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					                             Battery Principles
-A battery is a collection of cells, connected together electrically to produce a desired
voltage.
- Cells store electrical energy in chemical bonds and delivers this energy through
chemical reactions.
- A cell consists of a set of negative and positive plates immersed in an electrolyte
solution. They can be made of one or more pairs of plates connected in parallel.
- Thinner plates allow for a larger number and thus higher currents, but the depth of
discharge is lower.
- Chemical reactions during the battery charging produce gases –all batteries include
either cell vents or relief vents.
- When a battery is under an infinite-resistance load, this is know as steady-state.
- Lead-acid battery cells typically have an open voltage of 2.1 V.




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                           Capacity
- Capacity is the measure of the electrical potential of the
battery
-Capacity is commonly expressed in A-h but it can also be
expressed in W-h. For example, a battery that can deliver 3
amps for 40 hours has a 120 A-h capacity. If this current were to
be delivered at an average voltage of 12 V, then we can also
express this battery’s capacity as 1440 W-h (120 A-h x 12V).
- High temperatures decrease the useful life of a battery.




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                            Discharging
-As we connect a load to the battery, we create a path for electrons to
circulate through.
- The active material immerse in electrolyte will react and form a new
material that releases excess electrons. Conversely, the active material
connected to the positive terminal will require extra electrons – this
creates a difference of potential between the two terminals.
-Discharge rate is a ration of nominal capacity to the discharge time in
hours. For example, for a 100 Ah battery that delivers a constant 5 amp
current the Discharge Rate = 100Ah/5A = 20 and it is expressed as C/20.
One twentieths of the battery capacity is discharged each hour.
- The depth of discharge is the percentage of withdrawn energy in a
battery compared to a fully charged capacity. Why is this important?
- As the battery gets more discharge, the voltage will drop depending on
the discharge rate. As we connect larger loads, the voltage will be lower
at the same depth of discharge.

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                       Discharging
-Autonomy is the amount of time a fully charged battery system
can supply power to system loads without further charging.
- Internal electrochemical mechanisms will produce some self-
discharge proper to each battery –higher temp. => higher
internal resistance => higher self-discharge.




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                              Charging
-Charging is done by applying an electrical current to the cell or
battery in a direction opposite to the discharge.
- The voltage of the charging source must be higher than the battery
voltage to be charged. For example, a 12 V lead-acid battery will
charged at about 14.4 V and fully charged will have a 12.6 V open
circuit voltage.
- Charge Rate is quantified in the same way as discharge rate. That is,
a C/20 battery is a battery that is charging 1/20th of its capacity in one
hour.
- Bulk Charging is the charging that occurs for the first 80%-90% of the
battery capacity and it occurs at a relatively high rate
-Absorption Charging follows bulk charging and it occurs to a lower
rate
- Float Charging is the charging to counteract the self-discharge
-Equalizing Charging is a controlled overcharge to produce gassing in
order to ensure that every cell is fully charged. Most banks for PV will
be equalize every 8 weeks.
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-When gassing occurs water is decomposed in oxygen and
hydrogen, and more electrolyte should be added.
-Sulfation is the growth of lead sulfate crystals on the positive
plate of a lead-acid cell, which decreases the available active
material and consequently the capacity of the cell.
- Stratification: if the electrolyte is left unmixed, the reaction
processes will vary between the bottom and the top of the
plates. Gassing solves this problem.
- The battery life will depend on on battery materials, operating
temperatures, cycle frequency, depth of discharges, average
state of charge and charging methods.




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                       Battery Types
-They are divided into two big categories: primary and
secondary batteries. The main difference is that the latter ones
can be recharged.
- Traction batteries can undergo repeated deep discharge,
which makes them ideal for solar. These batteries have fewer
plates that are thick and durable.
- Starting/Ignition batteries have many thin plates allowing
them to deliver high currents for a short period of time.
-Stationary Batteries are designed to last for a long time stand
by but they cannot be cycled many times



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Flooded-Electrolyte Batteries: Lead-acid are the most most
typically used batteries in the industry. Lead-acid ones use
diluted sulfuric acid as electrolyte. Nickel-cadmium cells use an
alkaline solution made of potassium hydroxide.
- Open-vent batteries will have a longer life span than sealed-
vent batteries. Distilled water should be added to recup for the
decreased levels of electrolytes caused by gassing.
- Catalytic recombination caps recombine a significant portion
of the gas emissions back into water, which greatly reduces the
number of times that water should be added –These caps are
know in the industry as miser caps.
Captive-Electrolyte Batteries: the electrolyte is immobilized,
which makes them spill proof. Gelled Electrolyte and Absorbed
Glass Mat Electrolyte are two of the most common types .
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-Nickel Cadmium batteries: are well suited for PV systems
including long life, low maintenance, excessive discharge
tolerance, excellent low-temperature capacity retention, and
non-critical voltage regulation requirements. However, the high
initial cost puts Lead-acid batteries at an advantage.

Battery Selection

-Life time, deep cycle performance, tolerance to high
temperatures and overcharge, and maintenance requirements
are some of the aspects to be considered when selecting a
battery.
-Batteries of different types and/or ages should not be mixed in
the same system.
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                        Battery Bank
-A battery bank is a group of batteries connected together with
series and parallel connections, just like we could connect an
array of PV panels.
- Whenever possible, we will try to up the voltage of the battery
bank to reduce the gauge of the wires to equal load. P = V I so
I =P/V . The higher is V the lower is I.

- Series and Parallel Connections work just like in PV modules.
The voltage of each battery in series will add, and the capacity
of each battery/string in parallel will add.
- It is recommended to connect as few parallel strings as
possible as voltage differences received during the charge can
lead to decreased capacity in some strings.

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                          Enclosures
-An enclosure must be insulated, and they may have cooling or
heating mechanisms to protect batteries from extreme
temperatures.
- Racks must be made from rigid materials, either metallic or
nonmetallic, that are resistant to deterioration from electrolyte.
-Battery systems should contain means to contain the
electrolyte in the event of a spill.
-Ventilation of the battery enclosure is required.




                                                                     11
     Overcurrent Protection and Disconnects
- Proper DC-rated overcurrent protection and disconnects to
protect systems conductors and isolate the battery bank from
the rest of the system are required for maintenance purposes
and safety.




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posted:8/25/2012
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