Battery stores charge in the form of chemical energy and then converts it into electrical energy to utilize for a specific time. The amount of available charge is the capacity of a cell or battery which may be expressed in Ah (Ampere-hour). Moreover, in a charged battery, the numbers of molecules are limited to create a flow of electron in electric circuits, so, there must be a limited number of electrons in a cell/battery which they motivate through a circuit to fully discharge.
Now we have the option to rate the battery capacity in Number of flowing electrons for a specific time, but, it would be a headache, because there are a vast number of electrons in it. So we have another option (1C (Coulomb) = 6.25 x 1018 electrons, or 6,250,000,000,000,000,000 electrons.
In addition, 1A (Ampere) = 1 coulomb of electrons per second and,
1 hour (h) = 3600 Seconds
Therefore;
1Ah = (1A) x (3600s) = (C/s) x (3600s) = 3600 C.
∴ A (1 Ampere) = 1 Coulomb per second = C/s
But,
Why make up a new unit for battery capacity rating when an old one unit is doing just fine? Of course! To make your lives as technicians and students more difficult ;). As they do for electricity units… i.e. 1 Unit of Electricity = 1kWh = 1 board of Trade Unit.
Why Battery is Not Rated in VA (Volt Ampere) or Watts?
VA stands for volt amps. This is the measurement for apparent power one of the three types of power in AC circuits. The other two are true power, measured in watts and reactive power measured in VARS (Volt Amperes Reactive). The relationship between the three determines a high power factor which is what you want in an AC circuit. Batteries are DC where power factor is unity (i.e. P.f is not not applicable in DC circuits due to zero frequency).
In addition, battery is not a constant voltage source i.e. we only interested in battery capacity which is only for specific time, hence we can’t use Wh instead of Ah for battery as well. That’s why battery is not rated in VA or W instead of Ah.
First of all, keep in mind that both AC and DC voltage and Current are dangerous and hazardous. Both are our friends and worst enemies as well as they wont miss if you give it a chance.
Fig – 1 Difference Between AC & DC
AC is more serial killer as AC with less frequency (50 Hz in EU and 60 Hz in US) is more dangerous than the DChaving the same level of voltage. In other words, 230V AC (or 120V AC) is more dangerous than 230V DC or 120V DC respectively. But keep in mind that DC has the ability to roast you i.e. if we say AC is more dangerous, it doesn’t mean that DC will only play with you. Stay away and don’t trust both.
AC voltage and current with low frequency i.e. 50 Hz or 60 Hz is more dangerous than AC with higher frequency (say 500 0r 600 Hz). Same is the case i.e. AC Currents and voltages are three to five times more dangerous than DC having the same level of voltage.
In case of DC voltage and currents, It causes a single convulsive contraction (a jerky and uncontrollable process in which muscles become shorter and tighter) which pushes the victim away from the DC current or voltage source they touched.
In case of AC voltage and currents, it causes tetany (a condition marked by intermittent muscular spasms) or extended muscle contraction which leads to freeze the victim (or part(s) of the body) touching the AC voltage or current source.
Due to the alternating nature behavior of AC, it causes to heart’s pacemaker neurons into atrial fibrillation which is more dangerous than DC, where cardiac standstill (due to ventricularfibrillation) occurs in case of electric shock. In this case, there is a better chance for “frozen heart” to get back on the normal track as compared to fibrillating heart caused by AC. In that cases, defibrillating equipment (which supply DC units to halt the fibrillation and bring back the heart to the normal condition) are used as emergency medical service.
Generally, the final decision depends on multiple factors like, human body resistance, wet or dry skin or place, thickness of the skin, weight, sex, age, level of current and voltages, frequency etc.
If we consider the minimum level of AC and DC voltages, 50V AC in Dry condition and 25V in humid and wet places and up to 120V DC are considered safe in case of direct or indirect contacts with electrical installations. The above statement and the following table shows that AC Current and Voltage are more dangerous than DC.
For example, In case of AC, the safest limit is 50V (or 25V in humid) where in DC, the safe limit is 120V DC. Same is the case for current, i.e. lower currents are needed for the same effect on human body is compared to DC which is low. The following table shows the story of AC and DC and its effects on human body.
