What is charge equalization? Lithium battery pack equalization charging
principle
Charge balancing, referred to as equalizing charge, is charging that
balances battery characteristics. It means that during the use of the battery,
the battery terminal voltage is unbalanced due to individual differences,
temperature differences, etc. of the battery. In order to avoid the worsening of
this imbalance trend, It is necessary to increase the charging voltage of the
battery pack and activate the battery to achieve a maintenance method that
balances the characteristics of each battery in the lithium battery pack and
extends the battery life.
A lithium battery pack is composed of multiple single lithium batteries
connected in series. Due to the differences in the cells, inconsistent rise in
terminal voltage during series charging will cause some cells to be overcharged
and some cells to be undercharged. All batteries are connected in parallel
during equalizing charging and in series during normal charging and power use.
During balanced charging, the parallel voltages of all batteries are equal,
achieving forced balancing of each battery.
Lithium battery pack equalization charging method
When the lithium battery pack is produced and stored for a long time, due
to the different static power consumption of each channel of the protection
board and the different self-discharge rate of each cell, the voltage of each
battery string in the entire battery pack is inconsistent. Balance has the
function of equalizing the voltage of the lithium battery pack, so that the
battery pack capacity can be fully charged and discharged, so that the battery
pack can exert its maximum effect.
Commonly used lithium battery pack balancing charging technologies include
constant shunt resistance balancing charging, on-off shunt resistor balancing
charging, average battery voltage balancing charging, switched capacitor
balancing charging, buck converter balancing charging, inductor balancing
charging, etc. When a set of lithium batteries is charged in series, each
battery should be charged evenly, otherwise the performance and life of the
entire set of batteries will be affected during use.
1. Attach a parallel balancing circuit to each single cell of the lithium
battery pack to achieve a shunt function. In this mode, when a battery reaches
full charge first, the balancing device can prevent it from overcharging and
convert the excess energy into heat energy to continue charging the
under-charged battery. This method is simple, but it will cause energy loss and
is not suitable for fast charging systems.
2. Before charging, discharge each cell one by one through the same load to
the same level, and then perform constant current charging to ensure a more
accurate equilibrium state between each cell. However, for battery packs, due to
physical differences between individuals, it is difficult for each cell to
achieve completely consistent ideal effects after deep discharge. Even if the
same effect is achieved after discharging, new imbalances will appear during the
charging process.
3. Timing, sequencing, and individually detecting and evenly charging the
single cells in the lithium battery pack. When charging the lithium battery
pack, it can be ensured that each lithium battery in the battery pack will not
be overcharged or overdischarged, thus ensuring that each battery in the lithium
battery pack is in normal working condition.
4. Using the time-sharing principle, through the control and switching of
switch components, additional current flows into the battery with a relatively
low voltage to achieve the purpose of balanced charging. This method is more
efficient, but the control is more complicated.
5. Use the voltage parameters of each battery as the balancing object to
restore the voltage of each battery to be consistent. During equalization
charging, the capacitor is alternately connected to two adjacent batteries
through a control switch, receives charge from the high-voltage battery, and
then discharges to the low-voltage battery until the voltages of the two
batteries become consistent. This balancing method can better solve the problem
of battery pack voltage imbalance, but this method is mainly used in situations
where the number of batteries is small.
6. The entire system is controlled by a single-chip microcomputer, and each
battery has an independent set of modules. The module performs charge management
on each single battery according to the set program, and automatically
disconnects after charging is completed. This method is relatively simple, but
it will greatly increase the cost when the number of single cells is large, and
is not conducive to reducing the system volume.
Lithium battery pack parallel equalization charging method
1. Diode isolation parallel charging equalization method
Taking 6 single batteries connected in series as an example, turn off the
switches S1-S5 and then connect the charging power supply. The diode is
1N5401-5408. The measured forward voltage drop of the diode is 0.8V at the rated
current of 3A. When the forward voltage drop is 0.7V, the current flowing
through the diode is very small. Lithium iron phosphate battery has a maximum
charging voltage of 3.65V. In fact, in order to extend the battery life, the
maximum charging voltage is set to 3.5V. The charging voltage = 3.5+0.7+0.7=4.9V
plus the line voltage drop is very suitable to use a 5V power supply.
2. Direct parallel charging equalization method
The charging voltage for lithium iron phosphate batteries is 3.5-3.6V, and
the charging voltage for ternary and polymer batteries is 4.1-4.2V. The red
alligator clip leads are welded together and connected to the positive pole of
the charging power supply; the black alligator clip leads are welded together
and connected to the negative pole of the charging power supply. When balancing
charging, first disconnect S1-S5, and the red alligator clips clamp the positive
electrode of each single battery respectively; the black alligator clips clamp
the negative electrode of each single battery respectively, and all batteries
are connected in parallel.
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