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Sizing a Battery Charging Systems (BCS)
Electricity demand
When sizing a BCS it is important to consider the electricity demand of possible customers. On the individual scale (demand of a family) electricity demand in a newly electrified area can be estimated to lie at about 50 to 200 Wh/d if a flourescent light and a small radio with tape recorder is used. A commonly available 12 V battery of a useful capacity of 80 Ah has a stored energy of 960Wh.
However, depending on the type of battery (shallow cycle batteries as in automobiles vs. deep cycle batteries as in advanced PV systems) the capacity that can be used repeatedly in a sustainable way (i. e. without destroying the battery) varies enormously. Shallow cycle batteries, like the type used as starting batteries in automobiles, are designed to supply a large amount of current for a short time and stand mild overcharge without losing electrolyte. Unfortunately, they cannot tolerate being deeply discharged. If they are repeatedly discharged more than 20 percent, their life will be very short. These batteries are not a good choice for a PV system. Deep cycle batteries are designed to be repeatedly discharged by as much as 80 percent of their capacity so they are a good choice for power systems. Even though they are designed to withstand deep cycling, these batteries will have a longer life if the cycles are shallower. All lead-acid batteries will fail prematurely if they are not recharged completely after each cycle. Letting a lead-acid battery stay in a discharged condition for many days at a time will cause sulfation of the positive plate and a permanent loss of capacity.[1]
[! Please add a hint about how much of this capacity can be used from a solar battery and how much from a car battery ! ]Depending on consumption it must be recharged every 5 to 20 days. This lies in the required range for acceptance of the technology by the consumers.
Thus, the generation facilities for running a BSC should be sized in order to meet these minimum demands (that is about 200 Wh/d x targeted customer base).
Modular BCS
Principally, BCS can easily be operated in a modular way, implying that adding more batteries to the charging stations as demand grows is possible as long as electricity generation is sufficient. This procedure seems reasonable as it allows starting with small systems (pilot projects) that can be incrementally increased once the project is succesful.
Batteries should be recharged in parallel.
