I don't think you need to let the battery go that far down... Especially LEAVING IT THERE. The battery state that it should be (for the longest life) is 50% (or 40 to 60). You can leave it for weeks or more at that level. That is where you want the battery to spend the greatest part of its life. You also should not let it go down below 25-40% before recharging ASAP. If you will not use the car, bring it back to around 50%
I keep my battery, while at home, in the 40-60% range. I plug the car at night, in with the timer to start charging to 100% right before I leave for the day's driving. Try to only charge to 100% when you are going to drain it down to 60% soon. Otherwise, if you are only driving the car 15-20 miles per day, charge it to 80%, and recharge when it reaches about 40%.