Handbook for Sound Engineers

Power Supplies 699

However, because of packaging, the outward appear-
ance is similar to the zinc-carbon cell, with the same
terminal arrangement. Although this cell has an open-
circuit voltage of approximately 1.5 V, it discharges at a
lower voltage than the zinc-carbon cell. Also, the
discharge voltage decreases steadily but more slowly.
Alkaline-manganese batteries have 50–100% more
capacity than their zinc counterparts. Zinc-carbon cells
yield most of their energy above 1.25 V and are virtu-
ally exhausted at 1 V, while the alkaline cell yields most
of its energy below 1.25 V with a considerable portion
released at less than 1 V.
If the discharge rate is limited to 40% of the nominal
capacity of the cell and recharge is carried out over a
period of 10–20 h, alkaline-manganese cells can be
cycled 50–150 times. A typical discharge curve is
shown in Fig. 19-32.

19.10.14 Mercury Dry Cells

The mercury dry cell using a zinc-mercury oxide alka-
line system was invented by Samuel Ruben during
World War II. There are two kinds of mercury cells: one
with a voltage of 1.35 V and one with 1.4 V.

The 1.35 V cell has a pure mercuric-oxide cathode.

On discharge its voltage drops only slightly until close

to the end of the cell life when it then drops rapidly. The

1.4 V cell has a cathode of mercuric oxide and manga-

nese dioxide. On discharge, its voltage is not quite as

well regulated as the 1.35 V cell, but it is considerably

better than the manganese-alkaline or zinc-carbon cell.

Mercury cells have excellent storage stability. A

typical cell will indicate a voltage of 1.3569 V, with a

cell-to-cell variation of only 150μV. Variation due to

temperature is 42μV/°F, ranging from 70°F to +70°F

(56°C to +21°C), with a slight increase of voltage with

temperature. The internal resistance is approximately

0.75:. Voltage loss during storage is about 360μV per

month; therefore, a single cell can be used as a reference

voltage of 1.3544 V, ±0.17%. The voltage is defined

under a load condition of 5% of the maximum current

capacity of the cell. Normal shelf life is on the order of

3 years.

Recharging of mercury cells is not recommended

because of the danger of explosion. A typical stability

curve for a single cell over a period of 36 months is

shown in Fig. 19-33. The drop in voltage over this

period is 13 mV.

References

1. Kurt Mathews, Single IC, Power Factor Corrected, Off-Line Supply, Design Note 143, Linear Technologies
   Design Notes, Linear Technology Corporation.


2. Synchronous Rectification Aids Low-Voltage Power Supplies, Maxim Acation Note 652, Jan 31, 2001, Maxim
   Intergrated Products

Figure 19-32. Discharge characteristics of an alkaline- man-
ganese cell, on an arbitrary time scale.

1.2

1.0

0.8

0.6

0.4

Time

Cell voltage

Figure 19-33. Stability curve for a single-cell mercury

battery.

1.35701.3535 1.35101.34901.34701.3455

1.3440

Storage at or near 70°F

Time – Months

1.5

1.4

1.3

1.2

1.1

1.0

1% of initial EMF