today i am going to explain you how to work voltage in and battery charging section step by step using picture diagram.
BATTERY CHARGING
Introduction
The circuitry to recharge the batteries in a portable product is an important part of any
power supply design. The complexity (and cost) of the charging system is primarily
dependent on the type of battery and the recharge time.
This chapter will present charging methods, end-of-charge-detection techniques, and
charger circuits for use with Nickel-Cadmium (Ni-Cd), Nickel Metal-Hydride (Ni-MH),
and Lithium-Ion (Li-Ion) batteries.
Because the Ni-Cd and Ni-MH cells are similar in their charging characteristics, they will
be presented in a combined format, and the Li-Ion information will follow.
NI-CD/NI-MH CHARGING INFORMATION
In the realm of battery charging, charging methods are usually separated into two general categories: Fast charge is typically a system that can recharge a battery in about
one or two hours, while slow charge usually refers to an overnight recharge (or longer).
Slow Charge
Slow charge is usually defined as a charging current that can be applied to the battery
indefinitely without damaging the cell (this method is sometimes referred to as a trickle
charging).
The maximum rate of trickle charging which is safe for a given cell type is dependent on
both the battery chemistry and cell construction. When the cell is fully charged, continued charging causes gas to form within the cell. All of the gas formed must be able to
recombine internally, or pressure will build up within the cell eventually leading to gas
release through opening of the internal vent (which reduces the life of the cell).
This means that the maximum safe trickle charge rate is dependent on battery chemistry, but also on the construction of the internal electrodes. This has been improved in
newer cells, allowing higher rates of trickle charging.
The big advantage of slow charging is that (by definition) it is the charge rate that
requires no end-of-charge detection circuitry, since it can not damage the battery
regardless of how long it is used. This means the charger is simple (and very cheap).
The big disadvantage of slow charge is that it takes a long time to recharge the battery,
which is a negative marketing feature for a consumer product.
Slow Charge Rates
NI-CD: most Ni-Cd cells will easily tolerate a sustained charging current of c/10 (1/10 of
the cell's A-hr rating) indefinitely with no damage to the cell. At this rate, a typical
recharge time would be about 12 hours.
Chester Simpson N National Semiconductor
Some high-rate Ni-Cd cells (which are optimized for very fast charging) can tolerate
continuous trickle charge currents as high as c/3. Applying c/3 would allow fully charging the battery in about 4 hours.
The ability to easily charge a Ni-Cd battery in less than 6 hours without any end-ofcharge detection method is the primary reason they dominate cheap consumer products
(such as toys, flashlights, soldering irons).
A trickle charge circuit can be made using a cheap wall cube as the DC source, and a
single power resistor to limit the current.
NI-MH: Ni-MH cells are not as tolerant of sustained charging: the maximum safe trickle
charge rate will be specified by the manufacturer, and will probably be somewhere
between c/40 and c/10.
If continuous charging is to be used with Ni-MH (without end-of-charge termination),
care must be taken not to exceed the maximum specified trickle charge rate.
Fast Charge
Fast charge for Ni-Cd and Ni-MH is usually defined as a 1 hour recharge time, which
corresponds to a charge rate of about 1.2c. The vast majority of applications where
Ni-Cd and Ni-MH are used do not exceed this rate of charge.
It is important to note that fast charging can only be done safely if the cell temperature is
within 10-40°C, and 25°C is typically considered optimal for charging. Fast charging at
lower temperatures (10-20°C) must be done very carefully, as the pressure within a cold
cell will rise more quickly during charging, which can cause the cell to release gas
through the cell's internal pressure vent (which shortens the life of the battery).
The chemical reactions occurring within the Ni-Cd and Ni-MH battery during charge are
quite different:
The Ni-Cd charge reaction is endothermic (meaning it makes the cell get cooler), while
the Ni-MH charge reaction is exothermic (it makes the cell heat up). The importance of
this difference is that it is possible to safely force very high rates of charging current into
a Ni-Cd cell, as long as it is not overcharged.
The factor which limits the maximum safe charging current for Ni-Cd is the internal
impedance of the cell, as this causes power to be dissipated by P = I2R. The internal
impedance is usually quite low for Ni-Cd, hence high charge rates are possible.
There are some high-rate Ni-Cd cells which are optimized for very fast charging, and
can tolerate charge rates of up to 5c (allowing a fast-charge time of about 15 minutes).
The products that presently use these ultra-fast charge schemes are cordless tools,
where a 1 hour recharge time is too long to be practical.
The exothermic nature of the Ni-MH charge reaction limits the maximum charging current that can safely be used, as the cell temperature rise must be limited.
At present, there are no makers of Ni-MH batteries that recommend charging rates
faster than 1.2c (and the chances of that changing are not very good).
Fast Charge: Possible Cell Damage
Caution: Both Ni-Cd and Ni-MH batteries present a user hazard if they are fast charged
for an excessive length of time (subjected to abusive overcharge).
When the battery reaches full charge, the energy being supplied to the battery is no
longer being consumed in the charge reaction, and must be dissipated as heat within
the cell. This results in a very sharp increase in both cell temperature and internal pressure if high current charging is continued.
The cell contains a pressure-activated vent which should open if the pressure gets too
great, allowing the release of gas (this is detrimental to the cell, as the gas that is lost
can never be replaced). In the case of Ni-Cd, the gas released is oxygen. For Ni-MH
cells, the gas released will be hydrogen, which will burn violently if ignited.
A severely overcharged cell can explode if the vent fails to open (due to deterioration
with age or corrosion from chemical leakage). For this reason, batteries should never
be overcharged until venting occurs.
In later sections, information is presented which will enable the designer to detect full
charge and terminate the high-current charge cycle so that abusive overcharge will not
occur.
Fast Charge Current Source
Both Ni-Cd and Ni-MH are charged from a constant current source charger, whose current specification depends on the A-hr rating of the cell.
For example, a typical battery for a full-size camcorder would be a 12V/2.2A-hr Ni-Cd
battery pack. A recharge time of 1 hour requires a charge current of about 1.2c, which
is 2.6A for this battery.
A cost-effective method to design a current source for this application would be to use
an AC-DC wall cube to provide a DC voltage to a switching converter that is set up to
operate as a constant-current source.
Figure 1 shows a schematic diagram of a circuit which will fast-charge a 12V Ni-Cd or
Ni-MH battery at 2.6A and trickle charge it when the converter is shut off.
Note that the circuit must have a shutdown pin so that the end-of-charge detection circuit(s) can terminate the fast charge cycle when the battery is full (the LM2576 has a
low-power shutdown pin built in).
A temperature sensing end-of-charge detection circuit suitable for use with this charger
will be detailed later in this paper.
The LM2576 is a buck (step-down) switching regulator, used as a constant-current
source set to 2.6A. It provides good power conversion efficiency (about 80%) and operates from a wide input voltage range.
A constant-current feedback loop is established by holding the voltage at the Feedback
pin of the LM2576 at 1.23V.
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