12 volt Battery/Solar System Recharging Problem

Re: 12 volt Battery/Solar System Recharging Problem

Good Morning Mark,

I have deleted the second thread--It will just cause confusion as people may split their posts across the two threads.

Marco57 wrote: »

The system includes 5 solar panels (100W/each), a 2500 Watt Inverter/Charger (Trace SW), and 8 – L16 (6 volt) batteries wired for 12 Volt.

Lets look at what you have right now... 500 watts of solar panels and lets assume this L16's:

• Trojan L16RE-A 6 volt @ 325 Amp*Hours (20 hour rate)

Lets assume this is a 24 volt battery bank--But it does not matter that much--the numbers will be the same if it is 4x325AH@12volts or 2 strings x 325AH@24volts.

Your rate of charge with the solar panels and the L16's is (24 volt calc):

• 500 watts * 0.77 derating / (2x325AH * 24 volts) = 0.025 = 2.5% rate of charge

The first issue is, when charging purely with solar panels, you really do not have enough solar panels to fully recharge the battery bank in normal use.

We use several rules of thumb here... One is a 5-13% charge current/bank capacity... Below 5% you have problems mixing the electrolyte and it takes too long to recharge the battery bank (run risk of sulfate hardening).

Another is 10% minimum rate of charge for "tall batteries" like the L-16 to properly mix the electrolyte...

Over 13% rate of charge can overheat the battery bank--But for the most part, a good charge controller (with remote battery temperature sensor) will limit that problem. Leaving that 13%+ rate of charge just being expensive for power/panels/capacity you will hardly ever use.

So, for 8x L16's and 10% rate of charge (with 0.77 solar panel+charge controller derating), you would be looking at needing:

• 2x325AH * 24 volts * 0.10 rate of charge * 1/0.77 = 2,026 Watts of solar panels recommended

If you once a week recharged the bank at 10% rate of charge (~65 amps) from an AC genset:

• 2x325AH * 29 volts charging * 0.10 rate of charge * 1/0.80 charger eff * 1/0.50 generator loading = 4,700 watt genset (approx)

A 3-5kW genset running > 50% capacity will be fairly fuel efficient.

You could also add the "Mix charging/Equalization" by running a smaller genset during the middle of the day when the solar panels are charging too (if needed).

The system was designed for 3 day usage and 4 day charge. My cottage is only used on weekends. I use aprox. 100 amps daily (between 200 and 250 amps a weekend) I don't believe that I ever used the batteries more than 30-40% of their power.

The problem with small charging systems and large battery banks is the longer the batteries sit below approximately 75% state of charge, the more they will sulfate (lead sulfate crystallizes and permanently reduces battery capacity).

A day below ~75% is not going to kill your battery, but cumulative time of weeks and months below 75% will.

So, you have one of the common mistakes of having too much battery bank capacity for your charging capacity.

If you do not want to add solar panels at this time, at least running a genset to get the batteries > 80-90% state of charge when you leave would be a good idea (assuming no power usage while you are away--the current solar panels are, more or less, just trickle charging your battery bank).

In spring 2009, i had to return all 8 batteries to their manufacturer to be recharged, because the batteries were high in voltage but very weak in amperage (strength). All normal battery chargers were not able to bring them up in amperage.

Sounds like it can be sulphation--Does not sound like a battery manufacturing problem.

This fall i have the same problem. (Yes, it hasn’t been very sunny in my part of the country this fall, but I’ve been using my generator to compensate) I get to the cottage and the batteries are fully charged, but as soon as i start using a few lights, they drop down below 12 Volts. I've been so careful with my usage in the past year, and yet, i still have the same problem. I've been equalizing the system monthly as recommended by the manufacturer.

Now sounds like a good time to analyze your loads (watts peaks, and Watts*Hours use per day). Redesign the battery bank/solar array/generator to meet your needs, with a cost effective solution.

