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Battery Capacity C Ratings

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  • #16

    Re: Battery Capacity C Ratings

    Re: Battery Capacity C Ratings

    What is being considered is the Peukert effect. For estimating realized energy capacity, you want to match your loads (power draw) to the energy capacity of the battery at the current that will be pulled from it to supply that load.

    Using the C/xx approach is needed for battery banks because the wiring of the bank is going to determine the current drawn from a specific battery to provide an overall power from the bank.

    Note that C/xx is a rule of thumb and not a measure. It is a means to normalize current draw by battery size. As a contrast, the most common energy capacity ratings for batteries tend to involve time rather than current, i.e. the 20 hour rate being most common. For a 100 AH battery, the 20 hour rate implies a 5 amp 12v or 60 watt load.

    For the Peukert coefficient usually found in lead acid batteries (1.2 to 1.3 or so), a change in current draw means a change in battery capacity of about 15% over the typical useful range.

    For the topic here, the first thing to do is to figure out your use profile. What is the average power draw and how uniform is it? What is the peak draw and what is the median and well as the mean. These statistics will give you a target for your battery bank. You want it to supply peak loads without significant loss and you want it to be most efficient at the median load. From that, figure out the current you will be taking from a battery in various bank configurations and use that current to determine the effective capacity of the battery from the C/xx ratings that most closely match your loads.

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