What is battery capacity?
Capacity is the leading health indicator of a battery, but estimating it on the fly is complex. The traditional charge/discharge/charge cycle is still the most dependable method to measure battery capacity. While portable batteries can be cycled relatively quickly, a full cycle on large lead acid batteries is not practical for capacity measurement.
Capacity -Deep cycle batteries are rated in amp-hours. An amp-hour is one amp for one hour, or 10 amps for 1/10 of an hour. Stated mathematically (amps x hours). If you have something that consumes 20 amps, and you use it for 30 minutes, then the amp-hours used would be 20 (amps) x .5 (hours), or 10 AH. The the most common accepted AH rating time period for batteries used in solar electric and backup power systems is the "20 hour rate". This means that a battery is discharged down to 10.5 volts (100% capacity) over a 20 hour period while the total actual amp-hours it supplies is measured. The 100 hour rate is sometimes given just to make the battery look better than it really is. See Peukert Effect below.
The maximum total electrical charge, expressed in ampere-hours, which a battery can deliver to a load under a specific set of conditions.
Two 6V, 225Ah batteries are wired in Series, the voltage is doubled but the amp-hour capacity remains 225Ah (Total Power = 2700 Watt-hours [225A * 12V]).
Two 6V, 225Ah batteries wired in Parallel will have a total storage capacity of 450Ah at 6V (or 2700 Watt hours [225A * 2 = 450Ah] and [450Ah * 6V = 2700] ).
Series-Parallel looks and sounds more complicated, however the principal is the same. Consider, four 6V cells are wired in two "strings" of 12VDC that were then wired in parallel. Using 6V, 225Ah batteries, this system will have a storage capacity of 450Ah at 12V or 5400Wh.
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