Linden D., Reddy T.B. (eds.) Handbook of batteries

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SILVER OXIDE BATTERIES 33.11

FIGURE 33.5 Typical discharge curves of HR-5 silver

zinc battery at C /3 (2-A) rate. Curve 1—25⬚C; curve 2—

0⬚C; curve 3–⫺18⬚C.

FIGURE 33.6 Typical effect of temperature on en-

ergy per unit weight for silver-zinc battery (operated

without heaters). Curve 1—10-h rate: curve 2—1-h

rate: curve 3—20-min rate: curve 4—10-min rate.

33.12 CHAPTER THIRTY-THREE

FIGURE 33.7 Performance characteristics of silver-zinc batteries under various condi-

tions. (To ﬁnd the capacity and the plateau voltage of a silver-zinc battery, draw a straight

line between the discharge rate and the ambient temperature at which the battery is stabi-

lized.)

25°C

0°C

-10°C

-30°C

-40°C

FIGURE 33.8a Service life of silver-zinc batteries at various discharge rates and temperatures. (Amperes /

kg vs. discharge time in hours)

SILVER OXIDE BATTERIES 33.13

25°CC

0°C

-10°C

-30°C

-40°C

liter

FIGURE 33.8b Service life of silver-zinc batteries at various rates and temperature. (Amperes/ liter vs. dis-

charge time in hours)

TABLE 33.4 Nominal Life Characteristics of Secondary Silver Batteries*

High rate

(HR)Ag-Zn

Low rate

(LR)Ag-Zn Ag-Cd Ag-Fe

Wet shelf life

Cycle life†

6–18 months

10–50 cycles

1–3 years

100–250 cycles

2–3 years

150–1000 cycles

2–4 years

100–300 cycles

* These characteristics are nominal and vary with operating conditions and design of individual models.

† Cycle life characteristics are for deep (80–100% of full capacity) discharge cycles. Cycle life improves

considerably with partial discharges.

33.4.3 Discharge Performance for Cadmium /Silver Oxide Batteries

The open-circuit voltage of the cadmium /silver oxide battery is 1.38 to 1.42 V. Typical

discharge curves at 20

⬚C are given in Fig. 33.9, showing the two-step ﬂat discharge typical

of the silver oxide electrode. The discharge characteristics are similar to those of the zinc/

silver oxide battery except for the lower operating voltage; Ampere-hour capacities are about

the same.

The capacity and the discharge voltage of the battery are temperature-dependent, again

similar to the zinc/silver oxide battery. The effects of temperature and discharge rate on

voltage and capacity are shown in Figs. 33.10 and 33.11, respectively. The recommended

operational temperature range is

⫺25 to 70⬚C, with the optimum performance obtained be-

tween 10 and 55

⬚C. With heating, the temperature range can be lowered to ⫺60⬚C.

The performance characteristics of the cadmium /silver oxide battery are summarized in

Figs. 33.12 and 33.13a and b, which can be used to determine the capacity, service life, and

voltage levels under a variety of discharge conditions.

33.14 CHAPTER THIRTY-THREE

FIGURE 33.9 Typical discharge curves of silver-cadmium battery at various rates at 20⬚C.

FIGURE 33.10 Typical effect of temperature on plateau volt-

age for silver-cadmium battery (discharged without heaters).

Curve 1—10-h rate; curve 2—1-h rate.

SILVER OXIDE BATTERIES 33.15

FIGURE 33.11 Typical effect of temperature on en-

ergy per unit weight for silver-cadmium battery (dis-

charged without heaters). Curve 1—10-h rate; curve

2–h rate.

FIGURE 33.12 Performance characteristics of silver-cadmium batteries under various conditions.

(To ﬁnd the capacity and the plateau voltage of a silver-cadmium battery, draw a straight line

between the discharge rate and the ambient temperature at which the cell is stabilized.)

33.16 CHAPTER THIRTY-THREE

25°C

-18°C

-30°C

-40°C

FIGURE 33.13a Service life of silver-cadmium batteries at various discharge rates and temperatures.

(Amperes / kg vs. discharge time in hours)

25°C

-18°C

-30°C

-40°C

liter

FIGURE 33.13b Service life of silver-cadmium batteries at various discharge rates and temperatures.

(Amperes / liter vs. discharge time in hours)

SILVER OXIDE BATTERIES 33.17

33.4.4 Impedance

The impedance of the silver oxide cells is normally low but can vary considerably with many

factors, including the separator system, current density, state of charge, stand time, cell age,

temperature, and, importantly, cell size. In a study of the effect of storage time on the

impedance of silver-zinc cells, initial values of 5 to 11 m

⍀ for partially charged cells were

reported,

with the values rising to as much as 3 ⍀ following 8 months’ storage at 21⬚C

and9to15

⍀ following 8 months at 38⬚C. Cells stored in the fully discharged condition

retained their low impedance (ranging from 2 to 10 m

⍀) throughout the entire test period.

The high-impedance cells returned to normal low values, however, within several seconds

of the start of discharge.

The AC impedance of the silver oxide batteries is highly dependent on the frequency of

the load, with the impedance rising sharply above 5 kHz. The impedance of a six-cell, 350-

Ah silver-zinc battery, discharged to approximately 50% DOD, is shown in Fig. 33.14, and

the corresponding phase angle is shown in Fig. 33.15.

