Tubular Industrial Batteries: The Right Choice for Fast Charging
By Steve Spaar, Director of Marketing for the Americas, EnerSys
Fast charging industrial motive power batteries has revolutionized the way electric lift truck fleets are managed. Valuable warehouse space and personnel no longer need to be dedicated to battery rooms with spare battery storage, battery handling and changing equipment. Lift truck operators do not need to travel from their work area to the battery room to change out batteries, saving thousands of dollars in lost productivity. No longer are batteries weighing thousands of pounds hoisted in and out of lift trucks, reducing the potential for employee injuries.
Traditional thinking in the motive power battery industry has been to size a battery for an application by allowing for one “cycle” defined as 8 hours of work (discharge), 8 hours of charging, and 8 hours of rest or cool down time. Most standard motive power battery warranties limit a battery’s use to one cycle per day. Estimates of standard 8-hour charge times have been based on charging rates of about 14-20 amps for every 100 ampere-hours (AHs) of a battery’s rated capacity. Fast charging turns that old rule of thumb on its head by increasing charge rates to 40-50 amps per 100 AHs of a battery’s capacity. In addition, the battery is now charged many times during a workday in order to provide the power needed for a single battery to last for two shifts or more. This practice is referred to as Partial State of Charge (PSOC) or Opportunity charging. Since a battery exhibits the highest resistance to charge acceptance after reaching 80% of it’s rated capacity, special PSOC profiles are used to minimize heat created by this charge resistance by stopping a charge when the battery reaches 80% recharge levels. Even with a PSOC profile, fast charge current rates applied to batteries designed to only accept standard charge rates will cause excessive battery heat. Fast charging enhances thermal issues because there may not be time for the traditional 8-hour cool down time and the batteries are typically kept in an enclosed lift truck compartment during charge allowing for minimal ambient cooling. Excessive battery heat will significantly shorten the expected battery cycle life.
Several years ago, EnerSys® initiated a project to develop a battery that would address not only the thermal management impact on a battery, but also determine what battery technology would provide the best performance from both a charge acceptance and a lift truck operation point of view. Two areas were looked at: the battery’s top construction where most of the heat associated with a high charge current occurs and cell structure. Since EnerSys manufacturers several types of cell technologies from flat plate, tubular, high gravity, thin plate, thick plate, and various metallurgies, there was no preference for any specific battery design.
Cell Structure
While all cell technologies had advantages and disadvantages for fast changing, it was determined that a high capacity tubular plate was best suited for fast charging. Higher capacity achieved through the use of high gravity electrolyte proved to promote higher temperatures because of higher electrical resistance associated with higher electrolyte specific gravities. The standard flat plate batteries have capacities of 85 AH and 125 AH in standard 23” height and 31” height batteries respectively, while tubular plate batteries in these heights show capacities of 100 AH and 140 AH respectively. That’s 17% more capacity in the 23” and 12% more capacity in the 31” high battery. The main benefit of a higher capacity battery is more available energy in the battery “box”. This allows operators to run longer and or harder between charges, and allows some capacity leeway for peak operating times, or neglecting charging opportunities. Also, higher capacity allows discharged amps to be replaced faster then in lower capacity batteries. As stated earlier, fast chargers are typically set to charge anywhere from 40 to 50 amps per 100 amps of a battery’s capacity. Using a charge rate of 50 amps/100 amps of battery capacity (50% charge rate), higher capacity tubular batteries can accept more amps in a shorter time then flat plate batteries as outlined in Chart 1.
Conversely, if recharge time is not an issue, a tubular battery’s capacity advantage allows a charger to replace the same amps at lower charge rates. In this scenario, lower charge rates mean less heat is generated, less stress on the battery, and therefore longer battery life can be expected as illustrated in Chart 2.
Another advantage to higher capacity fast charge batteries is a concept called the “sweet spot”. (See diagram below.) As a battery is discharged and battery voltages decrease during a work shift, a typical DC lift truck will experience a gradual slowing of lifts and run speeds, which become more pronounced as the battery falls below 50% state of charge. Given the same discharge rates, a higher capacity battery will sustain higher voltages between charges and allow for a more productive lift truck. Considering a fast charge battery is operated in a partial state of charge due to the opportunity charge profiles used with fast charging, this extra capacity can have a significant impact on truck performance.
AC lift trucks also will benefit from high capacity tubular batteries in fast charging. One of the advantages of an AC lift truck over a DC lift truck is their ability to maintain constant lift and run speeds throughout the battery’s discharge cycle. However, to maintain lift and run speeds as a battery’s voltage drops, an AC motor will draw an increasing number of amps to compensate for the falling battery voltage. In a standard charging scenario, higher amp draws associated with AC trucks will discharge the battery faster and shorten battery run times, however the overall productivity of the AC truck per battery can be higher due the trucks ability to maintain run and lift speeds throughout the battery’s discharge cycle. Similarly, a high capacity tubular battery will ensure there is adequate capacity in fast charge applications as well.
Top Construction
The “top construction” of an industrial battery is critical to the thermal management capability of the fast charge battery design. On a standard industrial battery, the top construction consists of the lead post strap which connects the positive or negative plates in a cell with a post through the top of the cell’s cover, lead intercell connectors, lead charging cable termination connectors, charging cables, and the method of assembling these components. Several design concepts to improve the current carrying capacity of these components were tested. The combined impact of the design improvements resulted in a 10% decrease in battery temperatures during fast charge cycling. The post strap was increased in size from a standard ¾” diameter to a 1” diameter providing 78% more cross sectional area. Also, since copper is much more conductive then lead, a copper core was cast into the lead post. Similarly, new molds were designed for the lead intercell connectors in order to double the thickness of the connectors and allow for a thicker copper core. Because batteries exceeding 600 ampere-hours in capacity require two sets of positive and negative cables, a new dual cable termination connector was developed. The new lead cable connector is able accommodate the efficient positioning two sets of cables on top of the battery. Even the intercell connector covers were considered. By simply slotting the covers to vent heat off the top of the connector, heat built up under the cover can be reduced. Finally, 4/0 cables are used on fast charge batteries to minimize current resistance and reduce heat.
The result of this development project is the Express™ Fast Charge battery. The Express fast charge battery was designed from the ground up to be used exclusively in applications that push the limits of a battery and it’s components every day by pushing charge currents to the upper limit and rarely providing cooling or resting time. Doing more with less is the mantra in most operations today, and fast charging can provide significant operational savings if the system is managed correctly and the correct charger and battery are utilized.
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