Know how to maintain a battery fleet and eliminate the risk of unexpected downtime.
A battery performs well when new but the capacity begins to fade with use and time. To assure reliable service over many years, the design engineers must oversize the battery and include a reserve. The capacity of a well-managed battery ranges from 100 when new to 80 percent when aged, with 80 percent denoting retirement.
Besides allowing for the all-important capacity fade, a battery should also include spare capacity for extra activity when needed. System breakdown in an emergency is typically caused by a lack of spare capacity. Marginal batteries can hide comfortably during routine operation but will fail when longer runtimes than normal are required. These margins are often ignored by fleet users unless the batteries are maintained as part of quality control.
Figure 1 shows a battery that allocates 20 percent for fade and 20 percent for spare. This only leaves 60 percent for the actual capacity in a worst-case scenario, a ration most battery users will relax in favor of longer service life.
Figure 1: Calculating spare battery capacity
Reserve capacity must be calculated for a worst-case scenario. The allowable capacity range is 80-100%; a spare capacity of 20% is recommended for critical use.
To check the spare in your battery fleet, spot-check the capacity after a busy day. The Cadex battery analyzers provide this function on the “Prime” program by applying a discharge before charge. The first reading on the analyzer’s display is the spare capacity and the second is the full capacity after a charge.
If an older battery performing at 80 percent capacity comes back with 30 percent before charging, then the pack can be kept longer. This can be done by lowering the replacement threshold from 80 to 70 percent. Knowing the energy needs for a given application creates a sweet-spot between risk management and economics.
Batteries for medical and communications devices are typically replaced at a capacity threshold of 80 percent, but there are exceptions. Scanners in warehouses will often still provide a full day’s work with a capacity of as little as 60 percent and starter batteries in vehicles still crank the motor at a capacity of 40 percent. Even though the capacity is low, the battery still delivers full current, albeit for a reduced time. But the moment will come when the battery will lack the needed energy to turn the engine and the driver will get stranded. (See BU-902a: How to Measure CCA (Cold Cranking Amp))
Last Updated: 2015-07-06
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