First of all you only need two things. Knowledge (on this page) and a Digital Volt Meter.

These are available if you don't have one in many places including eBay.

All it needs to do is read VOLTS accurately.

The batteries in your wheelchair are 12v Lead Acid Deep Cycle batteries. In about 99 percent of all current powerchairs. They are generally wired in pairs, and connected in series. 

The chart below will apply to ONE battery only! You will need to double those voltage figures if you measure both batteries together Obviously.

Now how do you measure these voltages? And When?  Well to begin with you will need to switch OFF your Powerchair and wait at least 4 hours to get a true reading.

Overnight is better still. You cannot get very accurate reading just after charging or using your Powerchair. The voltage has to "settle" or "stabilise" so you just have to wait!  Although after say 30 minutes or so after you arrive home you will often get a reasonable idea.  With this in mind, how do you do a quick and dirty "read"?

Take a look at your powerchairs charge socket. In most modern powerchairs it generally has three holes for the "pins" in your chargers plug.

Two of these connect directly to the wheelchairs batteries. The third you can ignore.  It is used in programming, and inhibiting your chairs movement while charging.

The two we want are generally the bottom two. If you carefully stick the Volt meter probes into these two holes then it will read the true battery voltage.