I never used to equalize monthly, till i sent the batteries back to the manufacturer. My solar system provider, along with engineers from Trace Engineering, and the battery manufacturer informed me to start equalizing monthly. It's apparently the newest recommendation for solar systems with batteries.

Equalization has been a common recommendation (forever?)... There are several issues with equalization.

One is that end-users assume that equalization "fixes" battery problems and tend to equalize way more often and way longer (hours+) than needed. This can actually cause the batteries to fail much sooner (plate erosion, positive grid corrosion, etc.).

And, in your case, you may not have enough charging current (size of AC generator and battery charger, definitely not enough solar panels) to properly mix the electrolyte (stratification) in your tall battery cases. You can get a mix of problems--Dense electrolyte at the bottom cell (over-charged) and mostly water at the top of the cell (undercharged and sulphation).

My main consumption comes from a 3/4HP-220Volt water centrifuge well pump. It's peak is 3000 watts for a few seconds and then stabilizes at 1200 ... to fill up my tank, it takes aprox. 30 seconds and consumes aprox. 3 amps.

A large battery bank will certainly help when running large AC (and DC) and their starting surges.

A longer term solution may be to look at running a smaller DC (or AC) pump to reduce surges and keep other system components smaller.

Another solution may be to use a large cistern with the AC pump and genset (run once every few days). And use a small DC pump for pressurization in the cabin.

I've checked the system with a hydrometer, it's been stuck in the middle of white area for two months now. (All cells have identical readings) … also, the batteries were rotated in spring 2009.

Sounds like the battery wiring is OK (balanced):

Smart Gauge Battery Bank Wiring Guide

The system is fully maintained when I’m gone.

The water level in the batteries has never been below the plates.

I've always left the batteries in a warm place between 41 F in winter, and a maximum of 68 F in summer.

Sounds OK.

My supplier always has a different excuse. I use the system too little, or too many batteries, should go down to 4 instead of 8 etc...

I would guess 1/2 or 1/4 the battery bank with your existing solar panels

But, we also need to discuss your power needs by season... If you are in the Montreal Canada (or further north) area and use the system a lot in the winter--You will probably have to use the genset more than you are today.

It's been a 5 year trial and error situation. I'm tired of dealing with it.

It is frustrating... Lets start with how much power you use, by season, and where is the cabin located (You can look at the PV Watts website and pick a site in US/Canada that is close to your place).

Some random suggestions (read first--don't buy anything yet):

Deep Cycle Battery FAQ
www.batteryfaq.org
Battery Monitors (and Victron Battery Monitors)
Kill-a-Watt Meter (for measuring 120 VAC appliance power usage)
All About Charge Controllers
Read this page about power tracking controllers

I really like Battery Monitors--They let people look at one meter and figure out the state of charge of their battery bank. Great for family members/guests that you don't want going out and using a hydrometer to measure the bank's charge.

Another thing to do is look at your loads... We tend to stress conservation as being a better "investment" vs throwing more solar panels / batteries / generators at the problem.

Don't let coffee makers, hair driers, microwave ovens become a major set of loads unless you are willing to pay for their use (more panels, more batteries, larger inverters, etc.). On occasion, the spouse will give you a minimum set of requirements for their (and yours) happiness.

Also look at the genset / charger / battery sizing... Many times a large generator has a nice price for a large / reliable unit. However, for cabin use, a 8-15kW genset is way too large to efficiently power the day to day loads/battery bank charging needs.

Sometimes, a better solution is to get a second, smaller genset, (like the Honda eu1000i/eu2000i/eu3000i) for day to day use. Pick your genset to run > 50% rated load most of the time... Other than the Honda (and a few others) that idle down with light loads, most gensets use ~50% fuel flow for 0-50% electrical use. And diesel gensets should be operate at least 50-65+% rated load for long life.

I will stop typing here... I have made some pretty broad statements. They are intended to be starting points in our discussions--not the only possible end results.

Your thoughts?

-Bill

Re: 12 volt Battery/Solar System Recharging Problem

Sorry--I missed the Title with 12 volt system... Anyway, the wattage numbers still add up.

