It is possible for you to run a second battery in parallel with the existing battery but some things will need to be enlightened before hand. The light is designed to a specific draw rate for a certain amount of autonomy hours (hours of operation without sunlight). A panel was then designed to make up for this rate within a certain amount of exposure hours. Typically this exposure period is 3.5 hours of full exposure. Now when I say exposure I mean optimum exposure hours. You may get 6-8 hours or more of sunlight a day but only 3.5 hours out of that 6-8 hours are good enough for solar charging. The reason for this is because during certain points in the day the suns incidence angle to the ground is too high or too low to cast significant solar radiation that the solar panel needs. Meaning that at certain angles of the sun in relation with the earths atmosphere the light wavelength that solar panels require is not able to permeate the earths atmosphere and instead refracts off of it. Therefore this results in certain "windows" of exposure periods of which the solar panel can generate the power needed to recharge the battery. Adding a second battery will definitely increase your hours of operation but the panel is not sized to make up this difference in battery drain. What then will happen is the batteries will equalize there charge between one another and drain equally together. So the amount of charge that the solar panel is producing will only half charge the battery stack (battery stack is when multiple batteries are wired together) because the panel is designed to charge one of these batteries and your have doubled the battery size. The typical solution to this is to purchase a second panel and wire that in parallel with the first but this lights circuitry is designed to only withstand a certain amount of amperage from the solar panel (Amperage determines rate of charge) so adding a second panel to this system will ruin the circuitry and result in a defective light. Now when a battery is drained it turns the positive plates in the lead acid battery from lead oxide to lead sulfate. This chemical reaction is what produces energy storage. When a lead acid battery is left in a lead sulfate state for a prolonged period of time the lead plates can no longer turn back into lead oxide thus reducing the amount of charge it can store resulting in a useless dead battery within a year. So yes you can add an additional battery to prolong the operation times but it will yield worse results in the future unless you purchase batteries on a frequent schedule which in turn negates the reason for purchasing solar power in the first place.