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

Example350 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

• 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

• 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

• 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