Off grid question

geogrgeogr Registered Users Posts: 6
Hello to everybody,

I would be pleased to have the opinion of all the solar experts there..
We are dealing with an off-grid system which is conist of the following:
6 Shell solar PV 85W-P
1 Steca Tarom 245 charge controller
1 Siemens SP1500/24 inverter
8 Batteries AGM Banner 12V 69Ah
System voltage 24V
The irradiation is about 1,2kWh/kWp (Greece)

The loads are an A enrergy class (85W) fridge, many 11W lamps, and a small TV (vacation house).

The problem with this system is that there is no safety at the batteries against deep discharging except at the inverter side (switch off at <22V for longer than 30mins) which I think is not enough.

After a 1 year of satisfactory operation, the batteries are not operating quite well and the stored energy does not seem to be enough anymore for the above operations.

Do you thing that the use of a (pair) shunt could be a solution? Is there any way to imrove the efficiency of the batteries (someone has suggest me a hard manual boost charge every 6 months)?
Any suggestions are welcomed

Comments

  • mike95490mike95490 Solar Expert Posts: 8,768 ✭✭✭✭✭
    Re: Off grid question

    If your batteries have not been getting fully charged, that could be an answer.

    If AGM batteries are OVER charged, they vent H & O2, and you loose capacity as the fluid level drops. There is usually no way to restore this.

    The Manufacturer of the batteries "Banner" should be able tell you the proper voltage to charge the batteries to. If you have not set your charger properly this last year, you may have learned a very expensive lesson, and may have to buy new batteries.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Off grid question

    Using this site, set for Athens Greece, and setting the derating for 52% (because off-grid system), and I had to set 1kW worth of solar panels (lowest it would allow), then multiply by .510 kW of your system (510 watts of panels), I get:
    Month    Hr/day    kW/month
    1     3.22    24
    2     3.77    26
    3     4.68    35.5
    4     5.35    38
    5     5.84    42
    6     6.74    46
    7     6.97    48.5
    8     6.82    47
    9     6.43    44
    10    4.65    33.5
    11    3.04    21
    12    2.83    20.5
    Year 5.04    426.5
    

    So, you average around 40 kWhr per month 1/2 of the year, and 20 kWhrs per month the other half of the year...

    For the 40kWhr / month, that is about 1.3 kWhrs per day.

    Assuming you have a small but efficient fridge of 0.85 kWhrs per day (a very efficient US fridge is about 1kWhr per day--say your 85 watt fridge runs 10 hours per day in hot weather).

    1.3kWhrs - 0.85kWhrs = 0.45 kWhrs per day left over (or 450 watt hours).

    You add up the 11 watt light bulbs... say 10 of those 4 hours per night, 440 Watt hours... And that is about it...

    At this point, just guessing, much of the year your batteries are never getting fully charged. And during the less sunny months, your loads may be exceeding the panel's capacity by quite a bit.

    So, get a hydrometer and measure the specific gravity of your batteries.

    Get a better idea of how much load you are really using. And, you probably need to add more panels.

    Also, it would be nice to get a small generator and good charger to recharge the batteries when the solar is low. And equalize the batteries once or twice a month.

    If you have been running your batteries below 75% capacity for a long time, or they go below 50% capacity often--they are probably slowly dieing.

    Sorry, got to go now... Will be back later.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Off grid question

    You state that your inverter shuts off when the voltage drops to 22 volts for 30 minutes. Has it done that? Shut down because of voltage that low?
    If so, that is an indication that the batteries were very badly discharged and that will do them great harm. You should aim for not taking more than 20 % out of the batteries, leaving them still holding 80% of their charge, before recharging, which should take place ASAP. The very most that should ever be taken out is 50% and even going that far will shorten the life of the batteries. I've mentioned elsewhere on this form, that the term "Deep Cycle" is a misleading one, in that it leads folk to believe they can stand repeated draining until there's nothing useful left in them. Such deep discharging is considered abuse and will murder the batteries in short order, all the more so if they aren't recharged fully and right away, otherwise sulphation damage takes place, with the results you now have.
    I noticed that "BB" suggested using a hydrometer to check your batteries. He was in a hurry and missed where you said your batteries were AGM. In such batteries you can't use a hydrometer.
    Peace
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Off grid question

    Sorry, missed the AGM part...

