im interested of buying a used 4790k from a guy and he is claiming that he never oced the cpu, hes also selling the motherboard his was using with that i7 with a replaced battery, so i was wondering if a heavily oc can cause a battery failure and hes lying .
Nahh, Old age causes battery being dead. Shouldn't be an issue. The Cmos battery stores the settings. So yeah, oc'ing can cause issues with battery but u cant tell. It shouldn't matter much, if you are really worried , just run intel diagnostic utility (https://downloadcenter.intel.com/download/19792/Intel-Processor-Diagnostic-Tool) and it will check whether the processor is running fine or not.
Don't worry much about it, if you r getting the motherboard and processor for a bargain, just purchase it.
Oc does nothing to the battery. The power needed for cmos to save settings is the same no matter what settings it's saving. There is no settings that you can change to change that. That's a short time for a battery to last but it's likely just a bad battery. It happens.