Opportunity charging means plugging a traction battery into the charger whenever the truck pauses; done by design it can save a spare battery and a battery change, and done by habit it can wear the battery out early. We build lead-acid traction batteries, and we publish where charging in the breaks helps, because the answer depends on the fleet and not on a slogan. Our sources are two reference books, quoted with their own dates, so read the figures as illustrations of the principle and not as promises for your battery. Check your battery maker’s guidance before you change any charging routine.
What it is, and why fleets try it
Dell and Rand, in Understanding Batteries, define opportunity charging as the partial recharging of traction batteries at convenient times in the day when they are not in use: between shifts, while waiting for work, or during rest and meal breaks. They list three claimed benefits: no second battery is needed for a second shift or overtime, no labour goes on changing batteries between shifts, and a smaller, cheaper battery can serve a single shift. They also name the costs: daytime electricity is often dearer than off-peak, and you may need several charging points around the site as well as a central overnight area. Our guide to the spare battery ratio shows what a spare battery is worth, which is the money opportunity charging tries to save.
When it works: the state-of-charge window
Both books are cautious about timing. Dell and Rand say it works best when the battery sits between roughly 30 and 70 per cent state of charge, with overcharge and heavy gassing avoided by a suitable charging regime, and with discharge kept inside the usual 80 per cent depth-of-discharge limit. Crompton, in the Battery Reference Book, says there is little benefit in charging a battery that is already about 75 per cent charged or more, so a top-up should be reserved for a battery below that level. In his example that corresponds to an electrolyte relative density of about 1.240; the figure depends on the cell design, so take the value from your own cell’s datasheet, and see our specific gravity chart for how to read it. His table shows why: the capacity restored per hour of charging falls steeply as the starting state of charge rises, so a top-up from a nearly full battery buys little.
Crompton also reports that a battery opportunity-charged in two or three half-hour periods in a working day could gain between 12 and 25 per cent of its capacity, depending on its starting state of charge. That figure belongs to the particular charger he describes, so a modern charger will not necessarily match it.
The two ways it goes wrong
The first is heat. Crompton warns that too many opportunity-charging periods in a shift may cause unacceptably high battery temperatures, and that a battery worked for eight hours, charged for eight and worked again immediately will have its life considerably reduced. His advice is to give batteries a rest between charging and discharging so that they can cool. Our guide to operating temperature explains why heat costs life.
The second is overcharge. Crompton notes that opportunity charging was discouraged before electronically controlled chargers because of the risk of overcharging a battery connected again and again; with a charger that lets the battery dictate its requirement, each top-up receives only the overcharge it needs. Dell and Rand describe a factory with 120 electric trucks that moved from overnight charging to opportunity charging and, as reported, cut the batteries required from 290 to 135 with no observed fall in average battery life, although the energy passing through each battery more than doubled. They attribute the result to a computer-controlled system that prevented overcharge and deep discharge and warned of overheating. Treat that as one reported site, in an older text, with a particular control system, and not as a forecast for yours.
More charge passing through each battery also means more gassing and more water to replace in a flooded battery. That is our reasoning, not a figure from either book, but it is why the watering round, described in our battery water guide, and the ventilation of the charging area both need a second look before you switch.
Do not skip the equalising charge
Dell and Rand’s airport baggage-tractor example used opportunity charging through a long working day and still gave an extended balancing charge once a week. Cells drift apart whatever the daily routine, and the weekly or periodic equalise is how the weakest cell is brought back; our guide to the equalising charge explains it. Ask your maker how often it should happen for your cells.
A fair trial, and where lithium is better
Run a trial on a few trucks before you change the fleet. For a month, record each battery’s state of charge when it is plugged in, its electrolyte temperature at the end of the shift, and its water consumption, and compare them with a truck charged the conventional way. Ask your battery maker whether the cells are rated for the extra throughput and whether the charger is designed for partial charging; many older chargers are not.
If your fleet runs several shifts with natural breaks to charge in, we will say plainly that lithium is built for exactly this pattern and often fits it better than lead-acid; our guide to lithium or lead-acid for a forklift fleet sets out the comparison. For a conventional overnight cycle, our battery charging guide covers the routine. Tell us your truck count, shift pattern and charger make and we will tell you whether opportunity charging suits your fleet or whether a spare battery is the cheaper answer.
Sources quoted: R. M. Dell and D. A. J. Rand, Understanding Batteries (Royal Society of Chemistry, 2001), chapter 4, section on charging industrial traction batteries; T. R. Crompton, Battery Reference Book, 3rd ed. (Newnes, 2000), chapter 48, sections on opportunity charging.