What is Opportunity Charging?
To discuss opportunity charging, it might be best to define conventional charging. Conventional charging typically starts once a battery has reached a state of charge of ~ 20%. At this point the battery is connected to a Ferroresonant or SCR charger for an 8 hour charge cycle. During this charge cycle, the current delivery starts at 16-18 amps/100 Ahrs and progressively lowers to around 2-5 amps/100 Ahrs. This charge cycle allows the battery to be returned to a 100% state of charge. This type of charging generates a lot of heat and the battery must be allowed to cool for 8-10 hours before use. This type of charging greatly reduces your asset utilization. If you are depleting your battery to a 20% state of charge in 8 hours, you are reducing your utilization of that battery to 33% or less. If you only run one 8 hour shift per day, this process does not present a problem. The problem arises when you run more than 1 shift, or you have a fleet of batteries with capacity issues that will not allow for a full 8 hours of operation.
Opportunity charging is a process where the operator returns the battery to charge whenever there is an opportunity. Breaks, lunch, etc., are points where the lift-truck would be plugged in for a brief charging period. An opportunity charger is utilized when the state of charge is between 20% and 80%. At this state of charge, the battery can utilize higher current levels of 25-30 amps/100 Ahrs. To charge the battery from 80% state of charge to 100%, the charge profile changes and a lower current level is used for that final 20%. The final 20% takes longer and should be performed with an equalization charge once per week.
Is Opportunity Charging Bad for my Battery?
There is certainly some debate about the impact of opportunity charging on the life of an industrial battery. Some manufacturers and re-sellers will claim that a battery has a limited amount of charging cycles (~1500) and that any time you place your battery on charge you are using one of these cycles. The number of full 8-hour charging cycles a battery can handle is well documented and each manufacturer has performed performance testing on their products. The real question is whether a 5-30 minute charging session creates the same amount of product life impact as a full 8 hour charge. The damaging component of a charge cycle is the heat that is created during the process. A full charge cycle generates enough heat that the manufacturer suggests an 8 hour cool-down period. So, how much heat is generated during a 5-30 minute charging session? The truth is that it depends on the charger. Ferroresonant & SCR chargers increase battery temperature approximately 25°F in an 8 hour charge. An Infinity High-Frequency charger only increases the battery temperature approximately 15°F and requires a shorter charge period and cool-down period. When used as an opportunity charger, the Infinity only increase the battery temperature ~ 5.5% in an hour of charging. This minimal increase in battery temperature does create some lost life for your battery, but it is much less than the reduction that comes from a full charging cycle. It takes the accumulation of several short charging sessions to equal the effect of a charging cycle when the battery temperature is controlled.
What Makes the Infinity Different from Other Chargers?
All battery chargers convert an AC input into a DC output. The AC sine wave is manipulated with electrical circuitry to produce DC power. Ferroresonant and SCR chargers create an imperfect DC signal with rapid current fluctuations (electrical noise). This imperfect signal also produces efficiency loses in the form of heat. The Infinity High Frequency charger uses IGBTs that produce an almost perfect DC output which generates much less heat.
The differences in power conversion impacts the efficiency of each charger.
- Ferroresonant: ~85%
- SCR: ~80%
- Infinity HF: >91%
This much higher efficiency means that the Infinity requires less input power than the others. In fact, the Infinity requires 15% less power than a Ferro charger and 45% less power than an SCR charger. Stanbury, the manufacturer of the Infinity charger, used these statistics and regional electricity costs to generate a savings model. This model was designed to accommodate a conventional charging scenerio where two 1000Ah, 18-cell batteries were used per truck in a 2 shift operation. This model showed an average savings of $675/truck annually.
The Infinity is also very flexible. It can be configured with a taper profile for conventional charging, a PEI profile for cold-storage applications and an opportunity profile. The Infinity chargers can also operate with inconsistent supply voltages of 390-530V.