Inverter Battery Voltage

Discussion in 'The Projects Forum' started by jj_alukkas, Jun 28, 2011.

  1. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Hello,

    I have a 700va Inverter which I assembled 2 and 1/2 yrs back along with a 12v semi-Tubular non-sealed Lead Acid battery which is now 2 yrs and 4 months old. The Inverter uses a readymade pcb which along with the other components I just assembled into the cabin. It works fine but I doubt If I have a problem with my battery. The board came preset with a float charge cut-off voltage of 13.4 which worked fine till 2 weeks back but now, the inverter has started to float pulse more often like in 15secs or so. Previously it was a 2 sec long pulse in some 10-15mts. We took the battery to the dealer but he isnt that expert in servicing and said the battery is just fine and did a steady charge and gave it back to us. It still behaves the same way even after that. So the only problem now is that our electricity bill has went up quite a bit last month due to this useless charging. So now I have reduced the float voltage to 13.1 which gives around 5 mts for the charger to pulse again before the battery voltage drops below 13.1 again, but still Im confused. Is this operation normal for this battery? or do I need to adjust the voltage lower? Is my battery's life about to get over? Since its a semi-tubular, my dealer told it should last between 3-3 1/2 yrs.. now its just 2yrs 4 months. The battery electrolyte is fine, and even the float indicator shows green. I puzzled of why its float voltage went down after running for over 2 yrs. The inverter is never heavily discharged as it has a low cut off at 11v and we never needed to run it to run till it started beeping for low voltage as we have power cuts 2-3 times in a week for abt 1hrs for maintenance, so it is cycled properly for charge/discharge. Any help?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It sounds to me as if your battery is developing a short between one or more of its' cells. This can happen as a battery ages and the cells disintegrate. If the battery has a shorted cell, the float charger will overcharge any remaining good cells while the bad cell will have its' own charge drained off. Once a battery cell begins shorting out, there is little that you can do to save the battery; it will be only a short time before it is useless.

    Try operating the inverter for 5-10 minutes with a load on it; perhaps a few hundred Watts. Then measure the battery voltage. If it is still above 12.5v, it might be OK for a while. If it has dropped below 11v, you have a shorted cell.

    The float voltage really should be adjusted according to the battery internal temperature. The float voltage should be ~13.6v @ 25°C/77°F, with an offset of 18mV/1°C deviation from 25°C; the hotter the battery internal temperature, the lower the float voltage. You measure the battery internal temperature by the positive terminal. For example, if the battery internal temperature measured 30°C, you would calculate 13.6+(25°C - 30°C)*18mV = 13.6+(-5)*0.018 = 13.6-0.09 = 13.51v.

    You need to examine the manufacturer's datasheet for the particular battery that you are using in order to determine the exact float voltage to use, as it may vary a bit between manufacturers, and even different models of a given manufacturer.

    Your low-voltage cut-off is far too low at 11v. A lead-acid battery is considered to be 100% discharged when its' voltage drops to 11.4v. The service life of a battery will be reduced by 2/3 to 3/4 if you discharge it that low.

    To obtain better service life, you should not discharge the battery below 12v. It is best if you do not discharge the battery or batteries below 70% of its' capacity.

    So, let's look at your inverter power requirements. Let's estimate that your inverter is 80% efficient; so 700VA/80% = 875VA. 875VA will require ~73A at 12v. In order to not discharge the battery bank below 70% for 1 hour, you would need a deep-cycle battery or batteries wired in parallel rated for 73A/(1-0.7) ~= 243 AH or greater.

    I have a feeling that you may be underestimating the length of time that the power is off, as well as how deeply you have been discharging your battery.

    Note also that storing power in batteries is necessarily a "lossy" process. For every 1AH of power you put in, you only get about 0.75AH back. Then you also have the loss of efficiency in the inverter itself.

