Contents:
Source: http://www.gpbatteries.com.hk/catalogues/NiMH_technical.pdf
One crucial difference between the primary and secondary battery
is the ability to restore energy after discharging. This restoration of
energy is therefore a very important area to be considered in secondary
battery applications. Since different battery systems have their own characteristics
and applications have their own integrated electrical input/output requirements,
it is vital to select a charging method that suits both the battery system
and the application. Improper charging will lead to poor battery performance
or failure of the application.
Like NiCd, the main concern in charging a NiMH battery is the build-up
of temperature and internal pressure due to high overcharge rates. As previously
mentioned, the cell design applies the concept of oxygen recombination
in lowering the battery’s internal oxygen level during standard charging.
However, if the cell is subjected to severe charging conditions (such as
overcharging at a current rate over 1C), the rate of oxygen evolution from
the positive electrode increases rapidly, exceeding the recombination reaction
rate. As the oxygen recombination reaction is exothermic, this results
in excessive oxygen pressure and increased temperature. The excessive pressure
will then be released through the safety vent causing a reduction in the
cell electrolyte; the excessive heat will eventually degrade the cell’s
internal contents. These two factors are considered to be the major limitations
to the battery’s service life. For this reason, charge control is very
important in battery charging. GP NiMH cylindrical cells are designed to
be able to charge up to 1C rate. For applications that require higher charging
rates, please contact GP.
In secondary battery charging the two most commonly used methods are
constant voltage charging and constant current charging. As with the NiCd
system, constant voltage charging is not recommended for NiMH, due to thermal
runaway under overcharging conditions. As mentioned earlier, the heat generated
by the overcharge current can cause a significant rise in battery temperature,
which will cause a drop in the battery charging voltage. In constant voltage
charging, the overcharge current is determined by the potential difference
between the power source and the battery charging voltage. The increased
difference between the power source and the battery charging voltage, due
to the temperature rise, will also augment the overcharge current. This
increase in the overcharge current will lead to a further increase in cell
temperature. This positive feedback cycle of cell temperature and overcharge
current will not run down until the battery fails or until the current
limit of the charger is reached. For this reason, constant voltage charging
should not be used in charging NiMH batteries, and charge control should
be employed if this method cannot be avoided.
The advantages of the constant current charging method include high
charging efficiency, flexibility, and position control of input capacity.
GP NiMH batteries use constant current charging as the basis of
the charging method. Depending on different operational requirements, constant
current charging can be further classified according to the charging rate.
Charging at a current rate of 0.5C to 1C, or higher (up to 3C), is considered
fast charging. As explained earlier, if the charging current is too high
(1C or above), the cell internal pressure and temperature will rise at
the end, resulting in degraded cell performance and electrolyte leakage.
Various methods are recommended to help control charging, so as
to prevent gas pressure and temperature build-up due to overcharging. Proper
charge control will provide a longer battery service life.
a) dT/dt controlThe detection of the rate of temperature rise when the battery approaches a state of full charge (dT/dt control) is considered to be the best form of charge control. When charging at a current rate of 0.5C to 0.9C, a temperature rate change of 0.8°C/min. is recommended for charge termination; for 1C to 3C a higher rate of 0.8-1°C/min. should be chosen.
b) -dV control
Detecting the value of the voltage drop after reaching peak voltage is the most commonly used charge control method in fast charging GP NiMH batteries. A -dV value of 0-5mV/cell is recommended when fast charging GP NiMH batteries, while a -dV value of 2mV/cell is found to provide the best balance between charge termination and service life performance.
c) Charging time control (back up only)
An easier way to control fast charging of GP NiMH batteries is to control the elapsed time following commencement of charging. However, it is not recommended as the only cut-off method due to overcharging. A charging time equal to 105% of the cell nominal capacity is recommended.
d) Battery temperature control
As increased ambient and cell temperatures result in high cell internal pressure, it is highly recommended to have temperature control backup for safety and cell performance. When fast charging GP NiMH batteries, the cut-off temperature is recommended to be controlled at 45-50°C.
Apart from fast charging, GP NiMH batteries can also be charged
at a lower current rate of 0.1C. As this charging method is less severe,
charge termination at 160% nominal capacity input is recommended (to help
avoid extended overcharging of the battery). Also, in some applications
where overcharging is necessary, GP NiMH batteries can endure 0.1C continuous
charging for about one year.
In most applications - where cells and batteries need to be in a
fully charged condition - maintaining a trickle charge current to compensate
for the loss of capacity (due to self-discharge) is recommended. The suggested
trickle charge current to be used is 0.05C to 0.1C.
