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→Capacity of House Bank Required
When you have determined the <i>Total Daily Load</i> in AH, multiply it by the desired <i>Charging Interval</i> in days to determine the <i>Battery Drain Between Charges</i>.
<pre>Battery Drain Between Charges = [Total Daily Load ] * [Charging Interval]</pre>
Once a day seems like a common-sense choice. With less than a day, there will be a tendency for charging cycles to run into each other, along with all the extra fuss for your neighbours at the anchorage. With more than a day, you will need an ever bigger and more expensive house bank and alternator to carry over. With once a day, you exercise the system every day, keep the engine from rusting out, produce minimal fuss, and keep battery and alternator costs in a reasonable range.
Divide this number by the AH rating of your chosen battery type, to determine the number of batteries in the house bank. Typically, for a boat under 65 ft, the house bank will have four to ten 8D deep-discharge batteries with a capacity of 1,100-2,800 AH.
== Required Charging Capacity ==
The next step is to determine the <i>Required Charging Capacity</i> so you can size the alternator.
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]
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.