ELECTRIC VEHICLE BATTERY SYSTEMS MANUAL

7 ELECTRIC VEHICLE BATTERY PERFORMANCE THE BATTERY PERFORMANCE MANAGEMENT SYSTEM A typical electric vehicle (EV) traction battery system consists of a chain of batteries connected in a series, forming a battery pack with nominal voltages ranging from 72 to 324 V and capable of discharge/charge rates of several hundred amperes.

Owing to the fact that no two batteries in a pack are alike, or even that no two cells in a battery are identical or manufactured exactly the same, their parameters—such as capacity—may vary by a few percent.

In the case of a new battery, these factors may not be very noticeable, but as the battery undergoes charge-discharge cycles, later on in the battery life these factors determine the performance of the battery pack. In addition, some cells in the battery undergo a change in their parameters such as open-circuit voltage and internal resistance rather abruptly, due to internal dendritic shorts, corrosion, excessive thermal gradients, or loss of electrolyte due to gassing as in VRLA batteries.

Such phenomena can lead to hydrogen gas build-ups and may pose a fire or explosion hazard if not detected and acted upon early. This problem may be easily detected in a battery of up to 6 to 12 cells. A faulty cell can be easily disguised in a large battery pack consisting of tens or hundreds of series-connected cells. A similar problem exists for an excessively overdischarged (reversed) cell. Thus for the safety of the EV, it is essential to monitor the batteries individually and detect faults early.

In an EV, the battery of marginally lower capacity than the rest of the pack is the first battery to acquire and indicate a fully charged status.

On the discharge side of the cycle, this battery leads the pack and is the first to experience full discharge and reversal of plates. While this battery may not be weaker in any other sense than that it has a relatively lower capacity, it is now the weak link in the chain. This battery will be the first to undergo repeated overcharge and overdischarge, eventually resulting in the failure of the battery.

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