Solutions for active balancing and passive balancing of lithium battery
pack charging. The most important link we need to pay attention to when charging
and discharging lithium battery packs is the balancing link. Currently, there
are two solutions to solve the unbalanced situation of small lithium batteries
in the battery pack: active balancing and passive balancing. The balancing step
is to ensure that the input and output of the lithium battery pack are
reasonable to each lithium battery, so that it can be used reasonably and
effectively without damage.
Lithium battery pack charge balancing method
1. Traditional passive method: In a general lithium battery management
system, each battery cell is connected to a load resistor through a switch. This
passive circuit can discharge individual selected cells. But this method is only
suitable for suppressing the voltage rise of the strongest battery cells in
charging mode. In order to limit power consumption, such circuits generally only
allow discharge with a small current of about 100mA, causing the charge balance
to take up to several hours.
2. Active balancing method: There are many active balancing methods in
relevant information, all of which require a storage element for transferring
energy. If capacitors are used as storage elements, a huge switch array is
required to connect them to all battery cells. A more efficient method is to
store energy in a magnetic field. The key component in this circuit is a
transformer.
Lithium battery passive balancing
Passive balancing of lithium batteries is based on the charging state of
lithium batteries. Its premise is that before charging the lithium battery pack,
each small battery in the battery pack has a different residual charge. This
difference is determined by the different performance parameters of the battery.
Due to the performance differences of each battery Differently, some batteries
will overcharge due to their "weakness" under current impulse, causing
overvoltage. Overvoltage is the greatest damage to lithium batteries. It will
cause serious damage to the lithium battery pack and even make it impossible to
use it again.
The method of passive balancing is: during charging, the current that
causes the voltage of the small battery to rise rapidly can be shunted to the
bypass resistor. By shunting unnecessary charging current to a resistor so that
the resistor can dissipate the current, overcharging of the battery can be
avoided. This power dissipation function of passive balancing limits the current
drawn from the battery within a certain range, allowing all small batteries to
be fully charged.
The disadvantage of the passive balancing method is that because power
dissipation is used as a means, part of the balancing current appears as heat,
which greatly reduces the charging efficiency of the lithium battery pack and
increases the cost of manufacturing lithium batteries. In addition, it cannot
solve the problem of power imbalance in the lithium battery pack when it is
discharged and put on hold. Therefore, if you want to achieve the ultimate goal
of passive balancing - a comprehensive balance of charge and discharge of the
lithium battery pack, further measures need to be taken, but this will all
Increase the manufacturing cost of lithium battery packs.
Solution for active balancing method of lithium battery pack charging
Active balancing abandons the passive balancing method of consuming current
and changes it to a method of transmitting current. The device responsible for
charge transfer is a power converter, which allows the small batteries in the
lithium battery pack to transfer charge whether they are charging, discharging
or idle, so that dynamic balance can be maintained between the small
batteries.
Because the charge transfer efficiency of the active balancing method is
extremely high, it can provide higher balancing current, which means that this
method has better balancing ability when the lithium battery pack is charging,
discharging and idle.
1. Strong fast charging capability: The active balancing function can make
each small battery in the lithium battery pack reach balance faster. Therefore,
fast charging is safer and suitable for high-rate charging with larger
current.
2. When idle: Even if each small battery has reached the equilibrium state
during charging, due to different temperature gradients, the internal
temperature of some small batteries is higher and the internal temperature of
some small batteries is lower, which will also cause the internal temperature of
each small battery to Leakage speeds vary. Test data shows that the leakage rate
doubles for every 10°C increase in battery temperature. The active balancing
function ensures that small batteries in idle lithium battery packs "constantly"
regain balance, which is beneficial to storage in the battery pack. The power
can be fully utilized, so that when the battery pack ends, the remaining power
of a single small lithium battery is minimal.
3. When discharging: There is no lithium battery pack with a discharge
capacity of 100%. This is because the end of the working capacity of a group of
lithium batteries is determined by a certain small lithium battery that is
discharged first, and there is no guarantee that all small lithium batteries
will be discharged. Lithium batteries can be fully discharged at the same time.
Instead, there are individual small lithium batteries that retain unused
residual energy. Through the active balancing method, when the lithium battery
pack is discharging, the internal large-capacity lithium battery will distribute
electric energy to the small-capacity lithium battery. Therefore, the
small-capacity lithium battery can also be fully discharged, and there will be
no residual electric energy in the battery pack. However, the battery pack with
active balancing function has a larger actual storage capacity (that is, it can
release electric energy closer to the nominal capacity).
As a final note, system performance with active balancing methods depends
on the ratio between balancing current and battery charge/discharge efficiency.
The higher the imbalance rate of a set of lithium batteries, or the greater the
charge/discharge rate of a lithium battery pack, the higher the balancing
current required. Of course, this current consumption used for balancing is
quite cost-effective compared to the extra current obtained due to internal
balancing. Moreover, this active balancing is also conducive to extending the
life of the lithium battery pack.
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