Download Batteries

Batteries
Storing Renewable Energy
“Chemical engines used to push
electrons around”
Basic Terms
Voltage – Electronic pressure
Current – Flow of electrons
Power – Amount of energy being generated
How it Works
• Cells Contain
– Electrochemical Couples
• Two materials which react chemically to release
free electrons
– Electrolyte
• Transfers the electron between electrochemical
couples
• Sometimes electrolyte participates in reaction
(lead-acid) sometimes not (nickel -cadmium, nickel
Iron)
How it Works
• Polarity – One part of couple is electron
rich and other is electron deficient
• While discharging electrons flow from the
electron rich negative cathode pole to the
electron deficient positive anode pole
• While recharging process is reversed
Lets Look at the Atom
• Chemical bonding is the sharing or
exchange of electrons
• Sodium and Chlorine are chemical
elements
• When combined they become something
different – salt
• Chemicals made during the discharge
process are broken by the charging
process
Battery Capacity
• Measured in ampere-hours (amp-hours) at a
given voltage
• Depends on two factors:
– How much energy is needed and
– How long the energy is needed
Example
350 amp-hour battery can provide:
35 amps for 10 hours or
100 amps for 3.5 hours
Important!!!
A battery based alternative
energy system will not
be effective if it is not
sized correctly
Life Expectancy and cost
• At least 5 years
• Often over 10 years or 1500 deep cycles
• Shipping is expensive
State Of Charge
• Percentage which represents the amount
of energy remaining in the battery
• A battery is “deep cycled” when it reaches
20% or less state of charge
• A shallow cycle (car battery) will withdraw
less than 10%
• State of discharge is opposite so a battery
is “deep cycled” if it is at discharged to
80%
Rest Voltage vs. State of Charge
Temperature
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Batteries get sluggish at cold temperatures
Usable capacity drops radically below 40° F
Self Discharge happens rapidly above 120° F
Keep them between 55° F 100° F
Hydrometer
• Measures density of liquid with
respect to water
• The electrolyte has greater specific gravity
at greater states of charge
• So voltage can be an indicator
• Careful opening cells, contamination of the
electrolyte solution is possible
Rates of Charge and Discharge
• 50 amp load for a 100 amp battery is large
• But for 2000 amp battery – no problem
• So we combine current pulled (or added)
with capacity to get a rating scheme
– If it take 10 hours to fill a completely drained
battery then – C/10 charge rate
– If it takes 5 hours to drain a battery then C/5
discharge rate
Rates of Charge and Discharge
• Recommended rates are C/10 – C/20
• Using a C/5 rate will cause much more
electrical energy to be loss as heat
• This heat can damage battery plates
• Example –
– 440 Ampere-hour battery
– How many amps added for a C/10
– How many amps added for a C/20
Equalizing Charge
• After time individual cells vary in their state
of charge
• If difference is greater than .05 volts –
equalize
• Controlled overcharge at C/20 rate for 7
hours
• Turn off voltage sensitive gear before
equalizing
Self Discharge
• Temperature greater than 120° F results in
total discharge in 4 weeks
• At room temperature loss is 6% and will
discharge in 16 weeks
Storage
Fully charged
35 ° F - 40 ° F
Capacity vs. Age
If a battery is supposed to be good for 5 years
– This means it will hold 80% of its original
capacity after 5 years of proper use
Battery Care
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Don’t discharge beyond 80%
C/10 – C/20 rate
Always fill up when recharging
Keep batteries at room temperature
Use distilled water
Size batteries properly
Equalize every 5 months or 5 charges
Keep batteries and connections clean
Connecting Cells
• Power in battery can be increased by
arranging the cells in two ways
– Series
• One path for electrons to follow
• Connect + to –’
• Increases voltage
– Parallel
• Multiple paths for electrons to follow
• Connect (+ to +) and (- to -)
• Increases amperage
Series
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•
•
•
Each cell in lead acid battery is 2.1 volts
Nickel-Cadmium is 1.25 volts
Flashlight batteries are 1.5 volts each
A lead acid battery is typically 6 volts
– This is 3 – 2.1 volts cells wired in series
Parallel
• Increases Capacity
• Trojan L-16 are 350 amps and 6 volts
• Wire them in parallel and you will get 700
amps
• Wire two of these “700 amp batteries” in
series and you get one 12 volt, 700 amp
battery
One Trojan L-16
Where to connect?
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Right
Where to connect?
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Right
How to connect?
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Right
Wire Sizing for DC Applications
• Voltage drop is caused by a conductors
electrical resistance
• This voltage drop can be used to calculate
power loss
VDI Voltage drop Index
• Easier method for determining wire size
• What you need to know
– Amps (Watts/volts)
– Feet (one-way distance)
– Acceptable % volt drop
– Voltage
How to Use Formula and Chart
• Example: 1 KW, 24 volt system, 60 feet,
3% drop
• Amps = 1000 watts/ 24 volts = 41.67 amps
• VDI = 41.67 amps * 50 feet = 28.9
3% * 24 volts
VDI Chart
2 AWG wire
That’s pretty big wire
What if we make it a
48 volt system?
How to Use Formula and Chart
• Example: 1 KW, 48 volt system, 60 feet,
3% drop
• Amps = 1000 watts/ 48 volts = 20.8 amps
• VDI = 20.8 amps * 50 feet = 7.23
3% * 48 volts
VDI Chart
8 AWG wire
That’s better
Practical Considerations
• Lighting Circuits
– 10% drop in incandescent leads to 25% drop
in light output
– 10% drop in fluorescents results in 10% loss
in light output
– Suggested acceptable loss 2-3%
Practical Considerations
• DC Motors
– Operate at 10-15% more efficiently
– Minimal surge demands
– Some motors will fail to start if drop is too
great (Sun Frost)
AC Motors
• Exhibit high surges when starting
PV Battery Charging Circuits
• Need to be higher than battery voltage so they
are wired to be around 16 volts
• A voltage drop of 1 or 2 volts is significant
• A 10% drop will result in 50% loss of power in
some cases
• 2-3% loss is recommended