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float charge?

Discussion in 'Photovoltaics' started by bcps, Feb 8, 2007.

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  1. bcps

    bcps Guest

    As I mentioned in an earlier post, I'm charging a couple of 6 volt Exide
    3600 batteries in series. Even on overcast days, my batteries seem to hit
    float by 10 or 11am. This is a real bother for me because I know I'm not
    making enough use of the solar panels.

    As I said in that previous post, I was considering getting two additional
    Exide 3600 batteries to increase my storage capacity for times when it is
    overcast for several days and to take advantage of the power I'm certain I'm
    losing.

    There is another issue. The MPPT250 I am using is set to 14.1 volts float.
    Yet I see a lot of people reporting their batteries dancing around 15.2
    volts. The charger bounces on and off from 10am all the way to 5:15pm!
    When the charger is off, I only see 13.4 volts on the battery. I don't
    think this is efficient use of the batteries or the panels. Should I crank
    up the float voltage to 15v and maybe get a little more load on them to
    cycle them properly?

    Bart
     
  2. Steve Spence

    Steve Spence Guest

    The type of battery determines the charge voltage. Lead acid needs 14.4
    vdc and float at 13.6.
     
  3. bcps

    bcps Guest

    I thought that was bad for the batteries...
     
  4. George Ghio

    George Ghio Guest

    From Battery Energy's Manual.


    1. Float Charge When cells are operated with no load or light loads
    (less than 5% daily depth of discharge) the best method of charge is
    constant voltage float charge. The levels that the regulator float
    voltage should be set to are dependent upon average temperature and are
    shown below. At this temperature Voltage Setting Should Be 15oC 2.24V
    20oC 2.23V 25oC 2.22V 30oC 2.21V 35oC 2.20V Consult with
    Battery Energy for temperatures outside this range. If the average
    electrolyte specific gravity is slowly falling, then the float voltage
    should be revised upwards under advice from your installer or Battery
    Energy.

    2. Recharge Following Discharge and equalisation or refresher charge
    To bring the battery up to the full 100% charged state following a deep
    discharge either a constant current or a constant voltage charge can be
    given. Batteries are not 100% efficient and therefore they require
    approximately 15% more energy to be put back in the battery than was
    taken out on discharge. When giving an equalisation charge, this needs
    to be increased to 30-40% to allow for cell differences and possible
    stratification of the electrolyte.

    3. Constant Current If charging at constant current is employed, then
    the maximum rate at the start of charge is about the C/10 (where C is
    the 10H rate capacity). Once the gassing point is reached, this should
    be reduced to C/14. Gassing at high currents will cause the cell to
    heat up. Extended charging at temperatures above 50oC is not recommended
    and either a lower charge rate should be employed, or the charger
    switched off to allow the battery to cool.

    4. Constant Voltage Charging at constant voltage is the normally
    adopted method. To fully recharge a battery following a 100% discharge,
    the following times are required: Voltage Settings for Constant
    Voltage Charging 2.35 Volts 27 Hours 2.40 Volts 21 Hours 2.45 Volts
    17 Hours 2.50 Volts 13 Hours (Charge assumed to be C/10 maximum)

    Equalisation charge times can be calculated by taking
    the allowance for this recharge to gassing point (approximately 10H)
    from the above figures and increasing by two hours to allow for cell
    differences. The Specific Gravity only rises when gassing (2.35V per
    cell) has been reached and enough time elapsed (half to one hour) for
    the electrolyte to have mixed. If the battery is floated at 2.25V/cell
    it can take weeks for the gravities to come up to the specified 1240
    level. Regulators often switch off when a set voltage is reached (e.g.
    2.4V or more) usually 2.5V/cell. This is equivalent to 14.4 to 15v for
    a 12v system or 28.8 to 30v for a 24v system. This may not be
    sufficient time for the electrolyte to mix and the gravity can be
    G. CHARGING showing 1180, indicating a discharged battery, while the
    battery can be virtually fully charged in the plates, but not in the
    bulk of the electrolyte. If there is any doubt, then the battery should
    be left at a point above gassing for several hours and the gravity
    rechecked. Normally batteries will accept high rates of charge from a
    discharged state. If the system has an electrical problem leading to
    the batteries not being charged and the battery has been completely
    flattened at very low rate over a long period of time the charger
    voltage can rising sharply to the gassing point (2.35V/cell and above).
    If the specific gravities reading are not below 1100 the battery can be
    recovered by the application of a small charging current (trickle
    charge at approx C50), so that the voltage reading of each cell does
    not exceed 2.35 volts per cell. The alternative is to limit the voltage
    (2.35V/cell and above) accordingly until the battery has reached a state
    of charge when it can accept more current. THIS CAN LEAD TO
    DESTRUCTION OF THE CELL IF LEFT FOR A PROLONGED PERIOD UNDER THESE
    CONDITIONS.