    If you decide to use multiple batteries in parallel, then start out with both/all of the batteries being purchased at the same time. Don't try to parallel an old battery with a fresh one.
     
    russpatterson likes this.
  3. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    The battery is from one of the most reputable manufacturers in our country, so I hope It doesn't have a cell shorted. I will check the loaded voltage tomorrow and reply. The only voltage details in the manual given are to set the Inverter preset's low cut off to 11.0V and peak voltage cut of at 13.8V. Nothing else has been indicated. We dont discharge the batteries much as we operate only 3x20W tubes, a 10W Aquarium airator and maximum of a 75W fan, but that too not regularly for maybe half an hour. So we never had the need to discharge it much. The batteries we have is an 88Ah deep cycle semi-tubular battery, a single one. The rated life is 3 1/2- 4 yrs as by the dealer. So if it runs for a year more, it will be completing its life cycle. Now have to monitor the voltages on load and check as per your point. This is the battery : [​IMG]
     
    Last edited: Jun 28, 2011
  4. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Beautiful and practical calculation! I needed the second part badly as I knew only the first and considered a 100% discharge on the battery part. So the point is liked is discharging a battery to only 70%!! Thanks a lot for this explanation!!
     
  5. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    I observe a steady 12.22V when the battery is loaded with around 200W on the inverter. Voltage stabilized at this point, so I guess we dont have a shorted cell. Any idea about now what could be the problem?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Lets run through your numbers:
    3x20w=60w+10w+75w=145w, inverter efficiency guesstimate ~=80% =~181W load on the primary side, or 181/~12v = 15.1A current.
    To operate that load for 1 hour down to 70% charge remaining would require a 15.1A/(1-0.7) = 50.33AH battery. Since your battery is rated for 88AH, operating that load for 1 hour should leave it at ~88.3% charged; and at that small amount of discharge you should receive the maximum life from your battery, depending on core temperature.

    The higher the battery core temperature, the shorter the battery life. Keeping your battery core temp at 25°C is the best trade-off between available battery power and service life. The hotter the battery is, the more chemical reaction there is within the battery, which shortens the battery life.
    Have a read through this document:
    http://www.discover-energy.com/files/shared/Discover_temperature_effects_charging.pdf
    I do not know for certain what your average temperature might be where you live (eta: from your float voltage of 13.4v, I'll have to guess somewhere around 35°C-40°C), but as you can see in the chart in the document I linked to, if the average core temperature is 40°C, the battery life will be reduced by 50%; at 35°C, reduced by ~25%-30%.

    For your 200W load - so if your inverter is ~80% efficient, that's a 200W/.8=250W load on the battery, or 250w/12.2v=20.5A.

    Your battery might be getting sulfated, even though it has been maintained at a fairly decent float voltage - around 13.4v - it sounds like the battery hasn't been given periodic equalization charges to stir the electrolyte and remove sulfation.

    As a batterys' plates become sulfated, it reduces the ability of the battery to store and release a charge; eventually becoming so bad that the battery won't function at all. Plate sulfation begins when the battery voltage drops below ~12.5v, or ~2.083v in any particular cell.

    An equalization charge is a deliberate overcharge for a brief period of time; perhaps 10-20 minutes. There is some gassing in the cells, which is one reason you don't want the equalization charge to go on for very long; the gassing is hydrogen and oxygen gases.

    One of the best ways to check the condition of the battery is to measure the specific gravity of each cell using an accurate hydrometer, after the battery has been charged completely. You should measure/record the specific gravity of each cell weekly for a time, to see if the battery condition is changing. Make certain that the electrolyte covers the plates in each cell, but not much more than that. If you fill the electrolyte to the top, the electrolyte will be an overly dilute solution.

    If the specific gravity is low, the cells with the low specific gravity may be sulfated.

    You should try to find a datasheet for your exact battery model. You have shown an image of your battery, but after a brief search I was not able to locate a datasheet with the information that you provided. The tubular construction is different from the standard batteries, and the recommended charge current/float voltage may also be different.

