Know how to maintain a battery fleet and eliminate the risk of unexpected downtime.
Everyone wants a battery that is small and light, but a pack must provide critical additional attributes. Besides long runtimes, design engineer must satisfy load requirements and choose an architecture that is durable to deliver the needed cycle count in the environment deployed.
One of the best tools to calculate battery capacity is the Ragone plot. [BU-503: Determining Power Deliver by the Ragone Plot] It provides a snapshot of how specific energy in Watt/hour works hand-in-hand with power in Watts as the delivery vehicle.
Once energy and power requirements are satisfied in lab conditions, the works continues to find the limitations with a faded pack at hot and cold temperatures. These fringe areas must be defined and included in the specifications. Only by fully understanding the boundaries can battery and device deliver the anticipated performance for many years to come.
To approve and pass a device, device manufacturers prefer picking the best battery from the lot. This satisfies the present moment but ignores real field conditions. Capacity fade occurs naturally with age and cannot be ignored.
Once a new product is released and rubber-stamped, few provisions exist to hint at what capacity a battery should be replaced to maintain reliability. Capacity is the leading health indicator that also predicts the end-of-battery-life. Measuring the internal resistance, as could be done with older battery technologies, no longer provides reliable state-of-health results. The resistance of the modern battery stays low while the capacity decreases in a predictable fashion with use and age.
A typical capacity bandwidth of a well-managed battery fleet ranges from 80 to 100 percent. Packs below 80 percent should be replaced. In addition, the user should include 20 percent spare capacity to manage unforeseen activities. Figure 1 illustrates these requirements in the 20-20-60 capacity calculation.
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.
The proposed 20-20-60 capacity calculation is conservative and some battery users may consider relaxing the risk factor to keep the battery longer. To do this reliably, the spare capacity after a mission or a day’s shift must be known. On the “Auto” program, the Cadex battery analyzers accomplish this by applying a discharge before charge. The first reading on the analyzer’s display relates to the spare capacity and the second is the full capacity the battery is capable of delivering with a full charge.
The spare capacity in a battery fleet should be fine-tuning to find the optimal compromise between economy and risk factor. If, for example, the lowest performing battery in the fleet comes back with still 30 percent spare capacity, then the threshold capacity can safely be lowered from, say, 80 percent to 70 percent. This enables keeping batteries in service longer without adding increased risk.
Batteries powering medical and communications devices are normally replaced when the capacity falls from the original 100 percent to 80 percent. Scanners in warehouses can often provide a full day’s work with a 60 percent battery, and starter batteries in vehicles will still crank at capacity of 40 percent. A battery analyzer calculates the required capacity for a mission and calculates at what capacity threshold the pack should be replaced. This provides optimal service life without compromising risk.
Last Updated: 20141128