bcps wrote:
> 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
>
>
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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> 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?