Trickle charging
Encyclopedia
Trickle charging, or float charging, means charging
a battery
at a similar rate as its self-discharging
rate, thus maintaining a full capacity battery. Most rechargeable batteries, particularly nickel-cadmium batteries
or nickel metal hydride batteries
, have a moderate rate of self-discharge, meaning they gradually lose their charge even if they are not used in a device. Care must be taken, however, that if a battery regulator is not employed, the charge rate is not greater than the level of self-discharge, or overcharging and possible damage or leakage may occur.
varies significantly with the construction of the battery and the ambient temperature. With the appropriate voltage for the battery type and with proper temperature compensation, a float charger may be kept connected indefinitely without damaging the battery. With a 6-cell (nominal 12V) lead-acid battery the correct float voltage drops by about 0.15 V for a 5°C rise in ambient temperature. Not compensating for this will shorten battery life either by over- or under-charging.
, comprising twelve 2-volt flooded lead-acid cells, which has been deeply discharged, would normally be restored by a boost charge of approximately 2.4 volts per cell for a short time (perhaps around 72 hours). Once the collective cell voltage reaches a surface charge
of 28.8 volts (2.4 volts x 12 cells), the charge rate would be switched to the sustained lower float-charging rate of typically 2.23 volts.
Eventually, with the boost charge removed, the surface charge will diminish slightly and the battery voltage will stabilise at a preset float voltage, in the case of the example above to approximately 27 volts (2.23 volts x 12).
s), the battery may be designed to be discharged at a rate of C/8 or an 8-hour rate. The recharge rate may be at the C/8 rate or as fast as C/2 for some types of battery. A float or trickle charge might be as low as C/300 (a 300-hour discharge rate) to overcome the self-discharge. Allowable trickle charging rates must conform to the battery manufacturer's recommendations.
For a 12 V 60 Ah battery a C/300 rate would mean 60 A / 300 = 0,20 A = 200 mA
applications, where a relatively high current or power is required infrequently, charger costs can be minimized by applying trickle-charging principles. This can be an economy measure in cases where the charging method could be quite expensive if the full charging rate were employed, such as solar-cell
installations. Full battery capacity can be achieved at a low charging current over a long period of time to provide a high-power load for a short period.
Battery charger
A battery charger is a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it.The charge current depends upon the technology and capacity of the battery being charged...
a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
at a similar rate as its self-discharging
Self-discharge
Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes...
rate, thus maintaining a full capacity battery. Most rechargeable batteries, particularly nickel-cadmium batteries
Nickel-cadmium battery
The nickel–cadmium battery ' is a type of rechargeable battery using nickel oxide hydroxide and metallic cadmium as electrodes....
or nickel metal hydride batteries
Nickel metal hydride battery
A nickel–metal hydride cell, abbreviated NiMH, is a type of rechargeable battery similar to the nickel–cadmium cell. The NiMH battery uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide...
, have a moderate rate of self-discharge, meaning they gradually lose their charge even if they are not used in a device. Care must be taken, however, that if a battery regulator is not employed, the charge rate is not greater than the level of self-discharge, or overcharging and possible damage or leakage may occur.
Float charger
A float charger differs from a trickle charger in having circuitry to prevent overcharging. It senses when the battery voltage is at the appropriate float level and temporarily ceases charging; it maintains the charge current at zero or a very minimal level until it senses that the battery output voltage has fallen, and then resumes charging. It is important to note that the appropriate float voltageFloat voltage
Float voltage is the constant voltage that is applied continuously to a voltaic cell to maintain the cell in a fully charged condition. The appropriate float voltage varies significantly with the chemistry and construction of the battery, and ambient temperature...
varies significantly with the construction of the battery and the ambient temperature. With the appropriate voltage for the battery type and with proper temperature compensation, a float charger may be kept connected indefinitely without damaging the battery. With a 6-cell (nominal 12V) lead-acid battery the correct float voltage drops by about 0.15 V for a 5°C rise in ambient temperature. Not compensating for this will shorten battery life either by over- or under-charging.
Trickle charger
A trickle charger, on the other hand, will charge no matter whether the battery is fully charged or not, so it needs to be connected and disconnected periodically. Trickle chargers will work to keep the battery charged, if used once a month or so for a day or two, but the float chargers can be left connected indefinitely without potential harm to the battery.Example
For example, a 24 volt battery packBattery pack
A battery pack is a set of any number of identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density...
, comprising twelve 2-volt flooded lead-acid cells, which has been deeply discharged, would normally be restored by a boost charge of approximately 2.4 volts per cell for a short time (perhaps around 72 hours). Once the collective cell voltage reaches a surface charge
Surface charge
Surface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...
of 28.8 volts (2.4 volts x 12 cells), the charge rate would be switched to the sustained lower float-charging rate of typically 2.23 volts.
Eventually, with the boost charge removed, the surface charge will diminish slightly and the battery voltage will stabilise at a preset float voltage, in the case of the example above to approximately 27 volts (2.23 volts x 12).
Charging rate
Charging rates for a trickle charge are very low. For example, if the normal capacity of a battery is C (ampere-hourAmpere-hour
An ampere-hour or amp-hour is a unit of electric charge, with sub-units milliampere-hour and milliampere second...
s), the battery may be designed to be discharged at a rate of C/8 or an 8-hour rate. The recharge rate may be at the C/8 rate or as fast as C/2 for some types of battery. A float or trickle charge might be as low as C/300 (a 300-hour discharge rate) to overcome the self-discharge. Allowable trickle charging rates must conform to the battery manufacturer's recommendations.
For a 12 V 60 Ah battery a C/300 rate would mean 60 A / 300 = 0,20 A = 200 mA
Economy
In low duty-cycleDuty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
applications, where a relatively high current or power is required infrequently, charger costs can be minimized by applying trickle-charging principles. This can be an economy measure in cases where the charging method could be quite expensive if the full charging rate were employed, such as solar-cell
Solar cell
A solar cell is a solid state electrical device that converts the energy of light directly into electricity by the photovoltaic effect....
installations. Full battery capacity can be achieved at a low charging current over a long period of time to provide a high-power load for a short period.