Battery Charging Terminology
Battery Charging Topology
There are three main categories of rechargeable batteries available.
Automotive batteries
Used to supply primary power to start engines on cars, boats and other vehicles. They provide a
short burst of high current to get the engine started.
Standby/industrial batteries
Designed to be permanently connected in parallel with a critical load and a rectifier/charger system,
where the rectifier/charger forms the primary source of power for the load and the battery provides
the secondary source in the event of a primary source failure.
Portable batteries
Designed to power portable equipment such as consumer products and tools such as drills, mobile
phones, laptop computers and so on.
The latter two categories can be further broken down by the chemistries used in the construction ofthe battery.
Nickel-cadmium (NiCd) (vented & semi-sealed) - mature but have moderate energy
density. Nickel-cadmium batteries have generally been used where long life, high
discharge rate and extended temperature range is important. Nickel-cadmium batteries
contain toxic metals and are generally being phased out.
Nickel-metal-hydride (NiMH) - have a higher energy density compared to nickel-
cadmium at the expense of reduced cycle life. Contain no toxic metals.
Lithium-ion (Li-ion)- fastest growing battery technology offering high energy density and
low weight. Requires protection circuits to limit voltage and current for safety reasons.
Lead-acid (vented & valve-regulated) batteries
Most economical for larger power applications where weight is of little concern.
One of the main differences between the above battery types is the initial purchase cost of thebattery. However, when selecting a battery, the initial cost can give a very misleading impression ofthe total cost to the user during the system’s lifetime.
The selection of a battery based on cost alone can have a major impact on the life cycle cost of thesystem being supported due to such factors as installation, replacement, maintenance, testing anddowntime cost.
In many instances, the selection of the most suitable battery for a particular application can be a verycomplex calculation that can only be performed by the end user as a number of the factors relatingto life cycle cost are outside the control of the battery supplier maintenance for example. However,some basic logic can be applied by the supplier assuming some data has been provided, includinglocation and access to site, site ambient temperature and the nature of the application. It would belogical to select a nickel-cadmium battery for a remote unmanned site in the Middle East with anambient temperature of 45°C during the day and 5°C at night, where the load being supported is vital to the production of oil. However, if the equipment was to be housed within an air-conditionedroom in a well-maintained oil production facility it would be reasonable to utilize valve-regulated leadacid batteries.
Terms Associated with Standby Batteries
A cell comprises a number of positive and negative charged plates immersed inan electrolyte that produces an electrical charge by means of an electrochemicalreaction. Lead acid cells generally produce an electrical potential of 2V whileNickel-cadmium cells generally produce an electrical potential of 1.2V.
A battery is a number of cells connected together.
A battery string or bank comprises a number of cells/batteries connected inseries to produce a battery or battery string with the required usablevoltage/potential e.g. 6V, 12V, 24V, 48V, 110V.
The Ah or Ampere/hour capacity is the current a battery can provide over aspecified period of time, e.g. 100Ah @ C10 rate to EOD of 1.75V/cell. Thismeans the battery can provide 10 Amps for 10 hours to an end of dischargevoltage of 1.75V per cell.
Different battery manufacturers will use different Cxx rates depending on themarket or application at which their batteries are targeted. Typical rates used areC3, C5, C8, C10 and C20. Because of this it is important, when comparingbatteries from different manufacturers having the same Ah rate, to confirm onwhat Cxx rate this figure is based.
Example:An application requires a 100Ah battery at the C3 rate, based on a load profileof 33 Amps for 3 hours to an end discharge of 1.8V per cell. However, whentwo battery manufacturers are asked to tender for this project, the only datathey are given is that a 100Ah battery is required.
Standby Time (Hrs)
Manufacturer ‘A’ Ah (Cxx rate) 80.9 94.8 105.0 116.0 129.6 135.7 136.8 138.0
Manufacturer ‘B’
Ah (Cxx rate)
As can be seen above, both manufacturers can offer a 100Ah battery based onthe limited specification provided, but only the battery from manufacturer A iscapable of supporting the intended load profile.
