Replacing propane forklifts with lithium-powered trucks can reduce fuel handling and maintenance, but battery sizing matters. Four variables can help determine how much energy capacity the job actually requires.
Contributed by Max Khabur, Marketing Director for Eneroc USA
For warehouse operators considering a move from propane forklifts to electric trucks, the case increasingly extends beyond emissions. Maintenance requirements, energy costs, fuel handling and truck availability can all influence the business decision. But once a company decides to electrify, another practical question quickly follows: How large does the battery actually need to be?
Too little battery capacity can leave a forklift sitting before the workday is finished. Too much adds cost for energy capacity that may rarely be used. Proper sizing requires looking beyond nominal shift length and considering how the truck actually operates throughout the day.
Why fleets are looking at electric forklifts
A propane-powered forklift relies on an internal combustion engine, bringing with it oil changes, filters, spark plugs, belts, hoses, cooling-system components and exhaust-system maintenance. Electric drivetrains eliminate many of those components. Electric forklifts and their batteries still require inspection and service, but the drivetrain generally has fewer scheduled maintenance requirements.
Energy cost can provide another incentive. Depending on local electricity and propane prices, electricity can cost less per operating hour, particularly for trucks accumulating significant annual hours. There are operational differences as well. A propane forklift may require a cylinder change during extended operation, while a lithium-ion truck can take advantage of opportunity charging during breaks, lunches and other periods when the forklift would otherwise be idle.
Removing propane can also eliminate cylinder storage, fuel deliveries and associated inventory management. Indoors, electric trucks produce no tailpipe exhaust, avoiding the carbon monoxide emissions associated with propane combustion. The overall emissions benefit will depend partly on how electricity is generated, but electrification removes a direct source of exhaust emissions from the workplace.
Why lithium has changed the equation
Battery-electric forklifts are hardly new. What has changed is the capability available from lithium-ion batteries and, in particular, lithium iron phosphate, or LFP, technology.
Traditional lead-acid batteries remain widely used and can be economical in suitable applications, but they bring operational considerations including charging practices, watering, maintenance and battery changes in some demanding multi-shift applications. Lithium batteries can provide greater usable energy capacity, tolerate frequent opportunity charging and operate without the watering requirements associated with flooded lead-acid batteries.
That makes lithium particularly attractive when an operator wants an electric forklift to take over work previously handled by propane without routinely changing batteries. It does not, however, mean that the answer is simply to install the largest battery that will fit. The objective is to provide enough capacity for the application without paying for capacity the truck does not need.
Four factors that determine forklift battery size
Four variables provide a useful starting point when sizing a lithium battery for a propane-to-electric conversion: energy consumption, actual operating hours, battery aging and operating conditions.
1. Energy consumption per operating hour
The first step is determining how much electrical energy the replacement truck will consume while working. Forklift manufacturers publish standardized energy-consumption figures that can provide a useful basis for comparison. ISO 23308 establishes a test cycle for measuring energy consumption by industrial trucks under controlled operating conditions.
Using published specifications for comparable propane and electric forklifts from the same manufacturer, the examples analyzed for this article produced the following results:
| Rated capacity | Propane consumption | Electric consumption | Equivalent kWh per gallon |
|---|---|---|---|
| 4,000 lb. | 1.19 gal./hr. | 5.85 kWh/hr. | 4.92 |
| 5,000 lb. | 1.29 gal./hr. | 6.25 kWh/hr. | 4.84 |
| 6,000 lb. | 1.38 gal./hr. | 6.66 kWh/hr. | 4.83 |
| 7,000 lb. | 1.47 gal./hr. | 7.06 kWh/hr. | 4.80 |
Across these examples, roughly 4.85 kWh of electrical energy performed the equivalent standardized work associated with one gallon of propane consumption. That figure is best treated as a planning reference rather than a universal conversion factor. Actual energy consumption will vary with truck design, load weight, travel distance, lift height, attachments and duty cycle.
2. Actual working hours
A common sizing mistake is equating shift length with forklift operating time. An eight-hour shift does not normally mean eight hours on the hour meter. Forklifts spend portions of the day parked, waiting for loads, staging product or sitting during operator breaks.
If a truck records operating time during about 65% of an eight-hour shift, for example, the calculation becomes:
8 hours × 65% = 5.2 operating hours
That 5.2-hour figure, rather than the full eight-hour shift, provides a better starting point for estimating the truck’s energy requirement. Fleets with hour-meter records or telematics data can improve the calculation further by using their own actual utilization instead of an assumed percentage.
3. Battery capacity later in life
Battery sizing should also account for degradation. A battery that comfortably handles the shift when new will have less usable capacity several years later, so sizing only for day-one performance can create problems later in its service life.
Consider a 5,000-lb.-capacity, 80-volt electric sit-down forklift expected to accumulate 5.2 operating hours per shift. At approximately 6.3 kWh per operating hour:
5.2 hours × 6.3 kWh = 32.8 kWh per day
If the battery is expected to retain 70% of its rated capacity at the selected end-of-life point:
32.8 kWh ÷ 0.70 = 46.9 kWh nominal capacity
At 80 volts:
46.9 kWh × 1,000 ÷ 80 V = approximately 586 Ah
That would point toward a battery of roughly 600 Ah. The additional capacity is not simply excess reserve when the battery is new. It helps ensure that the truck can still complete its assigned work as the battery ages.
4. The operating environment
Standard calculations cannot capture every application. Cold-storage facilities, open docks and outdoor operations in winter conditions may need additional battery capacity because low temperatures can reduce available energy and affect charging performance.
Attachments also matter. Clamps, rotators and push-pulls place additional demands on the hydraulic system, while ramp work increases propulsion energy consumption, particularly when the forklift repeatedly travels uphill carrying a load.
Start with the calculation, finish with real data
A battery-sizing calculation is best treated as the starting point rather than the final answer. (Visit ENEROC USA at this link and scroll down to find their calculator.) Actual forklift usage data can substantially improve the result. Hour-meter readings, duty-cycle information and telematics can show how long trucks really operate, when they sit idle and whether there are realistic opportunities for charging during the workday.
Travel distance, load weight, lift height, gradients, attachments, temperature and charger capacity should also be considered before finalizing the battery specification. That helps avoid both extremes: buying too little battery and compromising uptime, or paying for energy capacity the operation will never need.
As fleets move from propane toward electric forklifts, the question is increasingly more specific than whether an electric truck can perform the work. The more useful question is how to configure the truck, battery and charging system around the work that actually needs to be done.
