RAM cells

At low temperatures down to -20 °C RAM cells function but their performance is decreased the decrease is more severe for higher rates. At higher temperatures up to 50 °C the low-rate performance is un-... 

Compared with nickel-cadmium and nickel-metal hydride systems RAM cells exhibit very low self-discharge, making them ideal for intermittent or periodic use without the need to recharge before using, even in hot climates. Figure 6 shows a comparison of the temperature characteristics, for various battery systems in the form of Arrhenius diagrams. 

However, even at room temperature, the shelf-life of batteries with nickel oxide cathodes (Ni-Cd, Ni-MeHy, and Ni-Zn batteries) is a source of difficulties for the consumer who relies on the state of charge of his power source when he needs it-without charging time available. Figure 7 compares the self-discharge of RAM cells with Ni-Cd and Ni-MeHy cells at 20 °C. 

Using external chargers, the discharged cells are removed from the battery-operated device and placed in an approved RAM charger where the cell terminals make proper contact for charging. This is the appropriate method when RAM cells are used in applications designed also for... 

Figure 7. Comparison of the self-discharge of RAM cells vs. Ni-Cd and Ni-MeHy cells at 20 °C.

Benchmarq Microelectronics, Inc., designed a special (more expensive) charging chip for RAM cells, connected in series [40]. The advantage is a complete disconnection of the load when the first cell reaches the predetermined cut-off voltage. Cell reversal is thereby eliminated. 

J. Daniel-Ivad, K. Tomantschger, Charge retention of Hg-free RAM cells, Proc. 184th Meeting of the Electrochem. Soc., Oct. 1993. 

Provided that the reduction does not exceed a level equivalent to Mn0li33 the reaction can be reversed and the cathode recharged. In practice this means limiting the discharge at 0.9 V. The rechargeable alkaline manganese (RAM) cell is discussed in Chapter 6. 

Fig. 6.14 Voltage profile for constant current discharge of a RENEWAL AA RAM cell. (By permission of Rayovac,)...

RAM cells are manufactured and shipped charged and have an initial capacity of about 1.8 Ah for AA-sized cells discharged at 50 mA (in comparison with, say, 2 Ah for an equivalent primary cell). This capacity falls to 1 Ah after storage for 3 years at room temperature. At higher drains, the initial capacity drops to about 0.6 Ah at 400 mA (Fig. 6.14). Cells are designed to operate within a temperature range of 0-65°C. The higher internal resistance of RAM cells limits their maximum continuous output current and also their peak output currents in comparison both with primary cells and with nickel-cadmium and nickel-metal hydride secondary cells. A new cell will have an internal resistance of approximately 0.1 2, but this will rise to 0.25 2 with use. 

Fig. 6.15 Effect of charge-discharge cycles on discharge curve of a RENEWAL A A RAM cell, Constant current discharge at 50 m A. (a) Cycle 1, (b) cycle 5, (c) cycle 10, (d) cycle 25, (e) cycle 50. (By permission of Rayovac.)...

The self-discharge rate of RAM cells is approximately 0.01% per day, which gives them a clear superiority over nickel-cadmium and nickel-metal hydride cells (Fig. 6.19). 

Fig. 6.18 Repetitive short discharge cycles for a RENEWAL AA RAM cell 400 mA constant current discharge for 10 minutes followed by standard recharge, (a) Cycles 1-4, (b) cycles 707-800, (By permission of Rayovac.)...

The commercialization of RAM cells has presented a rather novel type of power source to the consumer market. First, they are ready to use as purchased and retain their charge well until used or after charging (Fig. 6.19), and thus behave rather like a primary battery in this respect. Second, they are not much more expensive than an alkaline manganese primary and thus are very cost-effective, even if capacity fade limits their use to 20 or so cycles. It will be interesting to discover how deeply this new technology penetrates the primary cell market in the near future. 

Source Reprinted from K. Kordesch and Faistaucer, RAM Cells with Low-Cost Chargers May Compete with MN02-ZN Primaries on the Global Consumer Market in Batteries for Portable Applications and Electric Vehicles, Holmes and Landgrcbe, eds., Electrochemical Society, 1997, p. 924-925. Reproduced with permission of the Electrochemical Society. bZn-carbon plus Alkaline. 

Continuous research in primary alkaline manganese batteries ended up in the development of rechargeable alkaline manganese (RAM) cells. The design of these cells dated to 1975 [1], These batteries are basically an extension of the primary alkaline batteries. They also use zinc for the negative electrode, manganese dioxide for the positive electrode, and an aqueous solution of potassium hydroxide for the electrolyte. 

General Characteristics of RAM Cells with 0.025 % and Zero % Mercury... 

The major design changes made to primary alkaline cells were the use of improved cathode and anode formulations, the limitation of the anode capacity to approximately 1/3 of the cathode capacity to prevent overdischarge of the cathode, the application of improved separators and the integration of means to enable moderate cell abuse. Cell components (cans and closures) and raw materials (EMD, graphite, zinc) used are identical to the ones used in primary alkaline cells. The electrode capacity balance accounts for the reduced capacity of RAM cells when compared to primary alkaline cells of similar size. 

In genera] secondary cells have more stringent requirements for separator materials than primary cells. RAM cells typically apply two components, a non woven absorbent and a barrier material. The absorbent material used is formulated fiom polyvinyl alcohol and rayon fibers, acts as mechanical spacer between anode and cathode and provides as an electrolyte reservoir. The barrier is unglycerinated cellulose and prevents zinc dendrites from causing cell shorts. Prior to insertion into the cell the two materials are wound into a tube. Separator materials are selected which are not subject to oxidation in the alkaline electrolyte even at elevated temperatures and which combine chemical and mechanical stability with long life expectancy. 

The best developed types of RAM cells are cylindrical cells using sleeve electrode construction, which are simply discribed as cylindrical cells. They are produced in AA, C and D size types with a typical trend to small sizes. In the Figures 15 and 16 of chapter 3.1. the design changes of the different cell sizes are shown. The principal construction of the cylindrical RAM cell is identical with the equivalent PAM cell design consisting of the same cell components. Differencies between these systems exist mainly in the capacity limitation of the anode to approximatly 1/3 of the cathode capacity, the application of improved separators and better anode and cathode formulations as already mentioned in the above citied chapter. A lot of experimental data exist for low-mercury (0.025 wt % Hg) as well as for mercury-free (0 % Hg) RAM cells. They are summarized in Chapter 3.6.. 

Figure 22. Cycle performance of spirally rolled RAM cells (0.03 % Hg).

Principally two versions of spirally rolled RAM cells can be produced fully charged and nearly discharged cells. The Mn02 cathode can either be partly or completely prereduced in a batch process [48] and the zinc anode may consist mainly of zinc oxide. This special version is easier to manufacture and the shelf life in the nearly discharged state is probably unlimited. But initial charging is required for a 100 % discharge capacity of the first cycle. 

The rolled cell design has also been studied for the application in Mn02-H2 cells [49]. In that system the influence of zinc on the cycling behaviour is not present and the cycle life is improved. However, it should be noted that rolled Mn02 H2 cells are capacity-wise far below the 4 AA or 7 AAA RAM cell arrangement unless electrodes without supporting metal screen are developed. 

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