Page 125 - Lindens Handbook of Batteries

5.2 PRINCIPLES OF OPERATION

3. Overdischarge below the minimum recommended operating voltage of the cell (also includes
voltage reversal, or the discharging of the cell below 0 V)
4. Charging of primary batteries
5. Improper charge control when charging secondary batteries
6. Imbalance between series cells
These conditions may cause an internal pressure increase within the cells, resulting in an activation
of the vent device or a rupture or explosion of the battery. Internal cell shorts can also cause failures,
although these are rare. These may occur due to impurities being accidentally introduced into the cells
during manufacture. There are a number of means to minimize the possibilities of these occurrences.
Additional failure mechanisms can occur due to the assembly of individual cells into the battery pack
improperly. For example, poor cell connector tab welds, lack of proper insulation between tabs, and
improper case assembly can all lead to latent battery failures.
The use of high quality individual cells does not guarantee a safe battery pack assembly. All
factors must be considered carefully, including the mechanical assembly of the pack, any internal
protection devices or electronics, contacts, monitoring components, and the pack casing.

5.2.1 Charging Primary Batteries

All major manufacturers of primary batteries warn that the batteries may leak or explode if they
are charged. As discussed in Sec. 8.4, some primary batteries can be recharged if done under
controlled conditions. Nevertheless, charging primary batteries is not usually recommended
because of the potential hazards.
Protection from External Charge. The simplest means of preventing a battery from being
charged from an external power source is to incorporate a blocking diode in the battery pack, as
shown in Fig. 5.1. The diode chosen must have a current rating in excess of the operating current
of the device. It should be rated, at a minimum, at twice the operating current. The forward voltage
drop of the diode should be as low as possible. Schottky diodes are commonly used because of
their typical 0.5 V drop in the forward direction. Another consideration in selecting the diode is the
reverse voltage rating. The peak inverse voltage (PIV) rating should be at least twice the voltage
of the battery.

Protection from Charging within Battery. When multiple series stacks are paralleled within a bat-
tery pack, charging may occur when a defective or a low-capacity cell is present in one of the stacks
(Fig. 5.2a). The remaining stacks of cells will charge the stack with the defective cell. At best this
situation will discharge the good stack, but it could result in rupture of the cells in the weak stack.
To avoid this, diodes should be placed in each series string to block charging currents from stack to
stack (Fig. 5.2b).
When diode protection is used in each series stack, the diode will prevent the stack containing the
defective cell from being charged. The diode should have the following characteristics:
1. Forward voltage drop should be as low as possible, Schottky type preferable.
2. Peak inverse voltage should be rated at twice the voltage of the individual series stack.
3. Forward current rating of the diodes should be a minimum of

I
I min = op × 2
N
where I = device operating current
op
N = number of parallel stacks