In the EZ-Pass system, most though not all agencies allow you to use the transponder with other vehicles of the same class (e.g. a motorcycle transponder on a different motorcycle, but not a car). You must register the vehicle with your EZ-Pass agency, however. Otherwise, there is no way for the agency to verify that it should be associated with your account, and I'm not sure what grounds you would have for an appeal.
Some applicable sections of the I-Pass agreement include these:
- You agree to:
C. Use your I-PASS only on the vehicle(s) listed by make, model, and license plate and with the proper number of axles and tires specified on your application.
D. Promptly notify the Tollway of changes in your account name, credit card number and/or expiration dates, financial institution, e-mail or mailing address, vehicle make, model, and year, vehicle registration number, license plate type and number, and/or any other information required to keep your I-PASS account in good standing
H. Not assign or transfer your I-PASS without the written consent of the Tollway.
It's quite standard; compare at
You may use your transponder(s) on the vehicle(s) you specifically listed on your application for E-ZPass use.
You agree to use the E-ZPass transponder only on the vehicle(s) specified on your application.
North Carolina seems to be more of a stickler about one-vehicle-one-transponder:
You are required to purchase a TRANSPONDER for each vehicle listed on the ACCOUNT. Each ACCOUNT may only have as many TRANSPONDERS as the number of vehicles listed on the ACCOUNT. You may use the TRANSPONDERS only on the vehicles listed by license plate specified on your ACCOUNT.
So, you could try adding the rental car to your account before setting off. It's possible that the plate might already be registered to the rental agency's account; I'm not sure what happens in practice. If it works, do remember to remove it before returning the car so you don't end up on the hook for the next customer.
Most of the time, you can probably get away just using the transponder, as you have discovered. It's always possible, however, that it won't be read; there might be a problem with the reader, or the way you mount it in the new vehicle, or the battery might be low, or you might be going too fast, and so on and so forth.