Verifying Battery Capacity
To assure reliable service during the life of the battery, design engineers oversize the pack to provide spare capacity. This is similar to an airplane carrying extra fuel to enable a delayed landing when conditions dictate. No regulations exist as to the amount of spare capacity a battery should provide for each mission.
New batteries operate (should operate) at a capacity of 100%; replacement typically occurs when the packs fade to about 80%. Environmental conditions must also be considered as cold temperature lowers the capacity, especially with Li-ion. The capacity loss of a Li-ion (Energy Cell) is about 17% at 0°C (32°F), 34% at –10°C (14°F) and 47% at –20°C (–4°F). Not taking cold temperatures into account can leave a rescue mission in limbo.
Systems commonly fail during emergencies when increased demand is place on the battery. During routine events, marginal batteries can hide comfortably among their peers. A system is only as good as the weakest link and battery maintenance program as part of quality control assures that all batteries in the fleet meet the minimal required performance criteria.
Figure 1 demonstrates the breakdown of a battery that includes capacity fade and spare capacity. Adding 20% for fade and 20% for spare as a safety net leaves only 60% for the actual capacity. Such a generous allowance may not be practical for all cases, and effective battery maintenance will allow for tighter tolerances.
Figure 1: Calculating spare battery capacity.
Spare 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. Allow more capacity reserve when operating at cold temperatures.
To verify sufficient spare capacity in a battery fleet, identify older batteries that are close to retirement and spot-check their capacity after a busy day with a battery analyzer. Advanced battery analyzers (Cadex) provide a special program (Prime) that applies a discharge before charge. The first reading on the display reflects the spare capacity and the second represents the full capacity after a charge.
If older packs with fringe capacity levels come back from a full-day shift with less than 10% spare capacity, increase the pass/fail target capacity from 80 to 85% to gain five extra points. If, on the other hand, these old-timers come back with 30% before charging, keep the packs a bit longer by lowering the target capacity to, say, 70%. Modern battery analyzers (Cadex) offer adjustable capacity thresholds that can be set for each use.
Knowing the energy needs for each application during a typical shift makes power requirements transparent. This establishes the “sweet spot” between risk management and economics. Operations using such systems have reported payback periods of less than one year on battery savings alone, not to mention increased reliability and protecting our environment as fewer batteries are discarded.