    If you can justify the cash, the way to go is using a battery monitor. It is just about the equivalent to having a gas gauge in your car's fuel tank.

    Unlike a voltmeter which is only accurate after the battery has sat for several hours with no charging or load currents, the Battery Monitor can tell you the current state of charge of your battery bank at any time.

    Look at the XBM model as one that as been recommended by several people here as a model with a good range of features.

    As Mike said, you cannot over (or Boost) charge the AGM's... It will just destroy them if you over heat them (or over generate gas) from over charging.

    I am not sure what you mean by a "pair of shunts"... There are several electrical definitions of a shunt. 1) to short out or bypass a circuit path with another wire. 2) would be a large calibrated resistor that is put in the main current path of the battery bank (in this case) and connecting a small amp meter or battery monitor to measure the current flowing into or out of the battery bank.

    In any case, this will not give you any more power or efficiency--it will just tell you more about the state of your batteries.

    And, typically, AGM batteries are around 90% energy efficient when charging/discharging. Flooded (or Wet) cell lead acid batteries are about 80% efficient--so you are about as efficient as you can get there.

    Another place you may be able to increase efficiency a bit--is that a 1,500 watt inverter? That sounds a bit large for your installation--it is possible that if you used a smaller inverter, you may have less losses.

    Also, I did not find any specifications for that inverter... Is that a Pure Sine Wave Inverter or a Modified Square Wave Inverter (sometimes called by the marketing department a "modified sine wave inverter"). The MSW inverters cost much less, but when running with motors, the square wave will cause the motors to use, roughly, 20% more power than they would on a pure sine wave inverter... Also some electronic equipment (TV's, radios, etc.) will have more noise in the video/audio on a MSW inverter.

    Lastly, MSW inverters can cause some things to fail (motors, electronics, appliances, tools) before their time. Problem is that some 90% of devices will work fine and 10% may fail--but it is difficult to know which will work and which will fail.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • geogrgeogr Registered Users Posts: 6
    Re: Off grid question

    Thanks for your answers.

    - When I am saying shunt, I mean this large resistor for the mesurement of the flowing current. For the Steca (Tarom) controller they told me that I have to use it in combination with a data logger.

    - Yes, the inverter has shut down because of low voltage protection. Is there a practical way to increase this down limit (since the manufacturer deliver this preprogrammed) ?

    - About the battery monitor.. If I understand it rigth this can give to the user the real voltage of the battery (and a an estimation of the real capacity). However it does not deliver any automation to the system in protection from deep discharge.

    - Unfortunatelly the manufacturer "Banner" does not provide any datasheet in order to configure the charge controller accordinglly and charge the battery properly. Fot the time we are using the preconfigured voltage values for the equalizing - float charge.

    Taking as a fact that the batteries are already damaged, do you believe that a larger battery (2x the Ah) system could be a solution (keeping the old one as a back up) or is it mandatory to buy more solar power?

    Regards
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off grid question

    in general i'd say another pv or 2 certainly wouldn't hurt and more if you can as there aren't many who can say that they have so much in pvs that they can't use more power. i commend you on your going with agms, but they are sensitive to voltages going high and all batteries are prone to discharges going to low that depletes capacity and battery life. i believe you have a large enough system to justify going with an mppt controller and get the battery temperature sensor too.
    as to the batteries you can wait until they get worse or replace them now as this will depend on how well you think they are doing and if they are meeting or able to meet your needs. if you keep your present batteries for awhile you will need to monitor them closely as they could fail rather quickly one day. if one of those batteries goes then prepare to buy all of them anew. rethink the capacity to be sure that the capacity is meeting your needs as we don't recommend them being more than 50% depleted (dod) or less than 50% full (soc) and views don't depend on optimism here. keep in mind any reworking of battery capacity means a reworking of the pv capacity too. charge rates should not be less than 5% of the rate of battery capacity and upwards around 10% is even better. more than 13%, sure you can as long as you know what the maunufacturer says you can go up to safely and many agms will accept excesses over 13% rates with ease.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Off grid question

    Ask and you shall receive... Two new battery monitor products from Xantrex (not yet listed on Wind-Sun's store):

    The LinkLITE:
    Description
    Defining the amount of energy available in a battery is a complex task since battery age, discharge current and temperature all influence the actual battery capacity. High performance measuring circuits, along with complex software algorithms, are used to exactly determine the remaining battery capacity. A new shunt selection feature enables the LinkLITE to measure currents up to 1,000Amps respectively.

