confused??

farmgirl
farmgirl Registered Users Posts: 5
I am 2 months into living with my off grid solar system and I am quickly finding out how much I dont know about it.
I have 6 180W solar panels and 8 6v AGM batteries. I am using about 1500watts +/- of electricity a day. I cant seem to get the charge on my batteries to hold for long. On a good sunny day it may reach 52V, but usually drops down to 47ish at night, and on the cloudy days I often go in the red! I am not sure what I should do..do the batteries need to be charged up more and can I use a regular 6/12 volt battery charger to do that?

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: confused??

    What you really need to know is the Amp/hour capacity of the batteries. Roughly speaking, 1500 Watts on a 48 Volt system would be only 32 Amp/hrs - not hard to come up with. And your panels should be capable of keeping a 100 Amp/hr battery bank charged.

    The other questions would be: how do you know you're using 1500 Watt/hrs per day, and are you getting enough sun to fully charge the batteries?

    I'm sure others will have some observations/questions as well.:D
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    Re: confused??

    You need to reduce your loads on cloudy days. That's it. That's what I do. Having a lot of battery capacity helps as well. I have enough to power my loads for 66 hours until fully dead. This way I don't have to worry about going in the red all the time. I just check the log for lowest voltage before going to bed to know where my system is. There is also great tools to help visualize battery capacity in percents. Like this one: http://store.solar-electric.com/xalidubamosy.html

    52V is not full charge, 57.6V would be. But even then, you need to keep batteries at that voltage until amperage draw drops significantly. Then they are fully charged. You have to allow your batteries to reach full charge at least once a week. If you don't they will die sooner. How soon, maybe someone else could comment?
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
    Re: confused??

    You also need to watch out for phantom loads. Do you leave your inverter on all the time? I only turn mine on when I need it. I turn it off when I leave for as little as 2 hours and at night. Do your loads draw power when not in use?
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: confused??
    farmgirl wrote: »
    I am 2 months into living with my off grid solar system and I am quickly finding out how much I dont know about it.
    I have 6 180W solar panels and 8 6v AGM batteries. I am using about 1500watts +/- of electricity a day. I cant seem to get the charge on my batteries to hold for long. On a good sunny day it may reach 52V, but usually drops down to 47ish at night, and on the cloudy days I often go in the red! I am not sure what I should do..do the batteries need to be charged up more and can I use a regular 6/12 volt battery charger to do that?

    You did not mention the A-Hrs of your batteries. Also, I hope you mean 1500 watt-hrs per day. You should be making between 1500 watt-hrs to 3000 watt-hrs per day on average from you panels depending on location. You should have enough battery capacity for a couple of days of reserve. (save for a rainy day )

    An inverter on all the time could consume 500-750 watt-hrs a day even with no output load.
  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    Re: confused??

    Lots of info missing in your post, for example:

    1. Which panels do you have and how are they connected?
    2. What charge controller?
    3. What is the tilt on the panels?
    4. Which inverter do you have?

    I am assuming that you have the 6 volt GC batteries, so around 220 AH.

    If you are only using 1500 WH, then you have some real problems, as you should be getting over 4000 per day even this time of year.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: confused??
    farmgirl wrote: »
    I have 6 180W solar panels and 8 6v AGM batteries. I am using about 1500watts +/- of electricity a day.

    Using the PV Watts Website, 1,080 Watts (1.08 kW for program entry) of solar panels for La Crosse WI. Assume derating factor of 0.59 (AGM batteries + AC Inverter):
    "Station Identification"
    "City:","La_Crosse"
    "State:","Wisconsin"
    "Lat (deg N):", 43.87
    "Long (deg W):", 91.25
    "Elev (m): ", 205
    "PV System Specifications"
    "DC Rating:"," 1.1 kW"
    "DC to AC Derate Factor:"," 0.590"
    "AC Rating:"," 0.6 kW"
    "Array Type: Fixed Tilt"
    "Array Tilt:"," 43.9"
    "Array Azimuth:","180.0"

    "Energy Specifications"
    "Cost of Electricity:"," 9.1 cents/kWh"

    "Results"
    "Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
    1, 3.93, 83, 7.55
    2, 4.49, 81, 7.37
    3, 4.99, 99, 9.01
    4, 4.90, 87, 7.92
    5, 5.58, 100, 9.10
    6, 5.54, 93, 8.46
    7, 5.91, 102, 9.28
    8, 5.36, 93, 8.46
    9, 4.59, 80, 7.28
    10, 4.45, 84, 7.64
    11, 2.80, 53, 4.82
    12, 2.32, 47, 4.28
    "Year", 4.57, 1001, 91.09

    For November, you would be producing ~53 kWatt*Hours per Month of 120 VAC, or:

    53kWHper mnth / 30days per mnth = 1.767 kWhrs per day = 1,767 Watt*Hours per day

    So, if your using 1,500 Watt*Hours per day--you are running close to 100% of the system's average capacity (20 year average--of sunny and cloudy days). Assuming your hardware/controllers/etc. are setup correctly.

    Note:
    • Watts is the rate--like a 100 watt light bulb.
    • Watt*Hours is how much energy you use--like 100 watt light * 5 hours = 500 Watt*Hours = 0.5 kWH (kWH is the typical power unit charged by a utility.
    A useful tool for the AC side is a Kill-a-Watt meter. Use this meter on your AC loads for your home (usually for 24 hours per appliance) so you can capture average usage (refrigerator cycling, computer use during the day, etc.).

