Maintenance free lead storage battery

The weight of the negative active mass is below 80% of the positive active mass. Preferably there is one more positive than negative plates.

The invention relates to a maintenance-free lead storage battery whose 
electrode grid is made of pure lead or antimony-free lead alloys and which 
includes a sulfuric acid electrolyte which is absorbed in the pores of the 
separator and of the active mass. 
Maintenance-free lead storage batteries of this type are known in a wide 
variety of specific forms. In particular, they are used for small storage 
batteries such as are utilized in photo-flash devices. In such lead 
storage batteries the electrolyte is generally fixated so that no liquid 
electrolyte can escape even if the cell is turned upside down. For 
fixation of the electrolyte, there is generally used a polyelectrolyte, or 
else the electrolyte is fixated in thixotropic form through addition of 
silicon dioxide or other materials. This thixotropic electrolyte itself is 
then introduced, for example, in glass-fiber batting. Also in such storage 
batteries additives for the electrolyte are used, especially an additive 
of phosphoric acid. While this creates disadvantages concerning grid 
corrosion, yet under certain circumstances it enhances the cycling life. 
In U.S. Pat. No. 3,257,237, for example, there is described a storage 
battery with thixotropic electrolyte and a phosphoric acid additive in 
which the enhancement of the lift span is attributable to the combination 
of thixotropic electrolyte and phosphoric acid additive. The disadvantages 
of this construction lie particularly in that the introduction of the 
thixotropic electrolyte into the storage battery is difficult and 
complicated. 
As appears from German Pat. No. 1,180,434, in cells of the type under 
discussion it has also been proposed to control the dosage quantity of the 
electrolyte, e.g. in such a manner that, in the storage battery which 
still contains a certain quantity of freely mobile electrolyte the density 
of the sulfuric acid is at most 1.20 at the time the battery is placed in 
operation, i.e. at the start of charging. The excess electrolyte is needed 
to prevent too rapid a concentration through charging and over-charging, 
which would greatly shorten the life span. 
The importance of the electrolyte regime in storage batteries is known to 
those skilled in the art. In theory, 4.463 grams of PbO.sub.2, and 3.66 
grams of H.sub.2 SO.sub.4 are needed per ampere-hour. In practice, due to 
the realistically achievable utilization of the active mass, one expects 
to need 6 to 10 grams of positive mass per ampere-hour and about 13 
milliliters of 37% sulfuric acid per ampere-hour. Under these conditions, 
an acid density of 1.1 is not exceeded during operation of the storage 
battery. 
In all known storage batteries, the negative electrode is always 
over-dimensioned with respect to its capacity by 10 to 40%. This is 
intended to produce desirable low temperature performance, to counteract 
the pronounced tendency toward sulfation of the negative electrode, and 
also to facilitate an oxygen cycle. In practice this over dimensioning of 
the negative electrode is usually accomplished by providing the storage 
battery cell with one more negative than positive electrode. Thus, the end 
plates are always negative. As further reason for this construction, it is 
also recognized that positive end plates present substantial disadvantages 
in that their active mass is subject to greater volume changes during 
charging and discharging. Uni-directional loading of the end plates 
therefore places even heavier mechanical strains upon the plate. For these 
reasons, it has been generally accepted that the cells should always 
contain more negative than positive electrode plates. 
Accordingly, it is an object of the invention to provide a lead storage 
battery of the type under consideration, which has long life and high 
capacity relative to its volume, and which is particularly characterized 
by simple construction. 
This and other objects which will appear are achieved in accordance with 
the present invention by keeping the weight of the negative active mass at 
less than 80% of the weight of the positive active mass. The electrolyte 
quantity and concentration are so proportioned that, after 20-hour current 
discharge the acid density is at most 1.07 grams per cm.sup.3. 
The inventive proportioning of the electrode mass is preferably achieved by 
departing from the construction which is customary in the lead storage 
battery art and by building, for example, a cell which has four positive 
and three negative electrodes (inverted construction). This produces a 
capacity increase which corresponds to the positive mass increase. The 
greater acid dilution accompanying the increased capacity discharge is 
offset by the lower surface loading of the positive electrode. In this 
construction embodying the invention, the capacity is also limited by the 
positive electrode. The positive mass utilization has its customary 
values. On the other hand, the apparent mass utilization of the negative 
electrode is increased by this inverted construction. Such proportioning 
results in considerable enhancement of the cycling capability. This is 
attributable to the fact that the recharging takes place at considerably 
diluted acidity, and this promotes the reformation of .alpha.-PbO.sub.2.

Referring first to FIG. 1, in the data presented in that figure, the 
criterion for end of life is a drop below 60% of nominal capacity. 
Charging always takes place by the so-called I-U regime. This involves 
charging at constant current until the gas evolution potential is reached, 
and then continuing the charging at constant voltage. During the latter 
stage, the current declines. 
Curve 1 of FIG. 1 shows the performance of a maintenance-free storage 
battery in accordance with the state of the art, i.e. a storage battery 
which contains more negative than positive electrodes, whose electrolyte 
is not thixotropic, and whose electrolyte does not contain phosphoric 
acid. The electrolyte is fixated in a glass or plastic fiber batting, and 
the electrolyte quantity is proportioned in conventional manner so that an 
electrolyte density of 1.10 grams per cm.sup.3 is reached at the end of 
the discharge. 
Curve 2 of FIG. 1 shows the performance for the abovedescribed inverted 
construction, with reduced electrolyte density at the end of discharge. 
The condition is also met that the weight of the negative active mass is 
below 80% of the weight of the positive active mass. To the surprise of 
those skilled in the art, this yields a substantial increase in life-span 
and capacity. 
Curve 3 of FIG. 1 shows a storage battery of conventional construction, as 
described in connection with Curve 1, but whose electrolyte contains an 
additive of 30 grams per liter of phosphoric acid. 
Finally, Curve 4 of FIG. 1 shows the performance of a storage battery in 
accordance with the invention in which there is further added to the 
electrolyte 30 grams per liter of phosphoric acid. Here too, despite not 
using a thixotropic electrolyte, substantial improvement of the cycling 
capability is achieved. 
FIG. 2 shows the electrode plate assembly of a battery embodying the 
invention. This assembly includes three positive plates 10, and only two 
negative plates 11. Thus, there is one more positive than negative plates, 
in accordance with the inverted construction which characterizes the 
invention. 
Conventional separators 12 are positioned between adjacent plates 10 and 
11. 
Plate lugs 10a and 11a protrude from positive and negative plates 10 and 
11, respectively. 
This assembly of FIG. 2 may be enclosed in a conventional plastic battery 
case (not shown). The lid of such a case may have the construction shown, 
for example, in U.S. Pat. No. 3,836,401, except that the number of 
apertures in the lid of the battery case would, of course, be adjusted to 
correspond to the number of plates (and plate lugs) used. Pole bridges as 
shown in the said U.S. Patent may be used to connect the plate lugs of 
like polarity.