Maximum and minimum battery voltage

Re: Maximum and minimum battery voltage

kansas wrote: »

I've just started up my self designed system. I just need a couple of lights and use of a laptop and CD player from time to time in a remote cabin.

Welcome to the Forum Kansas--My first suggestion is to understand/measure your loads. Solar power is, relatively, expensive. It is easy to over design a system and over pay (and extra maintenance costs as your battery bank is over sized and costs you more money to replace after X years, etc.).

So--Along those lines, getting a meter or three to measure your loads is a good start. Your RM-1 meter may give you enough information--But these are also very handing:

Kill-a-Watt type meter (good for 120 VAC; world versions are good for 230 VAC)
DC Amp*Hour/Watt*Hour meters
AC/DC Current Clamp DMM (digital multi-meter).

Straight Ampere (and Watt) meters are OK--But getting meters that totalize the usage over 24 hours (or longer) is really nice for cycling/variable loads (refrigerators that may run 20 minutes out of every 60 minutes, etc.)... You plug in a WH or AH meter for 24 hours--And you know your daily loads.

I have two 140 watt Kyocera panels, wired in parallel; two 6 volt US batteries, wired in series; a ProStar 30 charge controller and a SureSine 300 inverter, both by MorningStar. I also have a MorningStar RM-1 remote meter.

I like to design a "balanced" system. The loads drive the battery bank, and the battery bank+loads drive the charging requirements (solar panels, available sun, backup genset, etc.).

So making a guess that these are ~220 AH @ 6 volt batteries, you have a 220 AH @ 12 volt battery bank. The recommended rate of charge is around 5% to 13% or so with solar power:

220 AH * 5% = 11 amps minimum
220 AH * 10% = 22 amps nominal
220 AH * 13% = 29 amps "cost effective maximum"

Your two standard Kyocera panels (good panels, good brand) will give you roughly a typical peak current of:

2*140Watt panels * 0.77 panel+controller derating * 1/14.5 volts charging = 14.9 Amps typical max charging current.

That falls between 5% to 10% rate of charge--Not a lot of charging current, but enough to keep the batteries happy with weekend/seasonal usage (and keep an eye on the battery state of charge). You could put 2 more panels (4 total) and have very nicely paneled system (probably would need a larger Amp rated charge controller).

Depending on when you visit the cabin and where it is located, typically you would see between 4-6 hours of sun (4 hours early spring/late fall; possibly 5-6 hours of sun in summer). In winter, it can be a lot less (again depending on location, weather, shading, etc.).

4 hour of sun * 2*140 Watt panels * 0.52 end to end AC system eff = 582 WH per day typically sunny day minimum available power.

If your laptop takes 30 watts, you could run:

582 Watt*Hours / 30 Watts laptop = 19.4 hours a day or better for 9 months of the year (guessing)

The meter should provide the current voltage, as well as the "maximum" and "minimum" voltage. The current voltage varies between 12.62 and 12.43 volts.

Lead Acid batteries are not "volt meter" friendly. Their voltage will vary with state of charge, temperature, and amount of current entering/leaving the battery. See this post for details:

http://forum.solar-electric.com/showthread.php?5556-Working-Thread-for-Solar-Beginner-Post-FAQ&p=125909#post125909

Basically, a (3 hour minimum, ~77F temperature) resting lead acid storage battery should be (very roughly) be around 12.3 to 12.7 volts. Below 12.3 volts, the battery is less than ~50% state of charge and should be quickly recharged >80% SOC. At 12.7 and above (again zero current flow), rest battery will be fully charged.

When charging the battery, if typical lead acid, you should see ~14.4 to 14.7 volts for 2-6 hours (absorb), then fall back to around 13.6 to 13.8 volts "float" or maintenance charging (in full sun).

If the battery is well discharged, it will be below ~14.5 volts for several hours while the charge is putting full amperage into the battery bank (Bulk charging stage). Absorb is a fixed voltage that is head for 2-4+ hours as the battery finishes charging.