Manufacturer A - 105Ah @ C3 Rate to 1.8V/cell ( i.e. 3 hour discharge rate)
Manufacturer B - 102Ah @ C10 Rate to 1.8V/cell ( i.e. 10 hour discharge rate)
EOD voltage
End of discharge voltage is the level to which the battery string voltage or cellvoltage is allowed to fall to before affecting the load i.e. 1.75V or 21V on anominal 24V system.
End of life factor
This is a factor included within the battery sizing calculation to ensure the batteryis able to support the full load at the end of the battery design life, calculated bymultiplying Ah by 1.25.
As the energy stored within a battery cell is the result of an electrochemical derate factor
reaction, any change in the electrolyte temperature has an effect on theefficiency or rate of reaction. i.e. an increase in temperature increases theefficiency/rate whereas a decrease in temperature reduces the efficiency/rate ofreaction. As a result of this, all battery manufacturers’ discharge data will bespecified at a recommended temperature (typically 20-25°C) with temperaturecorrections provided for operation above and below these values.
Typical temperature correction factors for Valve Regulated Lead Acid (VRLA) batteries
Temperature Correction Factors to be applied to 20°C data at:
charge rate
5 minutes to 0.800
59 minutes
to 24 hours
Typical reduction in design life against temperature
% Expected
Float Life
High temperatures will reduce the service life of VRLA batteries dramatically andcan, in extreme cases, cause thermal runaway, resulting in high oxygen orhydrogen gas production and battery swelling. Batteries will not recover fromthis condition and must be replaced.
As previously detailed, the energy stored within a battery cell is the result of an compensation
electrochemical reaction, so any change in the electrolyte temperature has an effecton the rate of reaction provided all other factors (charge voltage and current)relating to the reaction remain constant.
Therefore, if we alter these factors to compensate for the effect of temperature, wecan minimize the effect of temperature on battery life by maintaining the amount ofgas evolved within a VRLA or semi-sealed nickel cadmium battery to approximatelythe normal operating limit.
The simplest way of maintaining the rate of reaction within design parameters is toalter the charge voltage at a rate proportional to the change in temperature, i.e.
decrease the charge voltage with an increase in temperature above 20-25°C andincrease the charge voltage with a decrease in temperature below 20-25°C. Thetypical change in charge voltage is 3 mV / °C.
Boost charge
Charge given to a battery to correct voltage imbalance between individual cells andto restore the battery to fully charged state.
The process of replenishing or replacing the electrical charge in a rechargeable cellor battery.
Cycle life
The number of cycles (charge/discharge) a battery provides before it is no longerusable. A battery is considered non-usable if its nominal capacity falls below 60 to80 percent.
A non-metallic conductor of electricity between the positive and negative electrodesof a battery. The current is carried by the physical movement of ions.
Equalize charge
Fast charge
Term generally associated with NiCd batteries. The typical fast charge time isbetween one and three hours. The fast-charger detects the state of charge andswitches to trickle charge when full charge is reached.
Float charge
Similar to trickle charge. Compensates for the self-discharge of a lead acid battery.
Reversible capacity loss in NiCd and NiMH batteries. The modern definition of memory commonly refers to a change in crystalline formation from the desirablesmall size to a large size. Memory is often used to describe any reversible capacityloss on nickel-based batteries.
Nominal voltage
The cell voltage that is accepted as an industrial standard. (Cell voltages of 1.20and 1.25V are used for NiCd and NiMH batteries).
Quick charger
A charger that charges a battery in three to six hours.
Rapid charger
Capacity loss during storage due to the internal leakage between the positive andnegative cell plates.
Slow charge
Typically an over-night charge lasting 10 to 16 hours at a charge current of0.1CA (0.1 x Ah capacity in Amps). The battery does not require instant removalwhen fully charged.
A condition whereby an electrochemical cell will overheat and destroy itself through internal heat generation. This may be caused by overcharge or highcurrent discharge and other abusive conditions.
