Step One: Don’t do it if you can avoid it. It is simply not a good idea to set aside a conventional lead-acid industrial battery for a distant future use without taking steps to maintain it properly.
Batteries taken out of productive service and left idle for a year or so have been known to be unresponsive when eventually connected with a charger. Certainly in some cases it would have been better to sell off, or trade in, these surplus batteries to recoup some of their value while they were still serviceable.
Nevertheless, there are times when a lift truck and its battery are "between assignments", or awaiting resale, and it doesn’t make sense to pull the battery out and sell it off just because it might be a few months to a year before it is put back into service. There are some precautions that one can take to try to protect the health of the battery, but be forewarned, you may have to change the way you store equipment if you wish to follow this advice.
When was it last charged?
Lead–acid industrial batteries are designed to be used in regular daily charge/discharge cycles. They will perform for years in this way with surprisingly little attention and relatively inexpensive maintenance.
Under common industrial conditions, one cannot deprive a battery of routine charging without affecting the behavior of the electrochemical system, or possibly permanently compromising the battery’s active material. Wise buyers of used electric lift trucks will always ask the seller "When was this last charged?" or " How long since the battery was in regular use?"
An answer to these questions that indicates a lack of battery charging for many months pushes the value of the truck/battery package down because of the potential for battery problems.
A charged battery is a good thing
If a battery must be stored, it must be stored charged. It must be adequately watered so that the electrolyte always covers the tops of the plates. And make sure you have access to the battery throughout the storage period so that you can maintain the battery this way.
A lead acid battery will gradually revert to a discharged condition as it sits idle for a number of reasons. Internal chemical processes can and do move forward without any intervention on the part of the owner (even in a new battery) and although this internal self discharge can be a very slow process, periodic charging is necessary to maintain a fully charged condition.
Typically, battery manufacturers recommend that new batteries in storage be given a full, equalizing charge (the "weekly" or "daily + 3hr." setting on many chargers) whenever the electrolyte specific gravity drops below 1.240. Check your new battery’s specific gravity with a hydrometer every month if storage temperature is 80 degrees F or higher, every two months if stored at 60 F or lower. You may have to be even more attentive if you are storing a used battery, for reasons I will outline below.
Add water and charge
If the electrolyte level has dropped out of sight, add water and charge the battery to mix the water in. If the cells are topped off with water, and no charging is done, the water will sit atop the denser acid solution and interfere with accurate hydrometer readings. Also: added water that is not mixed in will freeze at 32 F, but mixed electrolyte in a fully charged battery will withstand exposure to temperatures below -40 F.
Your warehouse is no clean room
There are reasons, however, that a stored battery will discharge at a significantly faster rate sitting idle in your warehouse than a new battery would in a laboratory. A used battery, for example, usually has a significant amount of acid residue on top, the result of years of overflow and acid spray from the vent openings. Sulfuric acid and mixed corrosion products on top of a battery can conduct electricity from positive to negative terminals at many points since most cell connections are out in the open. This provides a discharge route in the same way that wire connectors do.
Also the battery is often left inside a lift truck that is being stored. If the battery remains plugged into the truck during storage, any current drain caused by truck components (heaters used in freezer trucks for example) will eventually bring the battery down to a completely discharged state.
It’s easy to counter these effects on battery storage: clean the battery, and store it with cables disconnected from the lift truck. Storage of the battery on a concrete floor, despite conventional wisdom, has no effect on the electrical behavior of the battery.
To summarize: keep your batteries charged, and keep electrolyte level above the plates. It also helps to store batteries clean and unplugged from the lift truck. In storage facilities where a lot of batteries reside, a multi-voltage charger could be kept fairly busy making rounds from battery to battery. A schedule that sees every battery get a full charge every three months is not out of line with common practice.