800 AH should look at next step up in voltage):

• 1,294 AH / 2 (going from 24 to 48 volt bank) = 647 AH @ 48 volt battery bank

For various reasons, this generally works better (smaller diameter copper cables and 2x more solar panels per MPPT charge controller because of the higher battery bank voltage). But there are some issues too with the higher voltage wiring and solar array that make things a bit more complex.

And then there is sizing the solar array based on your loads and where you live... Colorado Springs, fixed array, facing south
http://www.solarelectricityhandbook.com/solar-irradiance.html

Colorado Springs
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 51° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.55	4.97	5.65	5.77	5.90	6.12
Jul	Aug	Sep	Oct	Nov	Dec
5.81	5.64	5.99	5.76	4.73	4.36

So, the average harvest from your 1,600 Watt array would be:

• 1,600 Watt array * 0.52 off grid AC system eff * 4.73 hours of sun (November) = 3,935 WattHours per average November day

And this appears not enough hours of sun/solar panels to sustain your daily loads... If you where to up the array to a 10% array or 4,836 Watts:

• 4,836 Watt array * 0.52 off grid eff * 4.73 hours of Nov sun = 11,894 WattHours per Nov day from 10% array

And, for the most part, for base (required daily loads), I suggest that you plan on using ~50-65% of your predicted harvest (allow for some clouds, quicker recharging after bad weather, and general reduction in genset usage):

• 11,894 Watt*Hour harvest (10% charging and November sun) * 0.65 base load fudge factor = 7,731 WH "base load" from 10% November array

So, lots of numbers and assumptions... But my guess from 1,300 miles away, is you need to add dramatically to your present solar array, rewire the array to 2 panels (of the same type) in series for higher Vmp-array, and add a second charge (probably 80 amp or larger) MPPT solar charge controller.

Longer term changing to a 48 volt battery bank and larger AH batteries would be a suggestion--Especially if you want to upsize the system in the future.

Before you make any changes--I always suggest making some paper/back of the envelope designs first--Always cheaper to make mistakes on paper vs after you bought the hardware and did the installation/work.

My thoughts and guesstimates, which may or may not be accurate/correct for your needs. Your thoughts?

-Bill

Regarding storage of FLA batteries... Yes fully charge and equalize the batteries. Store in a cool place and either float charge them or recharge them ~1 day out of every month (self discharge). That is for ~75F batteries... For colder batteries, self discharge is less. For hotter batteries, self discharge is higher.

As I understand, your battery bank is 6 volt @ 428 AH... 2x parallel strings would be 24 volts at 856 AH:

• 856 AH * 29 Volts * 1/0.77 panel+controller deratings * 0.10 rate of charge = 3223 Watt array for 10% rate of charge for 2 strings of batteries...

Yes, I would add one panel (at least) to the existing string so you can setup 2s x 3p (6 panels total). The typical peak available charging current would be:

• 6 * 320 Watt panels * 0.77 panel+controller deraing * 1/29 volts charging = ~51 amps peak... Your 60 Amp controller would be fine
• 60 amps controller * 29 volts charging * 1/0.77 panel+controller derate = 2260 max over paneling of controller suggested
• 2,260 Watts max / 320 Watts per panel = 7.1 panels max

So, the 6x total panels for your 60 Amp controller is about the maximum cost effective array--Although, making it 8 panels in total (2s x 4p) would not be the worst thing in the world (MPPT controllers will safely limit their maximum output current--To 60 Amps in this case. You will have your output capped by the controller during the middle of clear/sunny/cool days--But at other times, the more panels will give you more overall harvest per day without spending $$$ on additional controllers.

And to add to the conversation, if you went to a 48 volt bank, the same 60 amp MPPT controller could have a 2x 2,260 Watt array (double the power support, with the same single controller). However, it will require some further research... Vmp-std rises in cold temperatures. 3 panels in series (3*Vmp-std=3*36.7volts=) 110 Volts Vmp-array-std... That is probably too high of voltage array for an FM 60 in Colorado (again, need to check the specs and string sizing).

And placing only 2x 320 Watt panels in series--Not really great for charging a 48 volt battery bank (59 volts charging * 1.6 MPPT and heat fudge factor = 94 Volt Vmp-std array voltage suggested).

In any case, still need a lot more solar (or generator support) to keep even 2/3rd bank "happy" at 10% rate of charge.

A 4kWatt AC inverter will draw (at maximum sustained power rating):

• 4,000 Watts * 1/0.85 AC inverter eff * 1/21.5 battery cutoff voltage = 219 Amps

As you see, that requires some pretty heavy cable (and breaker/fuse) to sustain that level of 24 VDC current at 100% rated power. And why we suggest going to next higher voltage battery bus for large loads (and/or large AH battery banks).

-Bill

Skip,

It will also depend on how much energy you are pulling concurenlty with charging... To properly/fully charge the battery bank, a 5% rate of charge is probably needed (as a minimum).

If you have other loads at the same time and charging with solar, the rate of charge to the battery will be less...

You may need to fire up the genset to recharge the battery bank (or dramatically reduce loads when the sun is up).

As a rough starting point, a wild guess would be approximately 3 kWH of charging for every 1 gallon of propane, or in AH:

• 1,000 Watt*Hour / 29 volts = 34.5 AH per kWH
• 34.5 AH * 3 = 104 AH per 1 gallon of propane

If your bank is at 50% state of charge, again very roughly:

• 856 AH bank * 50% = 428 AH to be recharged (2x parallel strings)
• 428 AH to recharge / 104 AH per gallon = 4.1 gallons of propane to fill 2 parallel strings from 50% to 100%

It is actually more complex than that, but it does give you very rough idea of what it costs to run the genset to recharge your bank.

My numbers may be wrong, because I think I am understanding a different bank AH capacity than you are telling me when pulling 1 string of batteries...

-Bill