Internal Top Construction
The distance between the center lines of two plates of the same polarity is
termed the plate center or strap center. Included in the plate center is the
thickness of one plate of each polarity and two thicknesses of the
separator-retainer combination. Short plate centers have lower electrical
resistance than long centers, because of thinner plates, thinner separators,
and shorter electrolyte path. Short centers are usually used in high rate
cells (SLI and UPS). Long centers are used in low rate cells (motive power and
The post is the conductor of all current into and out of the cell. When the cell is completely assembled, the post becomes the terminal of the cell. Its size and construction will be dependent upon the current rating of the cell. Multiple posts are commonly used for large capacity cells.
The strap is the section at the base of the post to which the plates are joined. The strap/post casting should be made of the same or compatible alloy as the grids. This will preclude excessive corrosion caused by dissimilar metals being burned together.
The joining of the plates to the strap/post is done by melting the portion of the parts to be joined, and adding lead alloy as necessary to bring the final strap thickness up to specification. In SLI units, the post/strap is molded on or "cast-on" to the plate lugs. A good joint here will result in best service performance and is especially critical for high rate cells. Strap thickness will also affect performance, more so for high rates than for long term, low rates.
The strap/post can be made in one of three constructions:
- Solid lead alloy
- Copper insert cast with lead alloy totally or
partially surrounding it
- Lead-plated copper.
The copper post will provide the most reliable service since there will be little, if any, cold flow at the connection.
In multi-cell units (used for SLI and aircraft), the connection from cell to cell in the unit can be either a burned connection over the partitions of the container or a connection made via high resistance welding through a hole in the partitions.
Motive power cells and certain types of sealed
stationary cells are packaged in multi-cell units in a steel tray. The tray
must have adequate mechanical properties to provide structural integrity
during all aspects of battery handling and operation. The design should be
compatible with intended battery handling equipment. Important factors to be
Strength and rigidity for the battery
Adequacy of welds for strength and battery
Acid resistance painting or coating
Lifting design to support the battery
Choice of material, usually steel, to meet
strength and shock requirements and to provide fabrication
If a tray cover is required, it must have adequate strength, rigidity, and impact resistance for the service. Abuse and rough handling by users is normal. The cover material is usually steel.
Intercell and terminal connectors may be burned or bolted, lead casting, flat copper, or cable. Connectors should be of proper cross-section and capacity for the service. Copper inserts may be used as required. The joints of bolted intercell connections must be torqued and suitably protected against corrosion. Cables must be welding grade and of sufficient capacity for the application. Cables and connectors should be sized such that the voltage drop through them is minimized.
Design Factors for Producibility
Producibility is defined as ease and repeatability of
production and not speed. While producibility is largely affected by process
design, product design factors have a significant impact as shown in Table
5-10. Battery designers should take these into consideration.