    5. Comments on Equalisation Charging Equalisation charging is employed
    to achieve several things: a) To return the batteries to a 100% state
    of charge. b) To even out differences between the cells that
    accentuate over a period of time where minimum recharge is used. c)
    To counter the problem of stratification. a) Stratification is where
    the electrolyte, particularly in tall cells with large volumes of
    excess electrolyte develops varying densities from the top to the bottom
    of the cell. b) If it is left unchecked, it can result in sulphation
    and corrosion at the bottom of the cell, with subsequent permanent loss
    of capacity. Equalisation charging must be carried out at least
    quarterly and more frequently if the system is being run down and
    exhibiting low specific gravity readings. Specific gravity readings of
    all the cells at top of charge should be above 1230.

    6. Setting of Regulators Most regulators are set to rise to a
    certain voltage/cell and then cut out. The batteries benefit from
    having an absorption time when the voltage cut-out point is reached.
    The most efficient setting for Suncycle batteries is between
    2.45-2.5V/cell. There needs to be an allowance for temperature
    compensation. An increase of 5mV/DegC should also be allowed for, i.e.
    at 35oC, the setting/cell is 50mV lower than at 25oC. If regulators are
    set at much lower values, e.g. 2.35V/cell or 14.1V for a 12V system
    then the capacities and gravities in the bank will slowly fall. Within
    the 2.45-2.5V range, the regulator setting should be adjusted according
    to the average amount of energy used on a daily basis, i.e. if 30% of
    the 10H rate is used then a regulator setting of 2.5v is set, if 10% is
    used a value of 2.45v is set

    7. Batteries not Performing/Perceived Capacity
    Loss When a Remote Area Power System is not performing well or giving
    full capacity the first component to be singled out is the battery
    bank. Our experience tells us that 9 times out of 10 that this loss of
    capacity is due to other reasons than faulty batteries. This is a
    system failure so a complete analysis of your system needs to be done.
    At this stage you should call the reseller or installer who installed
    and commissioned the system for you.

    There are many reasons for this failure mode some of which are listed
    below.

    a) System Sizing The system is undersized or does not have enough
    energy input. If more capacity is H. FAULT FINDING taken out of the
    system than put back in each day, the batteries will run down and less
    energy will be available. This can also arise from the addition of new
    appliances to the system or ancillary equipment such as solar panels,
    wind generators, etc. not operating to their full specification. The
    battery sizing should be based on a maximum daily average requirement
    of 33% of the 10H rate or 20% of the 100H rate. If the average is
    likely to grow, then this must be taken into account when sizing the
    system.

    b) The Low Voltage Cut Out The low voltage cut out is set too high.
    The voltage of the system is dependent on a number of factors. The
    Current The voltages at different currents can be seen from Figure
    ?. The state of charge. This can also be seen from Figure ? The
    Temperature Significantly less capacity will be obtained at lower
    temperatures to the same voltages. The factor to be used when
    calculating this is 1% per degree (i.e. around 10% less capacity at 15
    Deg C to 25 Deg

    C.). This is usually seen as moving the whole discharge curve
    downwards, so it will also be accompanied by a drop in voltage of
    between30 and 50mV/cell compared to the readings at 25DegC. The 10
    hour (C/10) capacity = AS650 = 340Ah C/10 = 34A The factor is
    accelerated at higher currents. For the optimum battery life a voltage
    cut out of around 1.92vpc is recommended. Refer to figure below. Low
    Voltage Cut-out Settings Per 2 Volt Cell 1.92 Volts per cell 12 Volt
    System 11.5 Volts 24 Volt System 23.0 Volts 48 Volt System 46.0
    Volts This is for 25oC average operation. Settings will need to be
    adjusted for other operating temperatures.
     
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