The illustration compares batteries in terms of safety

July 10 [Wed], 2013, 11:43
Battery manufacturers are tooling up for the electric vehicle, but what would happen if it failed? Could there be a déjà vu of the fuel cell in the 1990s, or the bio fuels in the last decade that cannot survive without heavy government subsidies? The US Department of Energy (DOE) has admitted that some critical Satellite A665D laptop battery parameters of Li-ion are not met. Newer NiMH batteries, which are cheaper and safer than Li-ion, are also suitable for the electric powertrain but these mature systems are often excluded from government grants for research.

There are no ideal contenders for the electric powertrain, and lithium-ion remains a good choice. Out of the five candidates illustrated in Figure 1, Li-nickel-manganese-cobalt (NMC), Li-phosphate and Li-manganese stand out as being superior. The popular Li-cobalt (not listed) used in consumer products was thought to be not robust enough; nevertheless, this high energy dense “computer battery” powers the Tesla Roadster and Smart Fortwo ED.

The illustration compares batteries in terms of safety; specific energy, also known as capacity; specific power, or the ability to deliver high current on demand; performance, the ability to function at hot and cold temperatures; life span, which includes the number of cycles delivered as well as calendar life; and finally cost. The figure does not mention charge times. All batteries offered for EV powertrains can be charged reasonably fast if a suitable electrical power outlet is available. A charge time of a few hours is acceptable for most users, and super-fast charging is the exception. Nor does the table reveal self-discharge, another battery characteristic that needs scrutiny. In general, Li-ion batteries have low self-discharge, and this parameter can mostly be ignored when the battery is new. However, aging when exposed to heat pockets can increase the self-discharge of the affected cells and cause management problems. Among the EV battery candidates, Li-phosphate exhibits a higher self-discharge than the other systems.

None of the five battery candidates in the figure above show a significant advantage over others, and the size of the spider fields are similar in volume, although different in shape. Focusing on one strong attribute inevitably Satellite C640 laptop battery discounts another. NCA, for example, has a high capacity but presents a safety challenge, whereas Li-phosphate is a safer system but has lower capacity. In the absence of a clear winner, car manufacturers include peripherals to compensate for the deficiencies. Battery manufacturers in turn assist by custom-designing the cell to strengthen the important characteristics needed for the application. Here is a brief summary of the most important characteristics of a battery for the electric powertrain.
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