The term used to describe the cycle life is the ‘turnover ratio’ or ‘Figure of Merit’ (FOM) defined as: The FOM is the summation of capacity removed from a cell while cycling, divided by the ampere hours of lithium in the cell. The FOM, therefore, depends on the efficiency of the lithium reaction, its stability and reactivity in the complex organic electrolyte and its DOD. The present cycle life goal for the Li-TiS2 system is 1000 cycles at 50% DOD at a C/3 discharge rate. The FOM required to achieve this is 80-100. Values as high as 50 have been achieved in 5 Ah engineering model Li-TiS2 cells. Charge Control A second area that requires additional technology development is the charge control process. Presently, the end of charge (EOC) and end-of-discharge (EOD) voltage of each lithium cell must be controlled to avoid undesirable reactions and safety issues. In the present spacecraft power systems, voltage is controlled on a battery basis. The Ni-Cd system allows this because it is forgiving with respect to overcharge and the depth of discharge is generally limited to 75% DOD in geostationary orbit applications. Even though there are less lithium cells in series than Ni-Cds for a 28 V bus, there could still be quite a divergence in cell voltage across the string. Controlling the voltage on a battery basis, as in the case with Ni-Cds, could result in cell voltage divergence which could cause cells to exceed the EOC and/or EOD voltage limits. One solution to this problem is accomplished by individual electronic cell voltage monitoring which is a cost, reliability and complexity issue. However, it is a viable solution if the lithium battery is considered an enabling technology for a specific application. A second and more desirable solution is a built-in reaction mechanism in each cell that will prevent the voltage from exceeding the identified limits of operation. The advantage of the Li-SO2, inorganic electrolyte, liquid depolarized cell is that this problem has reportedly been solved. Some success has also been achieved in charge control of the UP Li(Al)-FeS2 system. The reaction mechanism results in a controlled voltage of each cell below that of the undesirable irreversible reactions. Rate Capability For many applications high charge and discharge rates are not required. However, for most NASA applications where charge and discharge times are critical, rate capability is a factor. The design factors governing these rates are the surface areas of the positive and negative plates, which affect the current density, and the conductivitity through electrolyte and separator. The goal for charge and discharge rate capability is the C/4 to C/3 rate (the rate at which the capacity of the cell will be depleted in four to three hours). The C/2 rate is generally equal to a current density of 10 ma/cm2. Plate surface area, and therefore current density, can be maximized through the use of many thin plates. This requires developing processes for manufacture. However, the handling and assembly will become more complex as will the uniformity of current flow through the large number of plates. This does not appear to be a significant technical issue in any of the cell types.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==