40
If the contact points are placed asymmetrically, such as shown below:
Weld-strap material
(with intrinsic
resistance Rs per
unit length)
Vcell
1
Vcell
2
Vm
2
Vm
1
i (current)
Rs
Rs
Rs
Rs
Vm1 = Vcell1 –
3
x i x Rs and Vm2 = Vcell2 – i x Rs. The result of Vm1 – Vm2 would contain an undesirable offset in
it: Vcell1 – -2( i x Rs) where Voffset =-2( i x Rs).
This offset in the voltage readings would cause the BMS to read that there is more charge in one cell while the
current is flowing in one direction, and have less charge in it while current is flowing in the opposite direction. So
while the battery is discharging, the BMS would try to balance some of the cells, and while it is recharging, the BMS
would try to balance the others. This results in a great deal of wasted heat and energy that contributes to a
reduced performance and service lifetime of the battery.
Additionally, the balancing currents should not flow through the voltage sensing leads. Otherwise, the balancing
currents will affect the voltage reading proportionally, increasing the time needed to achieve proper cell balancing,
if not making it impossible.
