Make sure the PV module is 12 volts nominal as well. Be sure you have an easy and convenient way to disconnect the PV module from the battery as well as a way to protect the wire from PV to battery and a way to see at a glance the battery state of charge.
The main concern here is your safety.
If for example the battery is already fully charged and you happen to roll out into full sunlight and stay for a considerable length of time there is the possibility the batteries might get over charged and explode. Not a pretty picture. This why you need a disconnect and a battery state of charge indicator. An automated high voltage set point disconnect would be highly advisable. What if you were not able to disconnect it manually, say, if you fell asleep under those condition stated above?
Remember that, the wire will not only be carring power from the PV to the battery but all of the power stored in the battery can be unleashed all at once if the wire falls prey to a dead short causing a fire. Not a pretty picture either.
Be sure voltages are compatible.
A means of manual disconnect as well as high voltage automatic disconnect.
The correct size wire well protected from physical damage and fused properly.
A means of knowing exactly what the battery state of charge is.
A one way diode of the correct size will help keep power from the battery escaping to the PV module during low or no light levels.
Be sure that all devices wired inline are rated for open circuit PV voltage.
A 5 watt PV module at 12 volts nominal might produce as much as .65 amps under the right conditions.
Also, get to know your chairs battery pack. The type it is, probably a sealed cell, what its amphour rating is, and most importantly its charging criteria, what is its maximum charging voltage and all. Here are few websites that might be interesting to you.
http://www.treehugger.com/files/2008/05/solar-powered-wheelchairs_.phphttp://www.evparts.com/http://www.kronosport.com/Maybe a bit extreme but maybe interesting too and some cool weblinks as well.
http://www.speedace.info/index.htmhttp://www.nedra.com/