GP    Nickel Metal Hydride     Technical Hand Book

Contents:


Source: http://www.gpbatteries.com.hk/catalogues/NiMH_technical.pdf
 


3 Charging Method

 

3.1 Overview


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.
 

3.2 Charging Method


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 batterys 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 cells internal contents. These two factors are considered to be the major limitations to the batterys 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.
 

3.2.1 Constant current charging


The advantages of the constant current charging method include high charging efficiency, flexibility, and position control of input capacity.



 

3.2.2 Fast charging


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.
 

3.2.3 Charge control


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 control

The 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 0.8C/min. is recommended for charge termination; for 1C to 3C a higher rate of 0.8-1C/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 45-50C.
 

3.2.4 Standard charge


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.
 

3.2.5 Trickle charging


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.
 

3.2.6 Charging temperature


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 10C to 45C will yield the highest efficiency, which begins to drop at or above 45C. Conversely, repeated charging at less than 0C 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 0C to 45C under standard charging conditions, but preferably at 10C to 45C under fast charging conditions


 



 
 

6 Proper Use and Handling

 

6.1 Restriction on Usage


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:
 

6.1.1 Charging / discharging current


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: 0.8C/minute (0.5C to 0.9C)
  0.8-1C/minute (1C)
Temperature control: 45-50C
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.
 

6.1.2 Reverse charging


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.
 

6.1.3 Parallel charging


Parallel charging is generally not recommended, please consult authorized GP personnel for possible exceptions to connecting the batteries in parallel charging.
 

6.1.4 Charging / discharging temperature


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: 0C to 45C
Fast charge: 10C to 45C
Discharge: -20C to 50C
Storage: -20C to 35C

Look at the Graph2

High temperature series cylindrical:
 
Standard charge: 0C to 70C
Discharge: -20C to 70C
Storage: -20C to 35C

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.
 

6.1.5 Over-discharging / overcharging


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.