Transcript Basic Electricity, Cells and Batteries

Basic Electricity
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The intention of this lecture is to describe
basic electrical characteristics in a qualitative
way.
Much of the ‘skill’ is to understand the
associated jargon, especially as terminology
is often used for things not directly related to
its strict definition.
For example “D.C.” literally refers to Direct
Current, an electrical current which always
flows in the same direction.
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Charge is measured in coulombs and is the
amount of “electricity” present (or flowing).
Charge can be positive or negative. Like
charges repel each other. Unlike charges
attract each other.
In the vicinity of a charge there is an electric
field. The field points in the direction that a
positive charge would move.
Charge moves about and may be stored (e.g.
in a capacitor or battery).
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Current is a flow of charge; the rate of movement
of charge through a system. It is analogous to
the flow of water in a hydraulic system (in
litres/s). It is measured in amps; an amp is quite
big.
A.C. is alternating current, where the current
flows first one way then the other (repeatedly).
This does not mean it cannot transmit energy.
D.C. is Direct Current where a current flows in
the same direction at all times (normally implies
constant value as well as direction).
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1 Ampere is large current in
microelectronics!!
15 Amps can cook a turkey on domestic
voltage.
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It’s the volts that jolt
It’s the mills that kill !!
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Voltage is electrical “pressure”.
It is analogous to the pressure of a hydraulic
system (say like the height of a reservoir or a
waterfall). Voltage is measured in volts;
contrary to media opinion, volts do not flow.
240 volt mains power
5 volts electronic devices
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Impedance is the ‘resistance’ to current flow.
It is a very important concept in any electrical
circuit.
Impedance is a general term. For many
applications ‘resistance’ is equally applicable,
however capacitors and inductors have
different properties.
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Energy is a measure of work done. Power is
the work done per unit time. Thus a battery
contains so much energy it can power
something for a particular time.
If the power needs of the equipment is
reduced then the same energy can power it
for longer. (These two terms are not
restricted to electrical circuits.)
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Ohm’s Law
V=IxR
This is only true for resistive loads. Most
loads are more complex than this.
In general:
V=IxZ
where Z is the impedance of the load. This
may depend (for example) on the frequency
of an A.C. signal.
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What current goes in, goes out.
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Capacitors
A capacitor is a charge storage device.
It allows A.C. signals to pass through but
blocks D.C. signals.
Everything has some inherent capacitance.
Capacitance is usually the enemy in digital
circuits, slowing downsignal edges and
therefore circuit operation
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Resistors in series are added together –
resistance increases.
In parallel resistance is reduced
In particular if R1 = R2 then R will be half of
R1 (or R2).
Measures in Ohms (Kilohms - Megohms)
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Capacitors in parallel are added together –
capacitance increases.
In series capacitance is reduced.
Measured in Farads normally microfarads – μF
Be careful with capacitors which are not in
low milli farads. You can get a surprise!
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http://www.yenka.com/en/Yenka_Basic_Circu
its/
1946
1960s
1970s
1990
1991
1992
1999
Neumann: sealed NiCd
Alkaline, rechargeable NiCd
Lithium, sealed lead acid
Nickel metal hydride (NiMH)
Lithium ion
Rechargeable alkaline
Lithium ion polymer
Duracell batteries
Two cells
More precisely
9v battery
A real battery
6v dry cell
Another battery
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Size
◦ Physical: button, AAA, AA, C, D, ...
◦ Energy density (watts per kg or cm3)
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Longevity
◦ Capacity (Ah)
◦ Number of recharge cycles
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Discharge characteristics (voltage drop)
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Cost
Behavioral factors
◦ Temperature range (storage, operation)
◦ Self discharge
◦ Memory effect
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Environmental factors
◦ Leakage, gassing, toxicity
◦ Shock resistance
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Zinc carbon (flashlights, toys)
Heavy duty zinc chloride (radios, recorders)
Alkaline (all of the above)
Lithium (photoflash)
Silver, mercury oxide (hearing aid, watches)
Zinc air
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Features
+Inexpensive, widely available
◦ Inefficient at high current drain
◦ Poor discharge curve (sloping)
◦ Poor performance at low temperatures
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Features (compared to zinc carbon)
+Better resistance to leakage
+Better at high current drain
+Better performance at low temperature
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Features
+50-100% more energy than carbon zinc
+Low self-discharge (10 year shelf life)
±Good for low current (< 400mA), long-life use
◦ Poor discharge curve
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Features
+High energy density
+Long shelf life (20 years at 70°C)
+Capable of high rate discharge
◦ Expensive
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Nickel cadmium
Nickel metal hydride
Alkaline
Lithium ion
Lithium ion polymer
Lead acid
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Features
+Rugged, long life, economical
+Good high discharge rate (for power tools)
◦ Relatively low energy density
◦ Toxic
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Over 1000 cycles (if properly maintained)
Fast, simple charge (even after long storage)
C/3 to 4C with temperature monitoring
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Self discharge
10% in first day, then 10%/mo
Trickle charge (C/16) will maintain charge
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Memory effect
Can be overcome by discharges
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Features
+Higher energy density (40%) than NiCd
+Nontoxic
◦ Reduced life, discharge rate (0.2-0.5C)
◦ More expensive (20%) than NiCd
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Less prone to memory than NiCd
Shallow discharge better than deep
Degrades after 200-300 deep cycles
Need regular full discharge to avoid crystals
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Self discharge 1.5-2.0 more than NiCd
Longer charge time than for NiCd
To avoid overheating
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Chemistry
Lead
Sulfuric acid electrolyte
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Features
+Least expensive
+Durable
◦ Low energy density
◦ Toxic
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Low self-discharge
◦ 40% in one year (three months for NiCd)
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No memory
Cannot be stored when discharged
Limited number of full discharges
Danger of overheating during charging
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Ratings
CCA: cold cranking amps (Car battery)
RC: reserve capacity (minutes at 10.5v, 25amp)
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Deep discharge batteries
Used in golf carts, solar power systems
2-3x RC, 0.5-0.75 CCA of car batteries
Several hundred cycles
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Features
+40% more capacity than NiCd
+Flat discharge (like NiCd)
+Self-discharge 50% less than NiCd
◦ Expensive
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300 cycles
50% capacity at 500 cycles
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Chemistry
Graphite (-), cobalt or manganese (+)
Nonaqueous electrolyte
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Features
+Slim geometry, flexible shape, light weight
+Potentially lower cost (but currently expensive)
◦ Lower energy density, fewer cycles than Li-ion