The most common battery designs in use today are maintenance-free and non-maintenance-free (NMF). Non-maintenance-free batteries (also called a “wet-cell” battery) are easily identified by the removable filler caps on the top of the battery. Battery fluid in NMF batteries needs to be regularly checked and. Both non-maintenance-free and maintenance-free batteries contain sulfuric acid that can cause serious burns. Always wear gloves and eye. Without getting too technical, it all depends on a battery's chemical makeup or design. As an NMF battery discharges and recharges, battery fluid changes to a gas and evaporates. This requires the battery fluid to be refilled. If you've determined that your battery fluid is low, follow these steps: 1. Make sure the battery is completely charged. If you're unsure of the state of charge and there is no indicator eye, use a voltmeter to check the voltage across the.