As ambient temperature affects charging efficiency and cell reliability,
it is important to select a suitable temperature for optimising charging
performances.
Generally speaking, a temperature within 10°C to 45°C will yield the
highest efficiency, which begins to drop at or above 45°C. Conversely,
repeated charging at less than 0°C may cause cell internal pressure build-up,
resulting in electrolyte leakage as in high temperature conditions. For
these reasons, GP NiMH batteries can be charged at temperatures of 0°C
to 45°C under standard charging conditions, but preferably at 10°C to 45°C
under fast charging conditions
Knowledge of battery maintenance is crucial to a working battery,
helping to provide a longer period of operation. On the other hand, improper
battery handling or maintenance may lead to unnecessary battery defects
or problems, such as electrolyte leakage or cell bulging. In order to get
the most out of using GP NiMH rechargeable cells, special care in the following
areas should be considered:
For fast charging GP NiMH batteries, the current rate should be
0.5C to 1C. Trickle charging, which is common in various applications (such
as memory backup), requires a current charging range of 0.05C to 0.1C to
maintain the long-term standby power of the battery. In addition, GP NiMH
batteries can be trickle-charged at 0.1C continuously for one year without
leakage or explosions. Charging current rates higher than 1C are generally
not recommended. However charging with pulses higher than 1C is not uncommon
in some applications. Please contact authorised GP personnel to determine
the applicability of special charging schemes not mentioned in GP product
specifications.
Special attention should be paid to the charge termination method, which
is a critical element in providing an optimised cycle life, yet one which
is easily overlooked. Several charging cut-off mechanisms with related
parameters can be considered:
| Negative delta voltage: | 0-5mV |
| dT/dt: | 0.8°C/minute (0.5C to 0.9C) |
| 0.8-1°C/minute (1C) | |
| Temperature control: | 45-50°C |
| Timer control: | 105% |
Look at the Graph1
These charging cut-off mechanisms can be incorporated into the application
– either together or individually, with the choice of method depending
largely on the charging profile of the application. To avoid unnecessary
battery problems, which might look like quality issues, please contact
authorized GP personnel for implementing the appropriate charging cut-off
method. A wide range of required discharge current rates will be encountered
in different applications, and GP has a variety of battery types for specialised
use. Apart from the standard series for general applications, high temperature
and high drain series are specially designed for applications in high ambient
temperatures and discharge current rates respectively. The maximum discharge
current recommended for batteries of standard series is generally 3C. However,
there are situations where higher currents of shorter duration are permissible.
Reverse charging is one of the battery misuses that can appear to
be a battery defect. If the positive and negative polarities are reversed
when charging, the battery might bulge due to internal gassing. Electrolyte
leakage consequently results due to venting at the safety valve, which
leads to a decrease in capacity. Caution has to be exercised to avoid such
misuse.
Parallel charging is generally not recommended, please consult authorized
GP personnel for possible exceptions to connecting the batteries in parallel
charging.
It is impor tant to understand how ambient temperature affects the
charging and discharging of batteries, especially for obtaining maximum
efficiency in conditions that exceed room temperature. GP recommends the
following temperature range.
Standard, high drain and high capacity series - cylindrical / prismatic
/ 9V:
| Standard charge: | 0°C to 45°C |
| Fast charge: | 10°C to 45°C |
| Discharge: | -20°C to 50°C |
| Storage: | -20°C to 35°C |
Look at the Graph2
High temperature series – cylindrical:
| Standard charge: | 0°C to 70°C |
| Discharge: | -20°C to 70°C |
| Storage: | -20°C to 35°C |
Using or storing the battery beyond the recommended temperature range
leads to deterioration in performance. For example: leakage, shortening
of battery life, and lowering of charging efficiency may occur at higher
temperatures. At sub-zero temperatures, discharge capacity will decrease
due to lower mobility of the ions inside the battery.
Other than discharging C-rate and temperature, another factor affecting
battery life and performance is the discharge cut-off voltage. An appropriate
choice of end voltage not only determines the battery performance, it also
provides the bottom line to avoid over-discharging the battery. GP recommends
1V/cell as the end voltage in most situations. However, there are occasions
when slightly higher than 1V/cell is necessary (to avoid scenarios such
as over-discharge, when the number of batteries in the series is large).
In addition, discharge cut-off lower than 1V/cell should be considered
especially when the discharge rate is very high.
Overcharging also adversely affect battery life, the major cause of which is the extra heat generated by overcharging. When overcharging repeats from cycle to cycle, the accumulated heat will eventually degrade the battery life. Therefore, incorporating a proper charging cut-off mechanism is a critical element in ensuring a long battery life.