BasicBatteryCharging
Methods
Constant Voltage
A constant voltage charger is basically a DC power supply
which in its simplest form may consist of a step down transformer from the
mains with a rectifier to provide the DC voltage to charge the battery. Such
simple designs are often found in cheap car battery chargers. The lead-acid
cells used for cars and backup power systems typically use constant voltage
chargers. In addition, lithium-ion cells often use constant voltage systems,
although these usually are more complex with added circuitry to protect both
the batteries and the user safety.
Constant Current
Constant current chargers vary the voltage they apply to
the battery to maintain a constant current flow, switching off when the voltage
reaches the level of a full charge. This design is usually used for
nickel-cadmium and nickel-metal hydride cells or batteries.
Taper Current
This is charging from a crude unregulated constant
voltage source. It is not a controlled charge as in V Taper above. The current
diminishes as the cell voltage (back emf) builds up. There is a serious danger
of damaging the cells through overcharging. To avoid this the charging rate and
duration should be limited. Suitable forSLAbatteries only.
Pulsed Charge
Pulsed chargers feed the charge current to the battery in
pulses. The charging rate (based on the average current) can be precisely
controlled by varying the width of the pulses, typically about one second.
During the charging process, short rest periods of 20 to 30 milliseconds,
between pulses allow the chemical actions in the battery to stabilize by
equalizing the reaction throughout the bulk of the electrode before
recommencing the charge. This enables the chemical reaction to keep pace with
the rate of inputting the electrical energy. It is also claimed that this
method can reduce unwanted chemical reactions at the electrode surface such as
gas formation, crystal growth and passivation. If required, it is also possible
to sample the open circuit voltage of the battery during the rest period.
Burp Charging
Also called Reflex or Negative Pulse
Charging Used
in conjunction with pulse charging, it applies a very short discharge pulse,
typically 2 to 3 times the charging current for 5 milliseconds, during the
charging rest period to depolarize the cell. These pulses dislodge any gas
bubbles which have built up on the electrodes during fast charging, speeding up
the stabilization process and hence the overall charging process. The release
and diffusion of the gas bubbles is known as "burping". Controversial
claims have been made for the improvements in both the charge rate and the
battery lifetime as well as for the removal of dendrites made possible by this
technique. The least that can be said is that "it does not damage the
battery".
IUI Charging
This is a recently developed charging profile used for
fast charging standard flooded lead acid batteries from particular
manufacturers. It is not suitable for all lead acid batteries. Initially the
battery is charged at a constant (I) rate until the cell voltage reaches a
preset value - normally a voltage near to that at which gassing occurs. This
first part of the charging cycle is known as the bulk charge phase. When the
preset voltage has been reached, the charger switches into the constant voltage
(U) phase and the current drawn by the battery will gradually drop until it
reaches another preset level. This second part of the cycle completes the
normal charging of the battery at a slowly diminishing rate. Finally the
charger switches again into the constant current mode (I) and the voltage continues
to rise up to a new higher preset limit when the charger is switched off. This
last phase is used to equalize the charge on the individual cells in the
battery to maximize battery life.
Trickle Charge
Trickle charging is designed to compensate for the self
discharge of the battery. Continuous charge. Long term constant current
charging for standby use. The charge rate varies according to the frequency of
discharge. Not suitable for some battery chemistries, e.g. NiMH and Lithium,
which are susceptible to damage from overcharging. In some applications the
charger is designed to switch to trickle charging when the battery is fully
charged.
Float Charge
The battery and the load are permanently connected in
parallel across the DC charging source and held at a constant voltage below the
battery's upper voltage limit. Used for emergency power back up systems. Mainly
used with lead acid batteries.
Random Charging
All of the above
applications involve controlled charge of the battery, however there are many
applications where the energy to charge the battery is only available, or is
delivered, in some random, uncontrolled way. This applies to automotive
applications where the energy depends on the engine speed which is continuously
changing. The problem is more acute in EV and HEV applications which use
regenerative braking since this generates large power spikes during braking
which the battery must absorb. More benign applications are in solar panel
installations which can only be charged when the sun is shining. These all
require special techniques to limit the charging current or voltage to levels
which the battery can tolerate.