Sylmar Hang Gliding Association

My old (15 yrs+) brick Kenwood 2 meter uses 12 volts.

I recently replaced the latest worn out batteries with two cheapy Nitro Brand RC car batteries listed as 7.2 V (2x=14.4V). However, hooking them up to a voltmeter, they indicated 18 volts.

With little to lose, radio-wise, I have been running the radio on them anyway, with no apparent problem.

Is there a long term or less obvious problem with my doing so?

Comment welcome!

edited 6.2018