    Also, I don't know if your battery was marked with a manufacture date code. In the USA, batteries are stamped with a date code, either on the battery's case or an attached label. The vital information is usually in the first two characters—a letter and a digit. Most codes start with the letter indicating the month: A for January, B for February, and so on. The digit denotes the year: 0 for 2000, say. For example, B3 stands for February 2003. If your battery was made more than a few months before you put it into service, it may have been sitting idle in a warehouse. If it had already been filled with electrolyte, the battery would have been aging already.
     
    Last edited: Jun 29, 2011
  7. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Thank you for this detailed reply. The exact Battery model is Exide IN 880 Plus Inverter battery. Chances for finding a datasheet is very very less as I havent seen datasheets for this model. Even the manual gives inadequate information.
    After a lot of googling, found out the service tips by the manufacturer for their batteries here. Since its mostly related with specific gravity, it wont help me as I dont have a hydrometer. The battery is stored in a cool well ventilated area and room temperatures are around 25-32 °C. Here batteries are stamped with manufacturing month clearly including the week. The battery was manufactured on the first week of Dec 08 and put to service on Feb 09, so 2 months on the shelf isnt too long I guess considering the fact that when I received it, voltage was over 13.5V. Anyway, my only guess now is that I might have reached the approximate end of life of this battery and so battery is not giving the capacity it used to earlier. Also I doubt sulphation cos I dont happened to have given any equilization charge on a seperate charger other than what the inverter's built in charger does.
     
  8. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Also, everywhere I checked for this manufacturer, the recommended discharge voltage is 80% of its full capacity for the best life. So till what voltage across the battery can I run the inverter, say at 30°C and the same load?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to be able to check your batterys' individual cells' condition, you should purchase a hydrometer.

    According to what you stated previously, your inverter is rated for 700VA. I don't know at what rate it charges your battery, but it should likely be around C/10, or 8.8 Amperes or less. If it exceeds 8.8A, it may stress your battery.

    I don't have a datasheet for your battery. You might contact Exide, giving the exact model of your battery, and ask them where you can find a datasheet. Exact maintenance procedures will vary due to construction and battery chemistry. All I can do from here is to make guesses, and I don't like to do that.

    You also seem to be making guesses, as you appear to be giving me the temperatures of the room where the battery is, but not the temperature of the battery. If a battery gets heated up internally, it takes a very long time for it to cool down. That is why you don't want to charge it very fast, as fast charging causes heat.

    I also mentioned the equalization charge earlier. In the page you linked to, they're saying 2.75v per cell, which would be about 16.5v. They don't mention temperature compensation. 16.5v sounds too high. I would not go above ~15.5v.

    So, get the datasheet from the manufacturer, take accurate temperature readings from the battery positive terminal, take accurate specific gravity readings from each battery cell when the electrolyte level is correct and the battery is charged, and then we will have a place to start from. If you can't supply that information, we will be simply guessing, and guessing will cost you money.
     
  10. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    Thanks a lot for all the help. Anyway I have come up with a few conclusions from your points and from the manufacturer's site.
    # Not to buy a battery which has been on the shelf for long. They say a battery looses 4% of its capacity per week, mine was 2 months!!
    # Do a equalization charge once in a month
    # Try not to undercharge below 80% capacity.

    I will try contacting Exide for the datasheet, and thanks a lot for your help and valuable information which will be useful and can be kept in mind when I buy a new battery. :) Thank you.
     
  11. pistnbroke

    Member

    May 9, 2011
    32
    1
    your charge voltage is too low ..you need to get it up to 13.8 floated ...personally I would charge it to 14,4v ..I suspect you have sulphated it due to not chargind sufficiently
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    This is why I need to see the manufacturer's datasheet and also get the battery core temperature. I don't want to simply guess. The float voltage will change with battery chemistry/construction and temperature.

    The inverter he has most probably does not have an automatic equalization charge cycle built into it.

    It appears to be a single-stage type of charger; it merely charges to the preset float voltage at a given current, then turns off. Certainly not an ideal charger.
     
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