And I will forgive the double posts (including one in the wrong forum) this one time--that is normally sufficient cause for instant banning. ;)

Regarding using a smaller genset--The inverter/charger has both a maximum charge charging current and a maximum AC-2 (Generator Max Current) setting.

You could program both to better match a smaller genset...

However, you are still in need of a lot of current and a 1,600 watt genset may be pushing it:

• 4x325AH * 0.10 rate of charge= 130 amps
• 1,600 watt genset * 0.80 charger eff * 0.6 power factor * 1/14.5 volts = 53 amps max for 12 volt bank
• 52 amps / 130 amps = 0.32 = 3.2%

So, a Honda eu2000i (1,600 watt) would still be way on the small side of charging for your full bank.

One reason for a large battery bank is to support the surge current for starting your AC well pump... If we assume that the inverter is outputting 1,000 watts while running and ~2,500 watts into the motor while starting:

• 1,000 watts (run load) * 1/10.5 volts cutoff battery voltage * 1/0.85 inverter eff = 112 amps @ 12 volt bank
• 2,500 watts * 1/10.5 volts cutoff battery voltage * 1/0.85 inverter eff = 272 amps @ 12 volt bank

The maximum sustained load and surge currents should be around C/8 and C/2.5...

• 112 amps run * 8 hour rate = 896 AH @ 12 volts for run
• 272 amps start * 2.5 hour rate = 680 AH @ 12 volts for surge
• 4x325AH = 1,450 AH @ 12 volts your battery bank

So--Your bank is sized pretty nicely to support the well pump requirements...

And that is where we get into problems with "unbalanced" systems... Heavy surge currents are a real killer for Off-Grid systems. You have to size the hardware to support the very short and heavy current spikes. Then you have support the charging systems to recharge the extra large battery banks.

There are different ways around the issues... There are some soft start solutions for well pumps. But they are not cheap and you may need to change the AC pump out to a 3 wire or 3 phase motor.

Or, you go with a DC / Off Grid pumping setup (several potential solutions). But that is not cheap or easy either.

Then back to your 300 Amp*Hours per every two days? Or 150 AH per day?

That is not very much power. For a system around Montreal (PVWATTS), 0.52 system efficiency, fixed array and 1 kW of panels (minimum size for program):

"Station Identification"
"City:","Montreal"
"State:","QU"
"Lat (deg N):", 45.47
"Long (deg W):", 73.75
"Elev (m): ", 31
"Weather Data:","CWEC"

"PV System Specifications"
"DC Rating:"," 1.0 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 0.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 45.5"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 0.1 Can$/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (Can$)"
1, 3.24, 55, 0.05
2, 4.02, 62, 0.05
3, 4.40, 72, 0.06
4, 4.61, 67, 0.06
5, 4.77, 69, 0.06
6, 5.12, 70, 0.06
7, 5.62, 78, 0.07
8, 4.76, 66, 0.06
9, 4.92, 69, 0.06
10, 3.66, 55, 0.05
11, 2.01, 29, 0.02
12, 2.10, 34, 0.03
"Year", 4.10, 726, 0.63

Ok, lets assume deep winter is going to require generator backup. And pick the 4th month (October) to design the "nominal" system around--29 kWH per month (long term average).

And you are using 150 Amp*Hours * 12 volts per day:

• 150 Amp*Hours per day * 12 volts = 1,800 Watt*Hours per day

And the average collection from 1,000 watts of solar panels in October:

• 29 kWH per month / 30 days per month = 0.97 kWH per day = 970 Watt*Hours per day per 1,000 watts of solar panels

And to supply your daily load would require:

• 1,800 Watt*Hours per day load / 970 WH per day per 1,000 watts of panels = 1,856 Watts of panels

To size the battery bank based on average loads--We use 1 to 3 days of no-sun and 50% maximum discharge (this is based on a 12 volt battery bank):

• 1,800 WH per day load * 1/12 volt bank * 1/0.85 inverter losses * 1 day no sun * 1/0.50 max discharge = 353 AH battery minimum
• 1,800 WH per day load * 1/12 volt bank * 1/0.85 inverter losses * 3 day no sun * 1/0.50 max discharge = 1,059 AH battery maximum

Now--that does not account for the run/surge current from a 3/4 HP well pump which would push you over the 700 to 900+ Amp*Hour (20 Hour battery rating capacity) anyway.

