In recent years, a new generation of low-maintenance, flooded batteries has been introduced for automotive duties. These lose very little water and require almost no maintenance. The technological advance that made this possible was the replacement of the lead-antimony alloy used for the grids by a lead-calcium or a lead-calcium-tin alloy.
These grids are produced either by gravity casting or by slitting and expansion of alloy strip (Figure 2(a)). It is this antimony component of the traditional battery that gives rise to excessive gassing and water loss on charge: antimony dissolves (corrodes) from the positive grid, diffuses through the electrolyte, and deposits on the negative plate where it increases the rate of hydrogen evolution on charging.
By eliminating antimony, a great improvement is effected. Nevertheless, the batteries are still vented. Their life on deep-discharge cycling is generally shorter than that of the conventional lead-antimony batteries. Not only does the same shedding of paste occur at the positive plate, but also a highly resistive corrosion layer forms at the interface between the grid and the active material. The addition of tin to the positive-grid alloy moderates the influence of this “barrier” layer. In any event, low-maintenance batteries should not regularly be cycled to greater than 15% DoD, and never beyond 50% .
The tubular battery is a technology of long standing that is designed for deep-discharge cycling. The batteries are used extensively as power sources for many types of low-speed electric vehicles, e.g. forklift trucks, golf carts, milk floats, tractors. A lead-antimony alloy casting of parallel rods
replaces the positive grid of the automotive battery (Figure 2 (b)). These rods (or “spines”) are attached to a common header, rather like a coarse comb with well separated “teeth”. Each rod is inserted into a vertical tube made of braided glass-fibre that it surrounded by a sheath of perforated polyvinyl chloride. The active material is then packed into the tubes around the rods, which act as the current-collectors. The flexibility in the glass-fibre tubes allows for expansion and contraction of the active material during cycling.
As the positive active material is constrained by the tubes, the batteries can withstand deep-discharge cycling. They are, however, more expensive than flat-plate batteries and still require regular topping up with water. It is possible to minimise maintenance (water replenishment) by lowering the level of antimony in the positive grid, or by using lead-calcium alloys. The latter option restricts operation to shallow cycling.
At least one manufacturer does market such a battery specifically for solar photovoltaic applications. This battery would be suited to remote applications where the cost of servicing a conventional tubular battery would be so high that it is more economical to buy a battery pack several times larger than would be required for deep cycling, and then allow only shallow discharge, e.g. 20% DoD