Page 114 - Battery Reference Book

314 Lead-acid secondary batteries

to be added to these batteries during their entire life. Partially recombining lead-acid batteries of the
If, because of improper charging or large variations in same capacity can be operated in series or in par-
temperature, a gas pressure does build up, the safety allel. To charge batteries in series, a high-voltage
valves ensure that this gas can escape immediately. low-current source is needed, while units in parallel
Afterwards, the safety valves automatically shut off the require a low-voltage high-current source. The con-
electrolyte space from the outside atmosphere. Assum- stant potential setting for series charging is equal to
ing that the operating and environmental conditions the sum of that calculated for each battery while the
are satisfactory, this type of battery is sealed and does voltage setting for units in parallel is that required for
not gas. Sonnenschein Dryfit batteries, for example, one unit. Charging batteries in parallel is the recom-
meet the requirements of the German Physikalische- mended procedure.
Technische Bundesanstalt for use in hazardous areas Partially recombining sealed lead-acid batteries
containing combustible substances of all the explosive have a gravimetric energy density in the range
classes within the range of inflammability G1 to G5. 30-35 W h/kg at the 20 h capacity rate.
Their method of construction also meets the require- Comparisons of the energy density of such batteries
ments of VDE D171/1-69. with that of nickel-cadmium batteries are interest-
Usually, a gelled electrolyte is used so that no ing as in many applications these two types of bat-
emissions of acid or electrolysis gas occur during tery are competing with each other. Rectangular mass
charging or use. These batteries have a low internal and sintered plate nickel-cadmium batteries exhibit
resistance and consequently a high current delivery lower energy densities (17 and 25 Whkg respec-
capability. The batteries can be used in any position tively) than sealed lead-acid and cylindrical sin-
and they function at full efficiency in a completely tered plate nickel-cadmium batteries (35-37 W hkg).
inverted position, although this should not be made a Yuasa quote 21-30kgiWh energy density for their
deliberate feature in equipment design. range of partially recombining sealed lead-acid bat-
Many types of recombining sealed lead-acid battery teries (Table 50.26).
feature calcium-lead grids rather than the conven-
tional antimonial lead grids in order to reduce self- 3.3.2 Fully recombining sealed lead-acid
discharge rates and gassing on stand and towards the batteries
end of charge. The Gates product uses pure lead grids.
Partially recombining sealed lead- acid batteries Hawker Energy Inc. are developing a range of sealed
may be charged and discharged over an ambient tem- cylindrical 2.00 V lead-acid rechargeable cells and
perature ranged from -20 to +50"C. If continuous also batteries. The first cell in this range to be
operation at one or more of the extreme values is offered was the D cell, which is rated at 2.5Ah
required, a temperature sensor should be used to opti- at the 10 h rate. It is 61 mm in height and 34mm
mize charging. Compared to this, nickel-cadmium in diameter.
batteries, depending on their type, are severely lim- The construction of this cell is similar to that of
ited by an allowable temperature range of 0-45°C standard cylindrical nickel-cadmium cells. It is sealed,
for charging. To operate properly the lead-calcium has a safety vent and makes use of a cylindrical spiral-
acid battery must be matched with a constant poten- wound plate design.
tial charging system, that is, one in which the current Stable capacity/voltage in long-term overcharge is
acceptance becomes a function of the back e.m.f. of well suited to standby applications. Special alloy plate
the battery. It is necessary that the battery sustain a grids and efficient oxygen recombination enhance long
constant on-charge voltage initially and throughout its life during overcharge. Drying out is not a problem.
life. Constant current charging may also be used in Sealed lead-acid cells are available as individual cells,
trickle charge applications. standard batteries of convenient sizes and voltages, and
Under constant potential charging the maintenance- also as assemblies of custom battery packs.
free lead-calcium acid battery will accept relatively Life is a function of use and environment, but the
high currents initially when the system is most effi- spiral-wound sealed lead-acid cell combines all the
cient. This stage is followed by a second period when features - oxygen recombination, low impedance, seal
the back e.m.f. of the battery begins progressively to design, resealing vent, low alloy plate grid - to extend
increase and to control the amount of current accepted. the life of the cell beyond what other lead-acid designs
At full charge and under proper voltage regulation, the offer. The Hawker Energy Inc, sealed lead-acid
current accepted will be reduced to a few milliamps system offers a number of desirable discharge
input, thus restricting the degree of overcharge which characteristics. The spiral plate design enhances long
protects the unit from excessive electrolyte loss. life in both float and cycle applications. Since the
This type of battery further supports this function spiral configuration results in lower impedance, the
by the fact that it develops a very high hydrogen and cell may be discharged at higher rates. State of charge
oxygen overvoltage which enhances the efficiency of is conveniently determined by measuring open-circuit
conversion and ensures a sharp and reproducible rise in cell voltage after a short stabilizing period. A voltmeter
on-charge voltage to regulate current acceptance best. is all that is required to measure state of charge. The