So--based on various issues--It looks like a 700 AH battery bank @ 12 volts would be the minimum bank size for your existing loads (with the well pump probably being problematic at this level).

If / when you decide to replace your inverter--You should be looking at a 24 or 48 volt system. That will at least reduce the surge currents by 1/2 or to 1/4 the 12 volt battery bank value. Smaller wire gauges would make the next install better and reduce any DC wiring voltage drop issues.

Note--a 12 vs 48 volt battery system is no difference in costs... If you have, 8x 6 volt batteries in parallel (12 volt bank) vs 8x 6 batteries in series (48 volt bank)--You sill have the same amount of stored "energy". Just delivered at a higher voltage / lower current (power=volts*current).

Also, solar charge controllers are rated based on their DC output current, so on a 48 volt battery bank, a 60 amp solar charge controller can handle more solar panels (fewer charge controllers to purchase):

• 60 amps * 14.5 volts * 1/0.77 derating = 1,130 watts of solar panels @ 12 volts
• 60 amps * 29 volts * 1/0.77 derating = 2,260 watts of solar panels @ 24 volts
• 60 amps * 58 volts * 1/0.77 derating = 4,520 watts of solar panels @ 48 volt

If you could figure out the way to limit the power from the water pumping system--that would allow you to use a smaller battery bank.

However, you are still left with needing a large array for your daily loads. If you assumed that you only stayed there 2 days at a time and wanted to cut back to 1/2 the amount of solar panel... But you still need to make sure the battery bank does not spend much time below ~75% state of charge to reduce sulphation (if bad weather if forecasted after you leave--then running the genset before you take off to get the batteries over 80% State of Charge will help).

And, if you still need the high surge current... Using a smaller bank of AGM's may be a possible solution. AGM's can support much higher surge currents than flooded cell storage batteries (higher than C/2.5 rate I used in the above surge calculations).

-Bill

Re: 12 volt Battery/Solar System Recharging Problem

The inverter may last you 10-15 years total--and at that point, it is usually better to replace the inverter/charger with something new (10+ year old electronics are nearly impossible to find parts and repair shops for).

So, you will have your long term plan.

Short term, if this cabin is only used 2 days a week, see if you can find a used Propane Refrigerator (perhaps from a wrecked RV). That will reduce much of your daily power usage and reduce your needs for more solar panels/generator run-time.

It is a close call, but solar PV off-grid power usually makes good economic sense for a cabin/home lived in 9+ months of the year. If your place is not occupied as much--or much of your time in place is during the winter (less sun, freezing weather anyway)--A propane powered fridge may be less expensive to run.

Your next battery bank may be better to use batteries that are not as tall at the L-16s until you can size the array to at least ~10% rate of charge.

AGM's would be a very nice alternative--But they tend to be much more expensive (and may not last quite as long as a very good set of flooded cell batteries). The economics on AGM may be a bit murky for your setup.

As Marc says, using a Battery Monitor to check out your exact loads and charging profile--and use the Genset in the early morning (let the solar finish off the charging the rest of the day) to keep the batteries "happier". Don't feel you have to recharge the battery bank to nearly 100% full using the genset every day. That will be very expensive fuel wise and not very efficient (as the charging current tappers down as the batteries are >~80-90% state of charge).

I am not sure you need a really nice 3 stage charge controller (with float) unless you use it at home with utility power. You will never use float voltage when running with a genset (total waste of fuel).

You can also use small Battery Tenders or equivalent float chargers at home after you get the batteries fully charged too (just monitor water usage once per month--Even float chargers will still use water).

-Bill