    The LinkLITE selectively displays voltage, charge and discharge current, consumed amphours and remaining battery capacity. Using a clear backlit LCD Display and an intuitive user interface, all parameters can be recalled with just a button press. A second battery input is also provided to monitor voltage on a second battery.

    It is equipped with an internal programmable alarm relay, to run a generator when needed or to turn off devices when the battery voltage exceeds programmable boundaries.
    Performance Features
    • Read your battery bank like a fuel gauge
    • Provides critical information about the status of your battery bank
    • Displays voltage, current, consumed amphours and remaining battery capacity
    • Two battery inputs
    • Auto sensing battery voltage inputs
    • Large backlit LCD Display
    • Quick nut mounting construction
    • Programmable alarm relay
    • Shunt selection capability enables flexible system integration
    • Splash proof frontpanel
    • 500 Amp shunt included
    • CE and e-mark certified

    Interestingly, measuring or estimating the battery voltage is not near as important as measuring/summing/subtracting Current*Time. Batteries care more about the number of "electrons" going in and out than the temperature or voltage at which that happens.

    Getting a Battery Monitor is sort of like comparing watching the speedometer vs just reading the odometer for the number of miles driven.

    You might be able to adjust the low battery limit on the inverter (don't know anything about your inverter)--but, in reality, no matter where you set the limit it will be wrong--and the exact value will vary with load, temperature, and charge levels. The low battery alarm on the inverter is really just to protect the inverter itself (current goes up as voltage goes down--can cook inverter if voltage gets too low with heavy loads) or cause problems with your ac equipment (brownouts and power cycling would kill the fridge).

    A good Battery FAQ from Wind-Sun.

    From Concord AGM battery website:
    Initial charge or recharge: 2.37 to 2.40 volts per cell at 25°C (77°F). Float charge: 2.20 to 2.22 volts per cell at 25°C (77°F). Equalize charge: 2.40 volts per cell at 25°C (77°F). Temperature compensation = ±3.75 mV. per cell per degree C [Reference to 25°C (77°F)]. This is for battery temperature (not ambient temperature) and is useful for battery temperatures from 0°C (32°F) to 40°C (104°F).

    And this gets back into the discussion that if your solar battery charge controller has a Battery Temperature Sensor (BTS) option--you should get it... Many charge controllers have an internal sensor--but the charge controller may run hotter than the batteries themselves (internal waste heat)--so many times, charging without a BTS will leave the batteries under charged.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Off grid question

    It's my opinion, that before you replace your batteries, you should do one or both of the following, or you'll run into the same problems with the new batteries.
    a) At a minimum, double your PV's. That's a minimum.
    b) Reduce your consumption.
    The PV you have now is not able to replace the power that has been used and still properly recharge the batteries
    Good luck
    Wayne
  • mike95490mike95490 Solar Expert Posts: 8,768 ✭✭✭✭✭
    Re: Off grid question
    geogr wrote: »
    - Unfortunatelly the manufacturer "Banner" does not provide any datasheet in order to configure the charge controller accordinglly and charge the battery properly. Fot the time we are using the preconfigured voltage values for the equalizing - float charge.

    My guess is you have chronically undercharged the batteries, and on occasions, when there was enough power, and an equalize cycle started, you overcharged them.
    You should use the pre-configured values for AGM batteries, which don't get a monthly equalize, only an annual one.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • WindsunWindsun Solar Expert Posts: 1,164 ✭✭
    Re: Off grid question
    BB. wrote: »
    Ask and you shall receive... Two new battery monitor products from Xantrex (not yet listed on Wind-Sun's store):

    -Bill

    We don't usually list stuff on the store until we actually have it in stock, and right now it looks like lead times on these are 4-8 weeks.


