The development of new energy and electric vehicles will use lithium
batteries with relatively high energy density. When lithium batteries are used
in series, in order to ensure the consistency of the battery voltage, a voltage
balancing circuit will inevitably be used. Today I would like to share with you
some battery balancing circuits that I have used in my work. I hope it will be
helpful to you.
The simplest balancing circuit is load consumption balancing, that is, a
resistor is connected in parallel to each battery and a switch is connected in
series for control. When the voltage of a certain battery is too high, the
switch is turned on and the charging current is shunted through the resistor. In
this way, the charging current of the battery with high voltage is small, and
the charging current of the battery with low voltage is large. In this way, the
battery voltage is balanced.
However, this method can only be applied to small-capacity batteries and is
unrealistic for large-capacity batteries.
Schematic diagram of load consumption balancing
The second balancing method I have not tried is the flying capacitance
method. To put it simply, each battery is connected in parallel with a
capacitor. By switching the capacitor, the capacitor can be connected in
parallel to its own battery or to an adjacent battery.
When the voltage of a certain battery is too high, first connect the
capacitor in parallel with the battery so that the capacitor voltage is the same
as the battery. Then switch the capacitor to the adjacent battery and the
capacitor discharges the battery. Complete the transfer of energy.
Since capacitors do not consume energy, they can transfer energy
losslessly. But this method is too cumbersome. Today's power batteries often
have dozens of cells connected in series. If this method is adopted, a lot of
switches are needed to control it.
The operating principle diagram of the flying capacitance method only draws
the balancing principle diagram of two adjacent batteries.
The first time I did balancing was to charge a power battery pack. Two
groups of batteries with a capacity of 80ah were connected in parallel, and the
balancing current was required to be 10a. The balance principle I originally
knew was not enough. Such a large current is equivalent to one small module. In
the end, n small modules were actually used in series, and each battery was
connected in parallel with a small module. If the single battery voltage is low,
At the set value, start the corresponding parallel module to start charging the
low-voltage battery, supplement energy to increase the voltage, and complete
balancing.
The automatic equalization method can use the multi-channel output method
of a transformer I mentioned earlier.
If you want to use the circuit diagram below to make a multi-output flyback
power supply and use the output voltage of each module to balance the battery, I
guess you need a lot of skills to do it, because the cross regulation rate alone
is It's very difficult. However, using this circuit, we can change our thinking.
Each output does not need voltage regulation. Of course, in order to avoid open
circuit damage to the output capacitor, we can do a simple primary side
response. Then an electronic switch is connected in series between each output
and the battery. Since this balance works in conjunction with the battery
management system, each output only needs to be connected in series with an
electronic switch and controlled by the management unit. We will determine which
voltage is grounded. This electronic switch can be turned on, and the power
output will charge the battery until the voltage of all single cells reaches our
desired value.
Using this balancing method, we have done balancing of 1000AH, 7-string
batteries and 300AH, 80-string batteries. After the balancing is completed, the
voltage of all single cells can reach within 5mV.
Structure diagram of multi-winding transformer method
Automatic balancing can also use energy transfer methods. The so-called
energy transfer can either take energy from the entire set of voltages to
supplement the low voltage, or it can take energy from a battery with too high
voltage to react to the entire set of voltages.
I have used the second method to complete battery balancing in a
communication power supply system. The circuit schematic diagram is as
follows:
What we were doing at that time was the balancing of 16 strings of lithium
batteries, which were divided into two groups, with 8 batteries in each group
connected in series. Only 6 are drawn here to describe the working
principle.
If the voltage of battery B5 is too high, control Q5 to operate in PWM
mode. When Q5 registers, inductor L5 stores energy; when Q5 is turned off, the
energy stored in the inductor will charge batteries B1-B4 through D5, reducing
the battery voltage of B5 and raising the voltage of other batteries. Voltage,
the same principle can be used to analyze the working process of other battery
packs when the voltage is too high.
During the experiment, this method was used to balance the two groups. When
there is a discrepancy between the two groups, bidirectional DC-DC can be used
for energy conversion, which requires a smaller number of modules and is more
convenient to design.
I did not use bidirectional DC-DC at that time, but simply used energy
consumption to balance the batteries between the two groups. Judging from the
final test results, the battery balance is still relatively good.
During the balancing process, if providing one charging module for each
battery feels like overkill, and the energy-consuming model does not meet the
technical requirements, that is, automatic balancing is required, then the
method of one-to-one transformer with multiple outputs mentioned earlier , may
be more suitable for your needs, just choose a suitable transformer and make a
multi-output flyback power supply with primary side reaction and current
limiting.
In fact, with the development of the use of power batteries, not only
balance, but also the protection of battery overcharge and overdischarge, which
is what we often call protection boards, will become more and more widely used.
We know that for the original 18650 battery cells, it is common to use ICs for
protection boards of more than a dozen strings to realize short circuit,
overcharge protection, and over-discharge protection. But what if there are
dozens of strings of batteries? I wonder if any netizens who have had access to
this information can share with us.
These are the four battery balancing methods I have tried so far. The
balanced batteries range from 2AH to 1000AH, and the number of cells in series
ranges from 7 to 120.
My personal feeling is as follows:
1. For battery packs within 10AH, it may be a better choice to choose the
energy-consuming type, which is easy to control.
2. For battery packs with dozens of AH, it should be feasible to use a
one-to-multiple flyback transformer and combine it with the battery sampling
part to do battery balancing.
3. For battery packs with hundreds of AH, it may be better to use an
independent charging module, because the balanced current of batteries with
hundreds of AH is around 10 A. If there are more cells in series, the balanced
power will be very low. Large, the leads are outside the battery, and it may be
safer to use external DC-DC or AC-DC balancing.
Current balancing uses battery voltage unity as the end condition for
balancing, but as SOC calculations become more and more accurate, capacity
balancing should be the trend in the future.
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