What is a dry charged battery?
A dry-charged battery is a conventional non-sealed wet-cell battery (this is almost always a rechargeable type). Normally, this battery type is wet-charged, meaning that the battery is filled with electrolyte at the factory, charged, and then shipped with the electrolyte in the battery.
However, since the battery is not sealed, the electrolyte (either a strong acid or alkali) can spill out, which can be both a health and environmental hazard. The battery will also self-discharge at its normal rate during shipment and storage.
Dry-charging is a way to minimize these problems. Like a wet-charged battery, a dry-charged battery is filled with electrolyte at the factory and charged. However, the electrolyte is then removed from the battery. The battery is then washed out, dried out, and sealed. The battery may also be filled with an inert gas to minimize reaction of any remaining electrolyte, as in the U.S. military BB-451/U silver-zinc battery, which used a very strong alkaline electrolyte (40% potassium hydroxide).
The sealed battery is shipped and stored separately from the electrolyte. Because the electrolyte is in a sealed container, chances of spillage are reduced. The sealed battery will also self-discharge at a lower rate than usual, so it should still have useful charge up to 18 months after manufacture if stored below +90 degrees Fahrenheit (+32 degrees Celsius).
When the battery is needed, the electrolyte is CAREFULLY added back to the battery (eye protection and gloves must be worn and other precautions taken). The battery must be allowed to sit for some time afterwards so the electrolyte can soak around and through the battery's internal structures. The electrolyte temperature will rise and its specific gravity (SG) will drop during the soak. The manufacturer may recommend that the SG be measured after the soak time, and the measured value will have to be corrected for any difference between the actual electrolyte temperature and the temperature at which the reference SG was measured.
After the soak time, a (very rare) nonrechargeable dry-charged battery is ready for use, and some rechargeable dry-charged batteries may also be ready for use (like the BB-451/U). However, dry-charged lead-acid batteries will almost always need a low top-off or trickle charge. Rolls Batteries prescribes 5% of the 8-hour or 20-hour charge rate, to be reduced if the electrolyte becomes too warm or too much gas bubbles out of it. If the electrolyte in a rechargeable battery becomes too warm before charging, the battery will first have to be cooled down or allowed to cool naturally.