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Engineering Corner #3
Battery Chargers
By Danny Rockett, GTS Vice President of Manufacturing and Technology
Everyone at GTS knows that we manufacture battery packs for hundreds of different devices. What
some may not know is that GTS also designs and manufactures a wide variety of battery chargers.
These range from a simple charger for a single battery or device, up to chargers which can charge 20
or more battery packs at a time. We even make the unique and valuable Emergency Operations
Charging Case (EOCC), which makes it easy to charge batteries from remote locations, even under the
most difficult conditions.
These chargers are an important part of GTS’s business. The capability to provide both battery packs
and chargers is an excellent example of our complete capabilities—we can satisfy every need a
customer has.
As there is a huge variety of batteries, there is also a large variety of chargers. Different chargers are
needed for differing battery chemistries, battery capacities, and battery voltages. There are four
major battery chemistries, all of which GTS is involved in.
1. The first is the oldest commercial type cell, which are lead acid batteries. Normally GTS uses a
special type of lead acid battery called a SLA (sealed lead acid). This type doesn’t leak and is
commonly used in UPS (universal power supplies), emergency lighting, security alarm systems, etc. A
battery is typically made up of 3 to 6 cells per battery, each cell providing 2 volts each, totaling 6 to
12 volts nominal output voltage. These are widely used but almost always for stationary applications
because they are very heavy.
2. A second type of battery which was very widely used but is now in decline, is the Nickel Cadmium
Battery (NiCad). These cells typically put out 1.2 volts per cell, and can be stacked in large serial
strings of cells with output voltages sometimes being as high as 24 volts (20 cells). A typical GTS
NiCad battery pack is 6 cells in series, totaling 7.2 volts output. One advantage NiCad batteries have
is that they can deliver very large current surges, and if used properly can last a large number of
cycles. Still, NiCad is in decline due to lower capacity and higher weight than more modern
technologies.
3. The third type of battery GTS manufactures is a made up of cells using a chemistry called Nickel
Metal Hydride (NiMH). It puts out about 1.25 volts per cell, and can also be used in large serial strings.
A typical GTS NiMH battery will use 6 cells totaling 7.5 volts output. This type of cell is also in decline,
and is very similar to a NiCad in operation, but requires a slightly different type of charge than a
NiCad cell based battery.
4. The fourth and most widely type of battery GTS uses is two versions of a similar cell, one called
Lithium-Ion (Li-ion, which come in cylindrical and rectangular cases), and a new type of Lithium-ion
called a Lithium Polymer cell (Li-P), which comes inside a flexible metalized foil pouch and can be
rectangular or square. These cell types typically are 3.7 volts per cell, and can be put in parallel for
more energy capacity, or in series for higher voltages, 7.4 volt packs (two cells in series) being very
common. Both of these Li-ion cells are becoming more popular due to their low weight and large
energy storage capacities.
So why write about differing battery types, when the subject is battery chargers? The reason is that if
you noticed, the cells run at differing voltages:
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2 volts per cell for a SLA
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1.2 volts per cell for NiCad
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1.25 volts per cell for NiMH
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3.7 volts per cell for the Li-ion type.
The methods required for optimum charging are also different. In addition to the differing voltages,
there are many differing capacities, rated in Milliamp/Hours (1/1000 of an Amp per hour), or
Amp/Hours. A typical battery pack may contain 2 Li-ion cells in series (totaling 7.4 volts) at a
capacity of 3400 milliamp/hours (also noted as 3.4 amp/h). This capacity is called the ―C‖ value of a
battery.
