Time in Hours

At any particular instant the actual current delivered by any cell depends on the load. The Time in hours(H) theoretically would be

Time(H) = Capacity (Ah)/Current(A).

If current is measured in amps and Capacity of a cell/battery in amp-hour, then time is measured in hours for charging and discharging.

For instance,

A 20Ah battery pack delivering 0.5A will last for 40hours

A 20Ah battery pack delivering 1A will last for 20hours

A 20Ah battery pack delivering 2A will last for 10hours

A 20Ah battery pack delivering 20A will last for 1hour


The charging and discharging rate are usually specified by the current.

Example, charged at 10A and discharged at 20A

Charging Time

Charging time is the proportion of charge added to the battery per unit time.

In general scenario charging Time is found by,

Charging Time = Battery Capacity / Applied Current

For instance,

Consider 80Ah battery pack, if the applied current is 20Ah, then the

Charging Time = 80 / 20 = 4 hours.

Discharging Time

Discharging Time= (Battery Capacity x Battery Volt)/Device Watt.

For instance,

Consider a 3.7V 20Ah battery, device watt=20watt

Discharging Time = 20Ah*3.7V/20 = 3.7 hours

Li-ion/ / LiPo batteries with power efficiency of 90%


Discharging Time = (Battery Capacity x Battery Volt x 0.9)/Device Watt

For instance,

Discharging Time = 20Ah*3.7V*0.9/20 = 3.33 hours


Voltage – The potential difference is measured in terms of voltage. The SI unit is a volt and is defined as the measurement of electromotive force, or the difference in potential, which will cause a current of one ampere to flow through a resistance of one ohm.

 How to measure the value of voltage in a cell?

1] Set the multimeter knob to DC voltage measuring mode.

2] Connect the probes into the multimeter.

3] Connect the positive and negative end of the cell to the appropriate probe ends of the multimeter.

4] The voltage value of the cell will be displayed.

Ampere: The flow of electric current is measured in terms of Amperes and its SI unit is Amp.One Amp is defined as the amount of current generated by one volt of electromotive force acting through one ohm of resistance.

Ampere-Hour: The electrical energy stored in the battery pack is measured in terms of Ampere-hour or Ah. The value of current multiplied by time in terms of hours equates ampere-hours. One ampere hour is equal to a current of one ampere flowing for one hour. Also, 1 ampere-hour is equal to 1,000 mAh.

Watt – The total power of the battery pack is measured in terms of watts. Watts is the SI unit for electric power. The wattage of a battery is obtained by multiplying amperes by volts.

Watt-Hour: A Watt Hour is a unit of measurement for power over a period of time. One watt-hour equals one Watt of average power flow over an hour cycle. One Watt of power will be two Watt-Hours for two hours.

Cell – It is an electrochemical device, consisting of positive and negative plates and separated by electrolyte, which is capable of storing electrical energy. It is the basic “building block” of a battery.

Capacity – The capacity of a battery is an indicator or the measurement of the amount of energy that it can deliver in one discharge. Battery capacity is normally mentioned as amp-hours (Ah or mAh) or in terms of watt-hours.

Direct Current (DC) – The unidirectional flow of electric charge. It is the type of electrical current that a battery can supply. One terminal is always positive and the other is always negative.

Cycle – A cycle refers to one sequence of charging and discharging of the battery of its full capacity.

Charge – process of converting electrical energy, given in the form of a current, into chemical  energy and storing it within the cell or battery.

Discharge – The conversion of the chemical energy stored in the battery into electric energy.

Battery-Charge Rate – The rate at which the battery is recharged is known as the battery charge rate and is expressed in amperes (A) or milli amps (mA).

Cycle Life – The total number of charge/discharges cycles a rechargeable cell can sustain before its capacity is significantly reduced is the cycle life of a cell. End of cycle life is commonly considered when the cell or battery only delivers 80 percent of the rated capacity. A battery cycle is greatly influenced by cycle type depth (deep or shallow) and recharging process. Improper cutoff of the charging period will significantly reduce a battery’s cycle life.

Depth of Discharge – The amount of energy that has been harnessed or utilized from a battery (or battery pack). It is usually expressed as a percentage of the total capacity of the battery. For example, 80% DOD means eighty percent of the energy was discharged,  so the battery now only retains 20 percent of its maximum charge.

Energy Density: The volumetric energy storage density of a battery, expressed in Watt-hours per litre (Wh/l).

Power Density: The volumetric power density of a battery, expressed in Watts per litre (W/l).

State of Charge [SOC]: The amount of charge remaining in a cell as a percentage of the charge contained by the cell when it is fully charged. Since, one cannot accurately determine the SOC of a cell without fully discharging it first, the calculation for SOC is usually performed by subtracting the charge removed from the cell so far from the charge contained when it was last charged to 100% SOC.

State of Health [SOH]: It is an indicator of the amount of charge that the cells can hold as they age when compared to its initial stage. In case of Electric Vehicle, the capacity of the cells in the battery pack is the important parameter in determining the range which can be achieved in one charge.

Hence State of Health of an Electric Vehicle battery pack, that is SOHe, which is the amount of charge the cells in the battery pack can hold as they age is  calculated as,

Over a succeeding number of cycles of cells, the capacity of a lithium-ion cell will drop owing to a number of factors including loss of lithium inventory and failing of the electrodes inside the cell. Hence when an aged cell is charged and discharged to its maximum and minimum voltage values respectively, the charge obtained will be lower than charge obtained when the cell was new.