The only load has been the RM=1, the inverter, and a 9.5 watt LED bulb run for just a few minutes a couple of times. The weather has been sunny.

The inverter--That can draw ~6 watts just turned on and nothing plugged in, or >300-600 Watts when running a heavy load/starting a pump/etc.

300 Watts * 1/0.85 inverter eff * 1/12 volt battery = 29.4 amps--A pretty healthy load on your 12 VDC battery bank.

I tested the PV panels before installing and each showed 17 + volts in full sun. Everything powered up with lights blinking as per manual instructions.

Usually in full sun, the Kyocera panels should be near 20-21 volts or so with no load (Voc voltage open circuit). If they are very hot, Voc will be depressed (Voc from the Mfg. is spec'ed at ~75F degree panels, in full sun they get much hotter).

The RM-1 meter displays a maximum voltage of 12.70, and a minimum voltage of 0.87. The meter will not clear these numbers when I follow the procedure the manual says should clear them. Any ideas why these are the meter readings? Shouldn't the maximum be higher than 12.70? And what could 0.87 mean as a minimum?

I really do not know much about the controller. Hopefully somebody here can help you with reseting the max/min voltages. Most controllers will not work below ~10 volts--So 0.87 could be just disconnecting the controller from the battery bank as the controller has just enough power to log the voltage before its brains "go away".

12.7 volts--Yes, you are correct, at some point with full sun and charging system working as expected (and no or light DC/AC loads), you should see around 13.6 to 14.7 volts or so. If you are not seeing >14.0 volts, the system is not charging correctly (if the batteries are not fully charged).

The panels are pole mounted about 20 feet from the charge controller wired with #6 AWG building wire. Any ideas?

DC Current Clamp DMM (above). Very quick way for you to measure DC (and AC) current flow anywhere in the system. A great way to understand and debug your system. Not needed--But usually well worth the $60 or so price.

I guess I should check the battery state of charge at the batteries with my multimeter.

And get a glass hydrometer--That is the gold standard for measuring a flooded cell battery state of charge. If you want a really nice hydrometer, look at the Hydrovolt from Midnite (meter is out of Europe).

http://www.midnitesolar.com/productPhoto.php?product_ID=505&productCatName=Battery%20Accessories&productCat_ID=39&sortOrder=1&act=pc

Questions?

-Bill

PS: don't worry about my re-formatting--Just trying to help.

Re: Maximum and minimum battery voltage

kansas wrote: »

BB -

You can call me "Bill" (really is my name).

I put some effort and research into designing my system and think you've identified its strengths and weaknesses pretty well. Given the expense, I tried to design what I would call a "right sized" system with components adequate for their job, but no more than that. The fun of our cabin has been in making do and electricity will be a real luxury after years of reading by lamp light. Key to making the system work will be living within what's available; using the electricity intelligently. More panels would be better, but would sit idle most of the time. However, my daughter will be camped out in the cabin this winter trying to get some writing done, which has been the impetus to put in some sort of solar power, as I had planned from the beginning. I wired the cabin for AC when I built it, so an inverter was easy to justify. That, and the fact that powering an Apple laptop directly from DC is apparently unthinkable (you must be an Apple devotee to understand) made the inverter the only real option.

You did a great job of balancing the design for a weekend/seasonal cabin. Personally, with the price and ease of killing batteries--I tend to go with the idea of more panels/less batteries--And make up with genset (small Honda or Yamaha inverter-generator type--quiet and fuel efficient--if needed).

Using PV Watts for Topeka KS, fixed array, 40 degrees from horizontal:

Month    Solar Radiation (kWh/m 2/day)
1      4.00     
2      4.47     
3      5.07     
4      5.21     
5      5.47     
6      5.79     
7      5.80     
8      5.91     
9      5.46     
10      4.78     
11      3.96     
12      3.40     
Year      4.95

Assuming that Topeka is close/similar weather & sun--Still have pretty good sun for winter:

2*140 Watt panels * 0.52 system eff * 3.4 hours of sun = 495 WH per day average December day

The folks at Morningstar responded promptly to my question about their remote meter and it appears it may have a flaw...