    Odd that the Banner batteries do not have any specs. Not sure if the Tarom controller has an AGM setting or not (too late at night and I am lazy).
  • geogrgeogr Registered Users Posts: 6
    Re: Off grid question

    If I am not mistaken the equal charging happens every time the battery is getting to be fully charge and it is the 3rd and last phase of charging. This procedure is handled by the controller automatically. The truth is that until recently the Tarom was configured to regulate the charging by the SOC value. This off course was wrong because there were not any devices (shunts) to measure the exact current running out and in the battery. Now it is programmed to run according the to the voltage. Regarding the Tarom there is no special parameter related with agm batteries. You can only choose the type of electrolyte (fixed/liguid) and as so i guess the regulation is the same for the flooded and agm ones.
  • BB.BB. Super Moderators, Administrators Posts: 30,522 admin
    Re: Off grid question

    "Equalization" is where each cell in series is slightly overcharged--because one (or more) of the series cells only reaches 98% charge, instead of 100%... If this goes on long enough, the one cell that requires more charging will eventually fail from being undercharged. So, charging the other cells to a bit over 100% once in a while will also bring up the 98% cell to 100%.

    Another reason for equalization is to mix the acid--it can stratify--where the heavy acid (produced during charging) sinks to the bottom of the cell and the light battery fluid (mostly water) floats to the top. Equalization, properly done, will produce enough bubbles to mix the acid throughout the battery. This is especially important for "tall" batteries.

    For AGM and Gel cell batteries, they don't have liquid electrolite--so there is no reason to mix. Also, AGM (at least) only requires light equalization once a year or so...

    If you equalize an AGM battery just like a flooded cell, it will generate Hydrogen and Oxygen gases--however, the battery will not be able to recombine them back to water fast enough--plus the excessive charging current and recombination will generate enough heat that the pressure seals will let out the steam--quickly drying out the battery and causing failure.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off grid question

    eqing may be automatic on a few controllers, but is probably once a month or something you or the factory programs into it. others may have a manual eq, but most do not have this feature at all. this may be considered a 4th stage of charging by some as the first 3 are bulk, absorb, and float. as i stated, agms are voltage sensitive. an eq charge to an agm such as a concorde would just be an extended absorb charge with the voltage limited to absorb voltage levels for a few hours. eq charges to some standard lead acid batteries would usually see a higher than normal voltage for a few hours and may not be recommended to be done on an agm like those from concorde. manufacturers may vary on this parameter though and allow it.
    i believe your controller will up the voltage to higher levels designed for standard lead acid batteries and i don't recommend you do that on agms. if you are eqing after each charge you are destroying those batteries quickly. inequities can be sometimes addressed by physically rearanging the batteries and be sure that all connections and cables are good otherwise individually charging the batteries may be the only solution if they are far enough out of eq. voltages of the batteries after several hours of no charging or loads will show if there are differences.
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Off grid question

    geogr,

    I think you might be looking for a way to automate your system in order to keep the batteries from over-discharging. You can do that easily with a voltage-controlled-switch. The vcs must have an upper, and a lower setpoint. You would use the output of the vcs to switch the load on and off, based on battery voltage. I use a Morningstar Relay Driver, which is actually 4 vcs's to do just that on my system.

    This is how I've set it up on my 12-volt system:

    Program the vcs to switch the inverter on when battery voltage reaches 13.85 volts.

    The inverter output is applied to an Automatic Transfer Switch, which uses inverter voltage as the default. Loads are powered by the PV system once battery voltage reaches the upper threshold, 13.85 volts.

    Program the vcs to switch the inverter off when battery voltage drops to 12.25 volts. Under a relatively heavy load, this represents about a 80% depth of charge.

    When the inverter switches off, the Automatic Transfer Switch switches the loads to grid-supplied power instead of power from the inverter.

    With this simple automation, my battery voltage never drops below 12.25 volts, and my loads always have power. I've been using my refrigerator and freezer as the loads for the past few months, and so far I've had no spoiled food.

    The vcs also has a programmable delay. I've set the delay for 10 seconds in order to compensate for a motor-starting power surge that might otherwise trigger the vcs prematurely.

    I have more detailed information about this on my blog: http://solarjohn.blogspot.com

    John
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Off grid question

    you will find that the voltage controlled switch is also refered to as an lvd or low voltage disconnect for your search purposes.
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Off grid question
    niel wrote: »
    you will find that the voltage controlled switch is also refered to as an lvd or low voltage disconnect for your search purposes.

