Flooded, or wet-cell, lead-acid batteries have long been the standard for powering electric lift trucks. One reason for this preference is nearly a century of experience in industrial vehicles. Wet-cell batteries usually have a longer life, so long as they are properly maintained and charged, than do other, more recently developed types of batteries. On a cost basis, they're generally less expensive than these alternative types of batteries.

But wet-cell batteries are maintenance-intensive. They require watering, and specific gravity checks. Also, after charging, there's a cool-down period required.

Maintenance-free or sealed batteries are a more recent development than the wet-cell products. There's about a decade of experience with them in lift trucks, particularly gel-type sealed batteries.

These batteries have certain strengths of their own, moreover. One is found, very simply, in the name. Sealed batteries don't require watering; they're maintenance free in this respect. There's also no free electrolyte to spill and corrode because they're sealed.

Maintenance-free batteries also need little or no cool-down time after charging. And some degree of on-board opportunity charging for batteries up to 24 volts is acceptable, as opposed to pulling the battery from the truck and recharging or taking the truck out of service, then recharging. (Unless charging is carefully controlled, wet-cell batteries don't do well with opportunity charges. Their useful life may be shortened.)

Technically, maintenance-free batteries are sealed valve-regulated lead-acid batteries of two basic types, depending upon what's inside them. The electrolyte inside a sealed battery may be in the form of a gel or it may be in an absorbed glass mat (AGM) composition.

AGM batteries tend to cost more than their gel-type counterparts of the same voltage, while not lasting as long. Some in the battery industry maintain the AGM products are not as suitable for deep discharge, motive power applications as they are for non-motive uses.

Thus, we will focus the comparisons here on wet-cell batteries versus gel-type, sealed batteries, generally.

There are tradeoffs with selecting either a more traditional wet-cell battery or a newer, gel-type, sealed battery. But, depending upon productivity issues, safety concerns, and other matters, the wet-cell type may not remain the winner at your facility when all factors are weighed in the balance.

Executives from several industrial battery suppliers – companies that make both wet-cell and gel-type sealed lead-acid power sources for lift trucks – were asked about the advantages and disadvantages of each type to help you decide which is best for your operation.

The wet cell battery promises greater life expectancy, in general, they say. A high-quality flooded (wet-cell) lead-acid battery usually has a service life of 1,800 to 2,000 cycles, says East Penn Manufacturing Company's Ed Miller, a product support manager. (A cycle generally involves discharging the battery to 80% of capacity, then recharging it. In a typical shift, most batteries discharge to 80% of capacity.) The actual life of the battery, however, depends on variables such as depth of discharge, operating ambient temperature, charger performance, and maintenance.

In contrast, says Miller, the service life of a comparable maintenance-free or sealed valve-regulated battery is expected to be 1,200 to 1,500 cycles.

Woodrow McGill, marketing support manager for the EnerSys Inc. Exide battery brand, reports similar results from life cycle testing by his firm's technicians on sealed valve-regulated batteries of gel-type design.

Gel-type batteries lasted a minimum of 1,200 cycles. Some gel batteries, however, survived up to 1,500 cycles.

Other tradeoffs to consider include the higher cost of sealed batteries compared to wet-cell products, how fast a battery recharges, and the length of any cool-down period that is required.

Wet-cell lead-acid batteries also have a cost advantage over gel-type, sealed batteries.

The latter type of battery typically costs 40% more than the comparable wet-cell battery, says East Penn's Miller.

But the maintenance-free characteristic and labor savings over life of the battery are selling points used by suppliers arguing against selecting the conventional lead-acid battery and for choosing sealed products. In contrast, wet-cell batteries require someone or some specialized equipment to do the watering. Moreover, the acid level must be checked by specific gravity readings from time to time.

Sealed batteries obviously don't require water to be added, says Miller. Maintenance is reduced. There's no free electrolyte to spill and cause corrosion, moreover.

These lower maintenance batteries do have a longer charge time than their wet-cell counterparts. A sealed battery discharged to 80% of rated 6-hour capacity will require 9 to 10 hours of charge time, Miller points out, to fully recharge without causing excessive gassing. Shorter and higher current recharge times can cause dry out over time of the gel (or of an absorbed glass mat electrolyte as well). That condition leads to premature failure, he points out.

A comparable wet-cell product with the same 6-hour capacity needs 7 to 8 hours charging on a standard, 100% rated charger.

Wet-cell batteries require a cool down period before they're ready to run again, however, after charging. "Little to no cool down time is required after charging a sealed battery," Miller says.

Furthermore, sealed batteries can receive opportunity charging. But they must get a full charge cycle, says Miller, every 24 to 48 hours to avoid sulfation, a chemical process that eventually damages the battery.

Sealed batteries are more sensitive to battery charging requirements than are wet-cell products. "A particular charge algorithm must be followed," says Miller, "and each battery manufacturer's requirement is different."

Wet-cell, lead-acid batteries perform better than sealed products in high-temperature applications, say both Miller and McGill. When the thermometer reading is "over 100 degrees Fahrenheit, and in applications that create high temperatures, conventional wet-cell lead-acid batteries with traditional 8-hour cool-down periods are still best," says McGill, compared to sealed products. "Of course, wet lead-acid batteries demand maintenance: the regular addition of water to the electrolyte and specific gravity readings."