    Your question about can you use a 12 volt battery charger on your bank? Depending your electrical experience, yea--But it can be full of problems... Problems of sparks from clips (lighting off hydrogen gas), connecting incorrectly, interfering with the other chargers, unbalanced charging of cells, etc... The "safe way" would be to disconnect the cells back into 12 volt pairs. A pain and requires care in doing this...

    Since this is your home (I guess) and you will be there pretty much full time. You should get a 48 volt battery charger that matches the requirements of your battery bank and generator capacity. When winter rolls around--you will probably be using the generator fairly often. Matching Batteries to Charger to Generator capacity can help ensure that you don't waste fuel (i.e, huge 20 kW generator running a 1,500 watt battery charger).

    Regarding the details about the battery... Take a look at these battery FAQs:

    Deep Cycle Battery FAQ
    http://www.batteryfaq.org/

    As AntronX suggested, you really should look at a Battery Monitor... AGM/Sealed batteries do not allow you to measure specific gravity... And measuring battery voltage is accurate only after the batteries have rested for ~3 hours or so.

    Estimating battery capacity while under load/charge is difficult.

    Also, AGM's are very sensitive to over charging. If your solar charger supports a Remote Battery Temperature Sensor option--you should get it.

    AGM's typically don't like being charged above ~14.4 volts or so (57.6 volts at 77F). And should probably be charged at least at 56.0 volts--and held there for several hours and only after the charging current falls quite a bit, should the charger begin to ramp down on the voltage.

    Search the Internet for the charging instructions for your brand/model of batteries and make sure you are following their instructions.

    Most people use more power than they first guesstimate... You have your system now--Measure your loads and charging Amp*Hours / kWH. Compare with battery capacity (you generally don't want to cycle the battery bank below 50% all the time--it is hard on them and they will not last as long).

    In the end, you will probably have to cut your loads a bi and/or run your genset more (especially during November and December when solar output is low for your area).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • farmgirl
    farmgirl Registered Users Posts: 5
    Re: confused??

    thanks for all the info folks.
    here is some more details about my system that might be helpful in getting this figured out.
    I have 8 6volt discover AGM batteries that are rated at 312 amps (does that mean that is my total amps with all 8 batteries). I have an outback mx 60 charge controller. I have an outback flex invertor (GTFX ??). I have measured my daily loads with a killowatt meter, and approximate about 1500 watt hours a day. But I didnt calculate what the invertor was using, which one of you mentioned could be as much as 500-700 watts a day! so I will shut that down at night.
    I also just found out that my invertor is for a grid tied system, not an off grid system, which is what I have. so a replacement is on the way and then I will be able to connect my generator to my invertor to charge my batteries.
    In the mean time I have all my power shut down and I am praying for sun!!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: confused??

    Regarding the amp*hour calculations for your batteries... Each battery has a rating (typically C/20 Hour rating).

    Say each 6 volt battery is 100 Amp*Hour.

    If you add two batteries in parallel, the Amp*Hours add... 100+100=200 AH and the voltage remains 6 volts

    If you add two batteries in series, the voltage adds, and the Amp*Hours remain the same. So you have 6+6=12 volts and 100 AH of capacity.

    In your case, it sounds like each battery is rated at 312 Amp*Hours (at 6 volts). And you 8 batteries in series for 8x6v=48 volts and 312 Amp*Hours.

    Note: You are not losing/gaining anything with series/parallel connections... Still the same amount of energy storage in your battery bank. Typically higher voltage battery banks are better for higher powered inverters (keeps current lower in the wiring):

    Power = Voltage * Current

    So if voltage goes from 12 volts to 48 volts, current will be cut to 1/4 amount.

    Yes, the stand by loads of an inverter can add up if left on 24 hours per day 7 days per week.

    Many inverters have a "sleep" mode where they turn on very few seconds and see if there is a load > 8 watts... If it finds one (say you turned on a light), then it turns on for full power. Can be very nice--But you cannot leave loads plugged in (chargers, DVR's, and such that will draw power even when "Off").

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: confused??

    One drawback of Outback inverters is that they can be power hogs. Mine uses 20 watts while doing nothing! Multiply that over 24 hours and you've got 480 Watt/hours under zero load. And it doesn't show up on the Kill-A-Watt. If you're thinking about replacing the inverter, you should consider the Xantrex line. The XW 6048 is a very good deal in terms of $/Watt.

    Also, judging by the info you've given your panels are insufficient to do a good charging of 312 Amp/hrs of battery bank. You definitely should be seeing more than 52 Volts - like 56-58 for Absorb and a Float of around 54-56. Try recharging your bank with a generator and see if you can get the 'resting Voltage' (nothing connected, batteries left alone for at least 3 hours) around 50.

    You'll get differing opinions on proper battery Voltages. Different batteries/systems/conditions all factor in.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: confused??

    cariboocoot,
    actually the rate is just under the 5% mark and would be considered close to the good range, however, i think the problem is the loads are too much for this charge rate as a load on during charging will effectively draw down the charge current to levels that could impede the proper charging of the batteries. if the loads used are to be maintained and if there aren't any problems with the batteries or any other equipment, i would suggest expanding the amount of pv feeding the batteries to keep the charge rate up to snuff. i suspect they are deficit charged. if farmgirl can do without too much power for a few days then allow the batteries the opportunity to recharge and see what the story is then. this means conserve your power by shutting loads off or doing without for some time.