I am currently working on an Arduino lighting project. Everything is working correctly on it so far. Currently, The Arduino uses an AC adapter to power it. I would like to be able to power it via rechargeable batteries. The problem is that I don't completely understand how to calculate the lifespan of a battery. The batteries I am looking at are 3.7V 5000mah rechargeable lithium. In total it would be 7.4V because I want to use two of these batteries. If I understand correctly, I can calculate the battery life by using this formula?
Battery Life = 5000mah / used mah
Is this correct or would I combine the 5000mah from each battery?
Also, if this is correct, it would appear that I would only get about 25 Hours per charge on these batteries. Would adding more batteries and then decreasing the voltage allow for a longer use?
Thanks in advance,