I hope they do right by you.

At some point, I would suggest you look at a Trimetric or similar Battery Monitor (Victron is another good brand). Great for family/guests/kids that you don't really want checking the battery specific gravity (spouse factor, acid, mess, etc.). Not perfect--But usually good enough to help prevent killing the battery bank. Battery monitors are not cheap and hard to justify for smaller battery banks. But for larger battery banks, if it saves you from an "ah heck" situation--They can pay from themselves.

At least, start with a smaller/cheaper set of batteries until you work out the bugs and get experience for the next couple years--Then you can size/spec. your next bank for your known needs.

I would like to be able to monitor the charging current being delivered by the PV panels, but that option isn't available from MorningStar except for their MPPT controllers. The ProStar 30 controller has an equalizing charge every 25 days that I would also like to monitor. They have a 30 amp controller that has a built in monitor, but my controller, etc. and under the cabin in the "stoop" space, so a basic remote meter seemed useful. Any thoughts on how to expand my ability to keep track of the system remotely or otherwise?

Remotely, in the cabin or over the Internet? A Battery Monitor will tell you a lot. If you grow the system, there are quite a few MPPT charge controllers that can give you a lot more & remote data. Of course, that is several times the price a a PWM controller, and if you want Internet--That is more power too.

And, with a remote seasonal cabin--You may not want to put too much money onsite with lack of security... It is a tough balancing act.

I'm anxious to see how the system performs this winter. All it has to do is keep the 60 watt laptop charged for 8 hrs/ day, 5 days a week; and light a couple of 10 watt LED bulbs 4 or 5 hours a day if the sun shines. If it doesn't, the oil lamps are ready to go.

Probably more panels if you want to meet the above... For the computer, plug it into a Kill-a-Watt type meter. Many times, the computer will average 20-30 Watts when running--Pulling 60+ Watts is usually only during recharging (and/or a gaming/high power processor+memory+etc.). If writing, crank the computer down to high efficiency mode--And you might get down to 5 Watt range on a late model Macbook Air or similar.

Remember about 20% of the losses in the 52% end to end AC efficiency is based on recharging the main battery bank. If the computer is used mostly during the day, you will save some energy (0.52/0.80=) 0.65 end to end "typical" daytime efficiency (powering computer from array directly vs stored energy in battery bank at night).

On the expense issue, I will say this. When we started to use the cabin a lot I bought a propane refrigerator, which was not cheap. And though it doesn't use much propane on a daily basis, the cost adds up to a couple hundred bucks over the course of a year with the frig on all the time. If I had the choice to make now, I would have added more solar, bought a Frostfire frig, and avoided the continuing cost of propane. But I need propane for the gas range anyway, I guess.

Yep--It is a close call... Build out a significantly larger off grid power system (larger panels, batteries, AC inverter--If DC fridge, the inverter and battery bank can be smaller, but DC refrigerators are usually "scary" more costly). Propane, if easy to get to your cabin, is not a bad choice.

NEW QUESTION: Any ideas on powering the Apple laptop directly from the DC load center on the charge controller.

I am perfectly OK with running the laptop from an AC inverter--Personally, I usually prefer the AC inverter to other devices. Running 12 VDC around a cabin and to "12 VDC car adapters" is not easy. Lots of copper (12 VDC is 10x the current of a 120 VAC cable), and the wide range of 12 VDC (~10.5 to 15+ VDC) can kill many "car type" power adapters. You lose 15% through the AC inverter--Just need 15% more panels and batteries.

One issue with your battery bank--Ideally we are looking for 5% to 13% rate of charge into the battery bank... If you have day time loads, they can draw enough power to keep you from hitting 5% rate of charge minimum on the battery bank.

Adding a panel or two will let you run loads during the day and still ensure that the battery is pretty much getting its required charging power (10% rate of solar charging or better--Usually you don't have to worry about random daytime loads).

-Bill