In practical application, according to the requirements of a brand of
electric bicycle factory, the design and implementation of two groups of 36V8A.h
lithium manganate power lithium battery pack protection plate, 10 series,
parallel, 36V8A.H. The single lithium ion battery protection chip adopts the
S28241 of Japan Seiko Company. The protection board is mainly composed of the
main circuit, the control circuit, the shunt discharge branch, the filter, the
optical coupling isolation and the level conditioning circuit. The discharge
branch current is about 800mA, and 510 resistors are used to form a resistance
network.
The debugging work is divided into two parts: voltage test and current
test. Voltage test includes charging performance detection overvoltage,
equalized charging and discharge performance detection undervoltage two steps.
The battery analog power supply can be used instead of the actual battery pack
for testing. Because of the series of multiple batteries, the test cost of this
method is high. You can also use the assembled battery pack to directly test,
cycle charge and discharge of the battery pack, observe whether the protection
device is normal operation under overvoltage and undervoltage, record the
real-time voltage of each battery under overcharge protection, and judge the
performance of balanced charging. However, this method takes a long time to test
once. When the battery pack is tested for charging performance, the 3-bit
semi-precision voltmeter is used to monitor the charging voltage of 10
batteries. It can be seen that each battery is within the normal working voltage
range, and the difference between the monomers is very small. The voltage
deviation in the charging process is less than 100mV, the full charging voltage
is 4.2V, and the voltage deviation is less than 50mV. The current test part
includes two steps: overcurrent detection and short circuit detection. Over
current detection can be in the resistance load and power loop between a series
of ammeter, slowly reduce the load, when the current increases to the over
current value, to see whether the ammeter indicates disconnection. Short circuit
detection can directly short-circuit the positive and negative electrodes of the
battery pack to observe the ammeter status. Under the premise that the device is
in good condition and the circuit is welded correctly, the current can also be
directly tested by the status of the power indicator on the protection
board.
In practical use, considering the external interference may cause the
battery voltage is not stable, it will cause voltage extremely short time
over-voltage or under-voltage, leading to misjudgment battery protection
circuit, so the protection chip with a corresponding delay logic, when
necessary, can add the delay circuit on the protection board, this will
effectively reduce the external interference caused by the possibility of a
protection circuit misoperation. When the battery pack is not working, the
switching devices on the protection board are disconnected, so the static loss
is almost 0. When the system is working, the important loss is the on-state loss
on the two MOS tubes in the main circuit. When the equalization circuit is
working in the charging state, the thermal loss of the resistance in the shunt
branch is large, but the time is short, and the overall dynamic loss is at an
acceptable level in the normal working cycle of the battery pack.
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