Vape pen batteries come in a range of voltages and also in variable voltage styles. The common range that you will expect to find is between 3.3v and 4.8v. By adjusting the strength of your draw you may be able to draw as low as 3.0v. This is about the right voltage for the common polycarbonate style oil cartridge. Finding the right voltage will depend on the resistance from the atomizer, (the heater in the battery), to reach the optimal temperature which is from 280* to 350* for most oils. Lower voltage means more pure vapor but a lighter hit, so always start low and work your way up. Voltage numbers don’t tell you whether it’s a better product, they just tell you whether it is the right product for your cartridge and vice versa.
By inhaling for too long or too hard "ripping" you will over heat the cannabinoids in the cartridge causing the liquid to darken and flavoring the subsequent inhalations to have a "smokey-burnt" flavor.