Cpap voltages, what is acceptable?

Many Cpap machines run on 12volt DC and lots of us hook them up to batteries as a backup. My question is if it is known what voltage can damage a machine, whether too low or too high. The customer service at Puritan Bennett specifically cautioned against voltages over 13 and yet you can run the machine with a cable plugged right into the cigarette lighter which has 14.5 + volts when the car is running. Similarly battery packs decrease in voltage as they discharge, is there a voltage that is dangerous or damaging to these machines or will they simply stop working with insufficient voltage?
Is anyone aware of whether there is a voltage regulator in the respironics cigarette plug cable?
Thanks

I think Resmed says not to run there cpaps with the the power adapter pluged in the car and the engine running.