FIGURE 33.14 Impedance magnitude vs. fre-

quency.

FIGURE 33.15 Impedance phase angle vs.

frequency.

33.18 CHAPTER THIRTY-THREE

33.4.5 Charge Retention

The charge retention of the activated and charged silver oxide cells is better than that of

most secondary batteries, with retention of 85% of charge after 3 months’ storage at 20

⬚C.

As with other chemical reactions, the rate of loss of charge is dependent on the storage

temperature (see Fig. 33.16). Storage at

⫺20 to 0⬚C is recommended for maximizing charge

retention. In the dry and charged condition, properly sealed and stored cells will retain their

charge for as long as 5 to 10 years. Here again, low temperature storage is highly recom-

mended.

FIGURE 33.16 Capacity retention for silver-zinc

and silver-cadmium batteries at various storage

temperatures. Curve 1-⫺10⬚; curve 2-0⬚C; curve 3-

10⬚C; curve 4-20⬚C; curve 5-30⬚C; curve 6-40⬚C.

33.4.6 Cycle Life and Wet Life

The separator system and the solubility of the active materials play critical roles in deter-

mining the wet and cycle lives of the silver-based cells. The separator must have a low

electrolytic resistance for discharges at high rates, yet it must have high resistance to chem-

ical oxidation by the silver species as well as low permeability to colloidal silver, zinc,

cadmium, or iron.

Since cadmium and iron are relatively insoluble in concentrated alkaline electrolytes, the

life of the silver-cadmium and silver-iron cells is therefore limited by the rate of silver

migration through the various layers of the separator system. Failure (internal short circuit)

occurs when a metallic bridge is established through the separator between positive and

negative electrodes. Multiple layers of separator are used to extend the life capability, how-

ever, at the expense of higher internal resistance.

The life of the silver-zinc battery is further hindered by the high solubility of zinc in

alkaline electrolytes. The problem manifests itself in two failure mechanism: shape change

and growth of dendrites. Shape change is the migration of zinc from the electrode tops and

edges where it becomes depleted, to the center and bottom where it densiﬁes, resulting in

capacity loss. Dendrites, a sharp, needlelike structure of the metal, are formed during over-

charge. They may perforate the separators and cause internal short circuits. The capacity

degradation in silver-zinc batteries due to shape change is illustrated in Fig. 33.17. The

decline in capacity of silver-cadmium batteries is at a much slower rate, as depicted in Fig.

33.18.

SILVER OXIDE BATTERIES 33.19

FIGURE 33.17 Typical capacity degradation of low-rate

silver-zinc batteries discharged at C / 10 rate.

FIGURE 33.18 Typical capacity degradation of

sealed silver-cadmium batteries discharged at C / 3 rate

to 100% depth of discharge.

33.20 CHAPTER THIRTY-THREE

Aside from normal capacity loss due to extended wet or cycle life, dry-charged zinc-

silver oxide batteries may exhibit a one-time deviation in capacity performance (typically

less than 20% of initial capacity) during the second cycle discharge called ‘‘second cycle

syndrome.’’ The cause of this deviation is unknown but is generally recognized by the user

community.

The nominal life ratings for the silver-zinc, silver-cadmium, and silver-iron batteries are

given in Table 33.4. The life of the silver oxide cells will also vary greatly with operating

and storage conditions. High rates of discharge to 100% depth of discharge and high-

temperature exposure (for more than 30 days) will signiﬁcantly reduce the wet and cycle

lives of the batteries. Cold-temperature storage (at less than

⫺10⬚C), when not in use, on

the other hand, will greatly increase the life of the cells. The cycle and wet lives will also

increase with a decreasing depth of discharge.

In a study to evaluate the capabilities of silver-cadmium batteries for satellite applications,

an extensive test program was run on three-cell, 3-Ah silver-cadmium batteries at various

depths of discharge.

These results are summarized in Table 33.5, showing the increase in

cycle life with decreasing depth of discharge.

Another study on 250 Ampere-hour silver-

zinc cells, cycled at less than 1% depth of discharge, with 14 full-capacity cycles, resulted

in a cycle life of 7280 cycles over a 38-month period.

TABLE 33.5 Cycle Life vs. Depth of Discharge for 3-V, 3-Ah

Sealed Cadmium/ Silver Oxide Batteries

Depth of discharge, % Cycle life at ﬁrst cell failure

1800

3979

⬎5400 (375 days)

Source: Ref. 20.

33.5 CHARGING CHARACTERISTICS

33.5.1 Efﬁciency

The Ampere-hour efﬁciency (Ampere-hour output/Ampere-hour input) of the silver-zinc and

silver-cadmium systems under normal operating conditions is high—greater than 98% be-

cause practically no side reactions occur when charging at normal rates. The Watthour ef-

ﬁciency (Watthour output/Watthour input) is about 70% under normal conditions because

of the difference between charge and discharge voltages.

33.5.2 Zinc/Silver Oxide Batteries

The manufacturers of these batteries recommend constant-current charging at the 10 to 20-

h rate for most applications. However, constant-potential and pulsed charging techniques

have also been applied.

A typical charge curve at constant current is shown in Fig. 33.19. The two plateaus reﬂect

the two levels of oxidation of the silver electrode: the ﬁrst from silver to monovalent silver

oxide (Ag

O), which occurs at a potential of approximately 1.6 V; the second from the