Always Remember: Current Kills, Not the Voltage. But Voltage is must to drive the Current. I.e. Amperes are responsible for electrocution, Not the Volts.
AC in mA (50Hz)
DC in mA
Effects
0.5 – 1.5
0.4
Perception
1.3
4 – 15
Surprise
3 – 22
15 – 88
Let’s Go (Reflex Action)
22 – 40
80 – 160
Muscular Inhibition
40 – 100
160 – 300
Respiratory Block
More than 100
More than 300
Usually Fatal
Electrical Shock Hazards & Effects on Human Body
Why AC is More Dangerous than DC?
Following are some reasons showing that AC is more dangerous than DC.
RMS and Peak Value
The domestic supply in our homes are 230V AC (in EU) and 120V AC in US. It is the effective or RMS Voltage. It means that alternating voltage available has the same heating effect as 230V DC or 120V AC respectivly.
The equation of this alternating current is
V = Vm Sin ω t
Where
Vm = √2 VRMS
ω = 2πf … (f = 50 0r 60 Hz Frequency)
Putting the values and solving for Voltage:
230 x √2 Sin x 2 (3.1415) x 50Hz x t
230 x √2 Sin x 314 x t Volts.
Now the Peak Value of AC Voltage or Current, (this doesn’t apply on DC due to the alternating sinusoidal waves of AC).
VRMS = VPK /√2 or VRMS = 0.707 x VPK
Similarly,
IRMS = IPK /√2 or IRMS = 0.707 x IPK
Using the above formula, we find the value of AC Peak Voltage and Current as follow
VPK = √2 x VRMS and IPK = √2 x IRMS
To calculate the max or peak value of AC Voltage for our homes supply (where home supply is 230V or 120V AC)
VPK = 1.414 x 230V = 325V AC (or 170V Peak AC in case of 120V AC home supply).
The above calculation shows that our home supply voltage which is 230V AC or 120V AC are RMS voltages and the Peak voltages of these RMS voltages are 325V or 170V or 650 peak to peak or 320 peak to peak voltages. Respectively, whereas DC has only RMS value which is contact i.e 230V DC or 120V DC.
In other words, for both AC and DC having the same level of Voltage, AC turns out more about 325V or 170V i.e. its more than it appears and yes, the more voltage, the high chance of electrocution. In short, More DC voltage or current is needed to induce the same hazardous effect as AC voltage and current.
Capacitance
A victim body acts an insulating medium between the live wire and ground leading to capacitance. But we know that a capacitor blocks DC while AC can pass through it. Let’s see methamaticlly,
Frequency in DC = 0Hz
Frequency in AC = 50 or 60 Hz.
Resistance in DC:
XC = 1/2πfC in Ω
If we put “f = frequency” as zero, then the capacitive reactance (XC) would be infinite. That’s why capacitor blocks DC to pass through it.
Now resistance in AC (also known as impedance)
Impedance Z = √ (R2 + XC2)
If we put frequency as 50 or 60Hz, the overall impedance (i.e. resistance) would decreased. This way, AC has the ability to easily pass through the capacitor. It means AC is more dangerous than DC in case when human body act as a capacitor.
In short, the impedance and resistance in DC is lower than AC as it decreases when frequency increases. This way, AC is more harmful than DC.
Frequency
Some having the concept that DC is more dangerous than AC with the same level of voltage because AC changes it direction multiple times (i.e. AC touches the zero value 50 or 60 times) per second due to frequency and there is a chance for victim to skip the shock, whereas there is no frequency in DC.
Now if we consider the frequency as 60 or 50Hz, lets see how fast AC changes its direction.
T = 1/f
T = 1/60Hz = 0.20 Seconds.
It shows that AC touches the zero point in after each 0.20 seconds, where human brain is not that much faster (except the unintentional functions) to response to electric shock and move back from the voltage source.
50 or 60Hz frequency play an important rule and electric shock effect on the human body. For example, low voltage about 25V AC with 60 Hz are harmful (wet and humid body).
Note: Both AC and DC voltages and currents are dangerous. Don’t touch the live wires. In case of electric shock, try to disconnect the power supply and push back the victim body from the source (keep in mind that you should properly insulated before doing so). Only call the professional electrical in case of repairing or troubleshooting. In case of emergency, call the local authority ASAP.
