Page 249 - Battery Reference Book

Nickel-hydrogen and silver-hydrogen secondary batteries 19/15

to require an extended period of overcharge to achieve Table 19.3 Cycle test sequence of porous Teflon membrane in
this limit. sealed nickel-cadmium batteries: Eagle Picher ceil RSN-50
Although the test results on the advanced design (50 Ah capacity)
cell were not conclusive, the projected improved per-
formance characteristics of the advanced design were Cycle Cell potential Cell pressure End-of-discharge
realized. The split negative design successfully permit- on charge on charge voltage
ted the incorpclration of a low-porosity separator mater- (VI (i.w.g.) (VI
ial which would significantly increase the list of poten- Near Earth orbit cycle
tial candidate materials. In addition, the split negative Cycle: 56min; 14A charge 23min; 14A dischaxge 22min
design resulted in an improved oxygen gas recombinat- Temperature: 4.4"C
ion rate (in this case up to a C/5 overcharge rate). The 1 1.458 - 21 1.232
incorporation of a gas electrode was also successful 640 1.463 - 24 1.242
in the recombination of hydrogen gas (again at least 1270 1.460 -25 1.238
up to a C/lO overcharge rate). The application of a 1920 1.459 -25 1.236
thin Teflon film to the back of the split negative elec- 2650 1.462 -25 1.235
trodes was beneficial to all designs, particularly with Synchronous orbit cycle
respect to increases in electrolyte quantities. Although Cycle: 24 h; 5 A charge 22.8 h; 25 A discharge 1.2 h
all cells accommodated greater electrolyte quantities Temperature: 26.6"C
than would normally be tolerated in a conventional 3 1.405 - 14 1.217
cell, an even greater quantity could be accommodated 14 1.412 -2 1.194
in the Teflon film design. 27 1.413 2 psi 1.199
Developments of this type in nickel-cadmium cell 44 1.383 10 psi 1.197
design promise even better cell characteristics in the 110 1.396 10 psi 1.190
future. In the particular example of battery design Accelerated synchronous orbit cycle
improvement discussed above, only minimal further Cycle: 12 h; 5 A charge 10.8 h; 25 A discharge 1.2 h
Temperature: 4.4"C
development should evolve a system which offers both
greater reliability and a significant improvement in 156 1.474 -5 1.206
cycle-life capacity. 162 1.480 - 10 1.200
The teflonated negative plate referred to above was 185 1.480 - 10 1.202
studied as part of an investigation into the elimina-
tion of potential failure modes and the enhancement
of reliability cif the nickel-cadmium system intended Cadmium hydroxide is reduced back to cadmium at
for use in satellites. The presence of a thin layer of the cadmium electrode.
porous Teflon film wrapped around the surface of the 2Cd(OH), + 4eC -+ 2Cd + 40HC (19.7)
negative electrode was believed to offer the advan-
tage of reducing or eliminating cadmium migration, For these reactions to occur effectively the oxygen has
which is knou7n to occur in present systems over time. to diffuse from the positive electrode to the negative
The results obtained from cell testing have been very and to permit this to occur at the required rate, the
encouraging. The Teflon film does not affect the oper- electrolyte has to be contained in an adsorbent separ-
ating voltage of the cell on charge or discharge, but ator. These separators, as opposed to those used in
does significantly improve the oxygen gas recombinat- open frame type cells, lead to an increase in electrical
ion rate. resistance of the cell, i.e. a slight loss in performance.
After completior, of approximately 3000 low Earth During the past 10 years the nickel film plate based
orbit cycles followed by approximately 300 syn- on polypropylene felt has been introduced and has led
chronous orbit cycles (Table 19.3) the Teflon film to an improvement in performance. Plastic or rubber
design batteries were opened up and it was shown bonded plates have also been introduced. The use
that the nonwoven, nylon separator (Pellon 2505) of Teflon bonded plates as a replacement for pocket
was remarkably free of any indication of cadmium electrodes has not fulfilled its earlier promise due
migration. to problems associated with swelling of the positive
Very recent developments in minimum or zero electrode.
maintenance nickel-cadmium pocket plate cells is dis-
cussed below. 19.2 Nickel-hydrogen and
These cells use the following oxygen recombination
cycle to prevent water loss, i.e. avoid the need for silver- hydrogen secondary batteries
topping up. These batteries are still at or slightly beyond the
experimental stage. The basic nickel-hydrogen system
Positive electmde: 40H- -+ 02 + 2H20 + f4e- (19.6)
consists of a catalytic gas electrode (negative) coupled
Negative electrode: 2Cd + 02 + 2H20 + Cd(OH)* with a nickel electrode (positive). Electrochemically