The less current is delivered by a lead battery, the longer the battery lasts. The series connection of two identical batteries allows to get twice the rated voltage of the individual batteries, keeping the same capacity.
If at all possible, operate at moderate temperature and avoid deep discharges; charge as often as you can (See BU-403: Charging Lead Acid) The primary reason for the relatively short cycle life of a lead acid battery is depletion of the active material.
Charge will flow from one battery to the other two until they’re balanced. With a lead acid battery bank, the internal resistances are limiting to a point that you don’t have to worry about arcing or your battery cables overheating when you connect them (not the case with lithium-ion banks…).
The primary reason for the relatively short cycle life of a lead acid battery is depletion of the active material. According to the 2010 BCI Failure Modes Study, plate/grid-related breakdown has increased from 30 percent 5 years ago to 39 percent today.
That chemical reaction is fairly complicated – but we need only notice a couple of things about it: As power is drawn from a battery sulphuric acid is lost from the electrolyte and combines with the lead plates to form lead sulphate.
The most accurate method consists of measuring the density of the electrolyte. If you do not have a density meter, thanks to the following table you will be able to know the state of charge of lead batteries by measuring the open circuit voltage at their terminals with a common digital multimeter. How to connect multiple batteries together?