Trickle charge
Maintenance charge to compensate for the battery's self-discharge.
Minimum information required to select and size a battery
In order to size a standby battery the following data is generally required: System nominal voltage. The nominal voltage that the load requires e.g. 24V, 48V.
Load rating. Either current or power taken by the load during normal & primary power
source failure. If load rating is given in Watts, the battery should not be sized by
dividing the nominal voltage to convert to Amps as the specified battery will be too
small to support the load for the required standby period.
Battery standby or autonomy time.
Load voltage limits. The voltage range over which the load will safely operate. Load
voltage limits need to be evaluated to determine whether the system will need some
form of DC/DC converter/regulator between the load and battery to protect the load
from over or under voltage due to the voltage range of battery.
Normal operating or ambient temperature in which the battery will be operating.
Battery type.
Battery Sizing Constant Current Method
The following example demonstrates how a battery is sized using the constant current calculationmethod. This method is not recommended where DC-DC converters are utilized as the load.
A 24V control system requires 50 Amps constant current and operates satisfactorily down to aminimum of 21V. The battery is required to support the load in the event of a mains failure for 2hours in an ambient temperature of 20°C.The battery will be housed within the main equipmentpanel, which will be located within the site’s main control room.
As the battery is to be used in a controlled environment, the most cost-effective solution would be touse twelve VRLA cells. The following calculation can be used: Minimum allowable volts per cell =
minimum voltage/number of cells
21V/12 cells
Cell performance required is 50 Amps constant current to 1.75V/cell.
By referring to the sample constant current performance table below relating to 1.75 Volts per cell, itcan be seen that cell type “F” is the smallest available to perform the standby duty required.
If the specification is now modified so that the system is operating in an outdoor enclosure wherethe ambient temperature during the winter is known to fall to 0°C and the sizing is checked againusing the temperature correction factor table, it is found that: At 0°C the current available from cell type ‘F’ is reduced to 55 A x 0.86 = 47.3 A, and hence too low.
The next cell size “G” at 0°C has available 75.2 A x 0.86 = 64.6 A. The revised battery selectionshould comprise 12 x “G” cells.
Discharge Currents (Amperes Per Cell) at 20°C to 1.75V/Cell
Standby Time (Hrs)
Cell Type
Temperature Correction Factors to be applied to 20°C data at:
charge rate
5 minutes to 0.800
59 minutes
to 24 hours
Battery Sizing Constant Power Method
The following example demonstrates how to size a battery using the constant power calculationmethod. Recommended for DC-DC converter loads.
Example A
A 110V sub-station control system requires 5kW constant power and operates satisfactorily at aminimum voltage of 94.2V. Under full load conditions there is approximately 3V drop between thebattery and the control panel input terminals due to cable resistance. A battery is required to supportthe load in the event of a mains failure for an eight hour standby period in an ambient temperatureof 20°C. The batteries will be housed within a separate battery room.
Based on the above, either VRLA or vented lead acid cells could be used.
Minimum allowable volts per cell
min Voltage/no. cells
= (94.2+3V)/54
= 1.8V/cell
Watts per cell
total Watts/no. cells
5000/54 cells
Cell performance required is 92.59 W/cell constant power to 1.8 V/cell.
By referring to the sample constant power performance table below relating to 1.8 Volts per cell, itcan be seen that cell type ‘F’ is the smallest available to perform the standby duty required.
Discharge Power (Watts Per Cell) at 20°C to 1.8V/Cell
Standby Time (Hrs)
Cell Type
Battery Charging
The time taken to recharge any battery is dependent on the voltage and current applied. If therecharge current or voltage is too low, then the recharge time will be relatively long; if the current orvoltage is high then the recharge time will be short.
However, as all batteries involve an electrochemical reaction, care must be taken to ensure thecharging characteristics of both battery and charger are matched correctly.
These unique needs must be met to obtain reliable service and long life. The table below summarizesthe general need of each battery type and advises proper handling of each battery type.
