4.2.3　Fault and Safety Protection

Text（课文）

The proposed circuit in Figure 4.7（a）can be used to balance the battery cells during charging and to isolate faulty cells during charging or discharging.There are two transistors associated with each battery cell.These transistors are typical low on-state resistance MOSFETs，with typical resistance at the milliohms range.

During normal operation，the parallel-connected transistors are turned off and the seriesconnected resistors are turned on，as shown in Figure 4.7（b）.During fault conditions，assume B1 has a fault，transistor T1P which is parallel-connected to the faulty cell（B1 as shown in Figure 4.7（c））is turned on while the series connected transistor T1S for the same cell is turned off.

Figure 4.7　A balance and isolation circuit to manage battery pack in a PHEV

（a）Configuration；（b）Normal operation；（c）B1 fault or bypass

During charging，the balancing can be performed using the same circuit.When B1 is full，T1P is turned on and T1S is turned off，so all charging current goes through the bypass transistor T1P.This is more suitable for constant current charging because when one cell is discounted from the battery string，the whole pack voltage is reduced by one cell.Therefore，the charger voltage has to be adjusted accordingly.

The advantage of this circuit is that it does not consume any power（other than the transistor loss）during balancing，and it can isolate any faulty cell during the usage of the battery pack.The disadvantage is that the loss on the series connected transistors can be very large during high current operations.Hence，this design is not suitable for large power applications.For example，if the MOSFET voltage at 200 A is 0.2 V；（RDS＿ON＝0.001 Ω which costs＄2），the loss on the single MOSFET will be 2002×0.001＝40（W）.For a pack that is composed of 100 cells in series，the loss will be 4 kW at peak.However，when the current is reduced to 60 A，the pack loss due to the transistors will be only 360 W.However，the pack will only be able to supply 24 kW assuming each cell is rated at 4 V.

New words and expressions（单词和短语）

1.New words（单词）

2.Expressions（短语）

Notes to the text（难点解析）

★1.The proposed circuit in Figure 4.7（a）can be used to balance the battery cells during charging and to isolate faulty cells during charging or discharging.图4.7（a）中的电路可以在充电期间用来均衡电池组单位，并且在充电或放电期间隔离故障单体。

★2.During normal operation，the parallel-connected transistors are turned off and the seriesconnected resistors are turned on.在正常运行时，并联的晶体管关断，串联的电阻导通。

Exercises（练习）

◆1.Translate the following passages（expressions）into Chinese（英译汉）

（1）This is more suitable for constant current charging because when one cell is discounted from the battery string，the whole pack voltage is reduced by one cell.

（2）It can isolate any faulty cell during usage of the battery pack.

（3）turn on

（4）faulty cell

◆2.Translate the following passages（expressions）into English（汉译英）

（1）关断

（2）并联的晶体管

（3）缺点是在电流很大时，串联的晶体管损耗会很大。

3.Directions:Answer the following questions briefly according to the text.

Can you analyze circuit shown in Figure 4.7？

Reading material（阅读材料）

The fundamental problem is this:different cells in the string all get the same chaining current for the same length of time（they are in series so it cannot be otherwise），but individual cells have different capacities and start with different residual charge，so they will almost certainly need slightly more or less current/charge duration.The problem gets worse as the string gets longer.