Is return drain better than main drain?
A "return drain" in reality would be called a return fitting . It's the one where the filterer, heated water goes back to the pool thus RETURN. Main Drain is that grate looking device at the deep end of the pool on the bottom where water is sucked from the pool and sent to the pump - the part of the equipment with the motor and clear lid. The filter - sometimes wrongly referred to as the pump - is the large round or vertically rectangular shaped tank. You need both.
A "center" drain or main drain is "good" in that it is usually located at the deep end of the pool and will circulate the water in the lower half of the pool better than if you only had just the skimmers at the surface. You will get less complete circulation if you used only the skimmer - You would essentially be circulating only the first two feet of the depth.
Can a sink drain pipe that is after the sink trap and goes to the main drain be lower than the main drain?
No! unless you want to always have problems with your sink. The sink (if its higher then the main line) will drain for a while but it will plug up sooner or later because the water will equal out till the dirt and junk fills up. You always want to use common sence and remember the water loves to go down hill!
If the main drain in your pool does not drain it is either a minor or major fix. The minor fix is to first look at where the water comes out and see if there is something stopping the water from coming out. IF not you will need to rent a pump and drain the water......After that you would need to remove the grate in side the pool and reach inside to see if there…
Depends on the usage. In low power drain devices operated mostly at room temperature, alkaline batteries are probably better. In high power drain devices sometimes operated at high or low temperatures, lithium is definitely better. It is best to use what the manufacturer of the device the battery will be used in recommends.