Optimal handling may not always be practical. Deviations from the ideal are acceptable but will lowerthe life expectancy of the battery to some degree, heat being the greater detrimental factor.
Nickel-cadmium (NiCd)
Fully discharge the battery before charging. Do not leave battery in charger for more than two days
because of memory effect. Avoid getting battery too hot during charge.
Charge methods: Constant current, followed by trickle charge when full. Fast-charge preferred
over slow charge. Slow charge = 16h, Rapid charge = 3h, Fast charge = 1h+
Nickel-metal-hydride (NiMH)
Discharge every three months. Over-cycling is not advised. Do not leave battery in charger for more
than two days because of memory effect. Avoid getting battery too hot during charge.
Charge methods: Constant current, followed by trickle charge when full. Slow charge not
recommended. Battery will get warm towards full charge.
Rapid charge = 3h, Fast charge = 1h+
Lithium-ion (Li-ion)
Charge the battery often. The battery lasts longer with partial rather than full discharges. Do not use
if the pack gets hot during charge.
Charge methods: Constant voltage to 4.20V/cell (typical). No trickle-charge when full. Li-ion may
remain in the charger (no memory). Battery must remain cool. No fast-charge possible.
Rapid charge = 3h
Lead-acid (Sealed or flooded)
Charge the battery immediately after use. Lead-acid must always be kept in a charged condition.
The battery lasts longer with partial rather than full discharges. Over-cycling is not advised.
Charge methods: Constant voltage to 2.40/cell (typical), followed by float held at 2.25V/cell.
Battery must remain cool. Fast charge not possible; can remain on float charge.
Slow charge = 24h, Rapid charge = 10h
Nickel-cadmium (NiCd)
NiCd is one of the most hardy and durable chemistries and is not harmed by a full discharge cycle.
Nickel-metal-hydride (NiMH)
Avoid too many full discharge cycles because of wear. Use 80% depth-of-discharge. NiMH has
higher energy density than NiCd at the expense of shorter cycle life.
Lithium-ion (Li-ion)
Avoid full cycle because of wear. 80% depth-of-discharge recommended. Recharge more often.
Avoid full discharge. Low voltage may cut off safety circuit.
Lead-acid (Sealed or flooded)
Avoid full cycle because of wear. 80% depth-of-discharge recommended. Low energy density
generally limits the use of lead-acid batteries to automotive and fixed standby applications.
Service needs
Nickel-cadmium (NiCd)
Discharge to 1V/cell every 1 to 2 months to prevent memory.
Nickel-metal-hydride (NiMH)
Discharge to 1V/cell every 3 months to prevent memory.
Lithium-ion (Li-ion)
No maintenance needed. Loses capacity due to aging whether used or not.
Lead-acid (Sealed or flooded)
Apply topping charge every 6 months. Occasional discharge/charge may improve performance.
Nickel-cadmium (NiCd)
Best to store at 40% charge in a cool place. Open terminal voltage cannot determine state-of
charge. 5 years and longer storage possible. Recharge battery if stored longer than 6 months.
Nickel-metal-hydride (NiMH)
Store at 40% charge in a cool place. Open terminal voltage cannot determine state-of-charge.
Recharge battery if stored longer than 6 months.
Lithium-ion (Li-ion)
Store at 40% charge in a cool place (40% state-of-charge reads 3.75-3.80V/cell at open terminal).
Do not store at full charge and at warm temperatures because of accelerated aging.
Lead-acid (Sealed or flooded)
Store fully charged. Apply topping-up charge every 6 weeks.
Nickel-cadmium (NiCd)
Do not dispose of; contains toxic metals; must be recycled.
Nickel-metal-hydride (NiMH)
Should be recycled. Low volume household NiMH may be disposed of.
Lithium-ion (Li-ion)
Should be recycled. Low volume household Li-ion may be disposed of.
Lead-acid (Sealed or flooded)
Do not dispose of; must be recycled.


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