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Question 9.16: Battery Storage Calculation in a Cold Climate. Suppose that ......

Battery Storage Calculation in a Cold Climate. Suppose that batteries located at a remote telecommunications site may drop to −20ºC. If they must provide 2 days of storage for a load that needs 500 Ah/day at 12 V, how many amp-hours of storage should be specified for the battery bank?

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From Fig. 9.39, to avoid freezing, the maximum depth of discharge at −20ºC is about 60%. For 2 days of storage, with a discharge of no more than 60%, the batteries need to store

\text{Battery storage} \ = \ \frac{500 \ {Ah}/{day} \ \times \ 2 \ {days}}{0.60} \ = \ 1667 \ Ah

Since the rated capacity of batteries is likely to be specified at an assumed temperature of 25ºC at a C/20 rate, we need to adjust the battery capacity to account for our different temperature and discharge period. From Fig. 9.42, the actual capacity of batteries at −20ºC discharged over a 48-h period is about 80% of their rated capacity. This means that we need to specify batteries with rated capacity

\text{Battery storage} \ \left(\text{25ºC, 10-hour rate}\right) \ = \ \frac{1667 \ Ah}{0.8} \ = \ 2083 \ Ah
Figure 9.39
Figure 9.42

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