    It is important to have the capability to set both the high voltage threshold and the low voltage threshold. With only a low voltage disconnect, you'll have to reconnect the loads manually. To automate your system you'll need both, and they need to be set independently of each other.

    Consider this:

    Let's say that you set the low voltage (disconnect) threshold at 12.25 volts. A load will cause battery voltage to decline. When battery voltage drops to 12.25 volts, the load will be disconnected. Because the batteries now have no load, battery voltage will begin to rise. If the load is allowed to reconnect too soon, you'll have a situation where the load is connected and disconnected at a rapid pace (oscillation). You need to set a reconnect threshold that is considerably higher than the disconnect threshold.

    The high voltage (connect) threshold not only prevents oscillation, it should be set high enough to allow the batteries to recharge, avoiding a chronic under-charging situation.

    John
  • geogrgeogr Registered Users Posts: 6
    Re: Off grid question
    SolarJohn wrote: »
    geogr,

    I think you might be looking for a way to automate your system in order to keep the batteries from over-discharging. You can do that easily with a voltage-controlled-switch. The vcs must have an upper, and a lower setpoint. You would use the output of the vcs to switch the load on and off, based on battery voltage. I use a Morningstar Relay Driver, which is actually 4 vcs's to do just that on my system.

    This is how I've set it up on my 12-volt system:

    Program the vcs to switch the inverter on when battery voltage reaches 13.85 volts.

    The inverter output is applied to an Automatic Transfer Switch, which uses inverter voltage as the default. Loads are powered by the PV system once battery voltage reaches the upper threshold, 13.85 volts.

    Program the vcs to switch the inverter off when battery voltage drops to 12.25 volts. Under a relatively heavy load, this represents about a 80% depth of charge.

    When the inverter switches off, the Automatic Transfer Switch switches the loads to grid-supplied power instead of power from the inverter.

    With this simple automation, my battery voltage never drops below 12.25 volts, and my loads always have power. I've been using my refrigerator and freezer as the loads for the past few months, and so far I've had no spoiled food.

    The vcs also has a programmable delay. I've set the delay for 10 seconds in order to compensate for a motor-starting power surge that might otherwise trigger the vcs prematurely.

    I have more detailed information about this on my blog: http://solarjohn.blogspot.com

    John

    Solar John,

    Thanks a lot for your answer. The configuration you describe is exactlly what i was looking for! The only thing now is to adapt this mornigstar (or a similar one if there is at the market) device to my system. I have to start reading it's manual.
    Just something more...
    Do you apply the relay cut off at the inverters power or it is applied directly to the loads?
    And, about the excess current of PV when battery is overcharged.
    Did you reffered that you send it directlly to the inverter? Is this always possible or there are any restrictions on that (through charger, inverter input voltage range)?

    Thanks for your help.
  • SolarJohnSolarJohn Solar Expert Posts: 202 ✭✭✭✭✭
    Re: Off grid question
    geogr wrote: »
    Solar John,

    Thanks a lot for your answer. The configuration you describe is exactlly what i was looking for! The only thing now is to adapt this mornigstar (or a similar one if there is at the market) device to my system. I have to start reading it's manual.
    Just something more...
    Do you apply the relay cut off at the inverters power or it is applied directly to the loads?
    And, about the excess current of PV when battery is overcharged.
    Did you reffered that you send it directlly to the inverter? Is this always possible or there are any restrictions on that (through charger, inverter input voltage range)?

    Thanks for your help.

    I'll try to answer your questions:

    My inverter has "Remote On/Off" switching capabilities. In other words; I can wire in a switch for on/off control, that is a low voltage - low current circuit. You can look at the Exeltech user's manual for more information.

    I'm always looking for ways to improve the efficiency of my system. When the batteries become fully charged, the charge controller limits further charging to protect the batteries. There is no harm in this, but unused energy from the sun is wasted. I try to use the excess energy to charge my spare battery bank, or to power a load, but I haven't automated that process yet. I might be able to automate the process by using a Diversion Load Controller, or by using an unused channel in my Morningstar Relay Driver to switch on/off a load.

    One way to learn more about the Relay Driver is to download the software that controlls it. Even though you don't yet have the Relay Driver, you can see how it operates by trying the software. Check the Morningstar website for download instructions. I believe it is: www.morningstarcorp.com

    John
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