There are some forklifts that use or have been converted to multiple traditional lead acid batteries like 4D, 8D, or even deep cycle marine batteries like Group 31 units. These batteries are different from the heavy duty industrial batteries.
Because of the construction of forklift batteries, they have certain characteristics differing from smaller lead acid batteries. Lifespan of lead acid batteries is directly related to the thickness of the positive plates, the thicker the better for deep cycling life. This is why 6 volt golf cart batteries (thick plates) will last longer than the same amp hour pack made up of 12 volt batteries. Automotive battery plates are about .040 inch thick, while a golf cart battery will be about .070 to .100 inch thick. Forklift batteries ordinarily are .250 inch or thicker, and use lead-antimony for plate material. This material increases plate life, but increases water loss and gassing, so proper maintenance is mandatory for good battery life. Water levels should never be allowed to drop below the top of the plates. Rapid sulfation can occur, which decreases battery capacity and eventually life.
A lot of traditional lead acid batteries of 250 amp hours and under (deep cycle marine, etc.), are recommended to be charged at a level of .1C, which is 10% of battery amp hour capacity. Some batteries have this printed right on them. For example, .1C of a 100 amp hour battery is .1 x 100 = 10 amp charge rate. Forklift battery chargers are usually sized at .14C to .20C, or 14% to 20% of battery amp hour capacity. It is important to know the ah rating of your battery, if you aren't replacing an existing charger, or to check if the existing charger is sized correctly. You can go a little larger in the charger sizing, but risk overcharging the battery and decreasing its life if you go too large. You can undersize a little, but going too small risks undercharging the battery, and not stirring the battery chemistry sufficiently, not to mention working the charger harder than necessary. Forklift battery cells (and UPS backup type 2 volt cells) are taller than regular lead acid batteries, and the electrolyte can stratify over time, with different concentrations of acid at different levels. A good, properly sized charger deals with this, during the charge cycle, and with an equalising cycle.
Most forklift batteries are designed for 1500 cycles or more, a cycle being defined as discharged 80% (20% charge left in the battery). A complete discharge/charge cycle every day (5 days a week) works out to a little more than 5 years. Forklifts that are not used to this degree (a lot aren't) and are properly charged/maintained can see 15 years or more battery life. Forklift batteries used in solar setups or similar applications where the depth of discharge is not as severe can last 25 years or more. While some recommend waiting until a forklift battery is discharged 80% before recharging, there are others who believe the less deep the discharge, the more cycles to be expected.
Forklift batteries should never be completely discharged (basically the lift won't move). They should be charged while there still is 20% charge or higher left in the battery. Deep discharge can damage the battery and/or some forklift electrical components including the motor. Some chargers look for a specific battery voltage before they initiate a charge cycle (to be sure they are hooked to a battery), and a deep discharge may drop battery voltage below this thresh hold, so the charger won't work, requiring a service call. The battery may not take a full charge after this, and sulfate due to incomplete charge cycles.
Lead acid battery discharge is not a linear function, from nominal battery voltage (i.e. 36 volts) to zero. To put battery voltage versus charge state in perspective - a healthy, fully charged 12 volt lead acid battery will be 12.7 to 12.8 volts (high performance batteries even higher). At 11.9 volts, a 12 volt battery is effectively discharged. You can pull them lower than this (not much useable power), or if not charged they will continue to drop voltage, but this is where damage occurs. An 80% discharged battery (12 volt) measures about 12.0 volts. Multiply these numbers by 2, 3, or 4 for 24, 36 or 48 volt forklift batteries.
