Spillable battery: Wet cell batteries are considered spillable and special preparation must be made before being transported by air. The battery will be accepted for transportation providing the following items are met:

Battery has no obvious defects upon visual inspection, which includes removal of the battery where necessary
The battery is disconnected and terminals are insulated to prevent short circuits
The wheelchair or mobility aid can be loaded, stowed, secured and unloaded in an upright position
If the battery is removed, it must be carried in strong, rigid battery box approved by United Airlines under the following conditions:
The packaging must be leak-tight and impervious to battery fluid. An inner liner may be used to satisfy this requirement if there is absorbent material placed inside of the liner and the liner has a leak-proof closure.
The battery must be protected against short circuits, secured upright in the packaging and be packaged with enough compatible absorbent material to completely absorb liquid contents in the event of rupture of the battery.

Non-spillable battery: Dry cell and gel cell batteries are considered non-spillable and fewer requirements are necessary for their air transportation. If your wheelchair or mobility aid is powered by a non-spillable battery and can be stowed upright in the cargo bin of the aircraft, the battery should not need to be removed providing the following:

There must be at least one effective means designed into the wheelchair or mobility aid in order to prevent accidental activation or possible short-circuiting
If no effective way of preventing accidental activation of the device can be determined, then the battery cables will be removed and the battery terminals will be insulated to prevent short circuits
The battery must be securely attached to the wheelchair or mobility aid and fully enclosed in a rigid housing that is properly marked

Lithium ion battery: Lithium ion batteries are classified independently of other battery types. Lithium batteries may overheat and ignite in certain conditions and, once ignited, may be difficult to extinguish. Lithium batteries are considered a hazardous material for which the following restrictions apply:

There must be at least one effective means designed into the wheelchair or mobility aid in order to prevent accidental activation or possible short-circuiting
If you have a collapsible mobility device, the battery must be removed and cannot exceed 300 watt hours.
If you have a non-collapsible device, Federal Regulations allow for one large lithium ion battery to be transported installed in the device and up to two spare batteries to be transported in carry-on baggage. There is no watt hour limit for the battery installed in the device. If you have one spare battery, it must not exceed 300 watt hours. If you have two spare batteries, they must not exceed 160 watt hours each.
The watt-hour rating is a measure by which lithium ion batteries are regulated. If you are unsure of the watt-hour rating of your lithium ion battery, contact the manufacturer of your device. Lithium ion batteries manufactured after January 1, 2009, are required to be marked with the watt-hour rating.

Spare batteries must be effectively protected against short circuiting by:

Packing each battery or each battery-powered device, in fully enclosed inner packaging made of non-conductive material (such as a plastic bag)
Separating or packing batteries in a manner to prevent contact with other batteries, devices or conductive materials (e.g., metal) in the packaging
Ensuring that exposed terminals or connectors are protected with non-conductive caps, non-conductive tape or by other appropriate means
Lithium metal batteries (non-rechargeable) with more than two grams of lithium content are forbidden on commercial passenger aircraft.