Fast charging capacity

Nickel-cadmium sealed cells are now a commercially important consumer product. They find use, both as button and cylindrical cells, in portable cordless appliances such as power tools, electric razors and photoflash apparatus, and increasingly in hybrid mains/battery equipment such as portable tape recorders, radios and television receivers. Many of these cells are readily interchangeable with primary batteries. In recent years, advances in design have increased recharge rates cylindrical cells with sintered electrodes can now be fast charged from full discharge at up to the C/l rate of 80% of capacity. 

Capacity measurements are very useful in determining the coverage by adsorbed intermediates formed in fast charge transfer processes, which give rise to an appreciable adsorption pseudocapacitance. The experimental methods which can be employed have been reviewed recently, and will not be discussed here further. 

Tests were performed on three lithium-ion battery chemistries to determine the fraction of the Ah capacity that could he returned without current taper. The results of the testing are summarized in Table 3.5. The LTO chemistry has a clear advantage over the other chemistries especially compared to the nickel cobalt manganese oxide chemistry for fast charging. 

Table 3.5 Maximum Charge Capacity without Taper for Fast Charging of Lithium-ion Batteries of Various Chemistries...

The characteristics of the 50-Ah LTO cell from Altaimano used in the fast charging tests are given in Table 3.6. The cell had a resistance of about 0.9 mO resulting in a Cr value of 0.045, which is typical for lithium cells, but not particularly low for an LTO cell (see Table 3.2). The Ah capacity of the cell varied little with discharge rate up to 6C. Based on the test results in Section 3, it is reasonable to expect that the 50-Ah cell would have good fast charging characteristics. 

FIGURE 3.10 Life cycle data (cell Ah capacity) for the 24-V module for 1000 cycles with fast charging. (For color version of this figure, the reader is referred to the online version of this book.)... 

To prevent overcharge and lithium plating, batteries are typically manufactured with an excess capacity of the negative electrode. However, even with excess negative capacity, lithium can deposit if the potential drop between negative electrode and electrolyte reaches 0 V (vs Li- -/Li). This condition occurs in graphite electrodes upon fast charge, or... 

A fast charge is defined as a method of charge that will return the full capacity in less than 4 h. However, many applications require 1 h or less. 

These data show that the thin-plate VRLA battery can be fast-charged to 100% of the rated capacity in less than 1 h. A constant-voltage charger set at 2.5 to 2.55 V per cell and capable of the 3C to 4C rate of charge is preferred. It should be noted, as discussed, that charging at 2.7 V per cell for prolonged periods will damage the battery. 

NiMH, due to its KOH/H2O electrolyte, provides performance as function of temperature similar to nickel-cadmium with some exceptions. Capacity dependence as a function of temperature is strongly influenced by the active material selection of the metal hydride and nickel hydroxide, and typical commercial NiMH performance is presented in Fig. 30.15. At low temperature, NiMH has reduced discharge capacity and power compared to NiCd because the metal hydride electrode is polarized due to the generation of water on discharge. On the other hani whereas NiCd has some difficulty with charging at cold temperatures, and particularly fast charge at eold temperature, NiMH is mueh less sensitive to this effect. 

---