In charging a battery, the amount of current which can be put in over time is usually a fraction of the
total C rate of a battery. Therefore a charge for a 4 amp hour battery, put into the battery at a 1 amp
rate would be called a C/4 (spoken as ―C over 4‖) charge rate. The charge rate which a battery can
withstand depends on the cell type’s chemistry, size, and capacity. If too much current is put into a
battery too fast, the battery cannot effectively absorb the charge and overheating and damage can
occur to the cells. A charger therefore has to deliver a charge current tailored to the capacity needs
and capabilities of a particular cell. The charger also has to regulate the current going into the
battery in what’s known as Constant Current Mode (CC Mode). This current regulation is important or
a battery will initially absorb very high currents, which can destroy both the charger and the battery.
Lead-based chemistries typically use a combination of constant current and constant voltage mode.
Constant current regulates the current to a preset value, e.g. 1 amp, and Constant Voltage Mode (CV
Mode) regulates the voltage at a preset value. For charging a 12V SLA battery, the voltage would
typically be about 14 volts. What this means is the current will be regulated at 1 amp until the
voltage on the battery reaches close to 14 volts; at that point the current is reduced and the voltage
is allowed to stay at 14 volts until the current drops to a very low level. At that point the charge is
considered complete. A SLA charger can also perform a ―float‖ charge, which keeps a battery fully
charged to compensate for the battery losing charge over time, by a phenomenon known as ―selfdischarge.‖ There is usually a timer included in the charger to ensure if something goes wrong in
detecting the termination current the charger will automatically cut off to avoid damage to the
battery.
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The nickel-based chemistries (NiCad, and NiMH) use a constant current mode charging method which
charges at a given C rate (often C/2), until the battery reaches a preprogrammed voltage or a
temperature rise is detected. One method based on voltage looks for the battery pack to reach a
peak voltage and then begin to decline again (a quirk of Nickel chemistry); this drop in voltage after a
peak is called a – delta V charge termination method. The voltage which has to be detected is very
small, and the charger has to be very accurate to measure the voltage properly. The charger then
reduces the charging current for a short period and then stops the charge. Another method which is
sometimes used is based on the temperature rise of the battery, but this technique is not very
common. Like the SLA charger, there is usually a timer to ensure that the charge stops before
damage occurs, should the termination detection method malfunctions.
The Lithium chemistry-based batteries, like SLA batteries, use a constant current (CC)/constant
voltage (CV) charging method. When the battery initially charges, the current is regulated to a safe
value. Once the voltage on the battery begins to get close to 4.2 volts per cell (8.4 volts in our
typical 2-cell pack), then the current drops down and the voltage is held steady at this preset voltage,
e.g. 8.4 volts. Once the current drops below a minimum value, the charge is considered complete and
the charger stops. Here again an additional safety timer is usually included in the charger to prevent
damage if something goes wrong.
Some chargers can charge more than one chemistry. Typically these multi-chemistry chargers are
called ―Tri-Chem‖ chargers and they can charge NiCad, NiMH, and Lithium chemistries. GTS makes
several versions of ―Tri-Chem‖ chargers. These chargers are ―intelligent‖ and they detect the
different cell chemistries and adjust the charge technique to optimize for that specific battery type.
A typical GTS charger includes a power supply to convert the input voltage, 12 VDC for vehicles, 120
VAC or 240 VAC for wall outlets, and then converts this input voltage to a value which is optimum for
the charger. We commonly use 13.5V. (The EOCC also works off of 24 VDC sources.)
This voltage is then used by the charger and regulated in CC or CV Mode (or both), and controlled by a
charging control microchip. Usually there is a microprocessor (tiny computer) that controls the
charger. This microprocessor may also ―talk‖ which the battery via a digital communications pin(s) on
the battery pack. These types of chargers and battery packs are called smart chargers and smart
battery packs. GTS is expanding its line of both since the market is demanding batteries and chargers
with ever-increasing capabilities.
GTS is devoted to ongoing advancements in both charging methods and battery technology to ensure
that we stay ahead of industry trends. GTS marketing and engineering stays abreast of these
developments and works very hard to be the market leader in chargers and batteries. At GTS,
delivering cutting-edge mobile power technology is a team effort.
Until next time, stay charged!
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