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→Required Charging Capacity
The <i>Charging Factor</i> determines the required capacity of the charging system (alternator). This rate of charge will damage the battery if it is too high. If it is too low, the batteries will be chronically under charged. The rule of thumb is to charge a deep-discharge flooded-cell battery at a rate of 25% of the listed AH. A gel cell can be charged at 40%; while an AGM can take an unlimited charge.
To determine the <i>Basic Charging AH</i>, multiply the <i>House Bank Required</i> in AH by the <i>Charging Factor</i>. To this, add the battery load while charging, i.e., the <i>Fixed DC Load</i>, <i>Fixed AC Load</i> and <i>Other DC Loads</i>. This total gives you the <i>Required Charging Capacity AH</i>. The larger this is, the bigger and more expensive the alternator required.
<pre>Daily Charging Period = [Battery Drain Between Charges]/[Required Charging Capacity]</pre>
Finally, we need a reality check. How long will it take each day to re-charge the batteries? An hour would be nice. Several hours would be insufferable, and counter-productive. To determine the <i>Daily Charging Period</i>, divide the <i>Battery Drain Between Charges</i> by the <i>Required Charging Capacity</i> (other loads net out). In the example shown, a flooded cell bank will take 55 min to charge, a gel cell 34 min and an AGM cell 28 min. Obviously a gel or AGM is the way to go, provided you can manage the larger alternator and charger system. Remember that these times are for a hypothetical house bank of 1100 AH capacity. A real example is likely to be several times larger.