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Battery Pack Control (Monitoring and Management)
When joining cells together, A123 recommends using a Battery Management System
(BMS) to accurately monitor cell voltage, current, impedance and other conditions of
the cells. The BMS may be implemented as discrete circuitry and/or through a
microcontroller. This section covers the following topics:
•
Cell Monitoring
•
Supervising Battery Pack Behavior
•
Cell Balancing
•
Fuel Gauging
•
Integrated Circuits
Cell Monitoring
Cell Voltage
To ensure optimal performance, safety, and durability of the pack, the Battery Management System must monitor
the voltage of each individual series cell in a battery string. The voltage monitoring connections shall be in a place
where they are not affected by high currents going through the interconnection elements.
The diagram below depicts an optimal positioning of the voltage monitoring contact points in an idealized setting:
Weld-strap material
(with intrinsic
resistance Rs per
unit length)
Vcell
1
Vcell
2
Vm
2
Vm
1
i (current)
Rs
Rs
Rs
Rs
In this case, Vm1 = Vcell1 – 2 x i x Rs and Vm2 = Vcell2 – 2 x i x Rs. The result of Vm1 – Vm2 would be exactly what
is desired, and that being Vcell1 – Vcell2.
Control
Protection
Structure
Cell Configuration