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→‎Example Calculation
== Total Daily Load ==
Load is measured in total daily amp-hours (AH), which is simply the average current drawn per hour times 24 hours. Calculating this is a major task subject to much second-guessing. The first step in calculating load is to determine the combined DC and AC AH -DC load for all ‘appliances’. Use a spreadsheet to list each item and its wattage or current draw, depending on which is available. For the DC, make sure you work entirely in either 12 or 24 V. For the AC start with watts and divide by the VDC to approximate the DC AH. See [[ElectricalCapacityDC]] and [[ElectricalCapacityAC]].
For each item, estimate the duty cycle (how long it will be used each day). Do a separate tally for fixed loads (e.g., instruments) and intermittent loads (e.g., coffee maker). If in doubt it is safer to over-estimate the duty cycle. But don’t go overboard. If you over-estimate too much you might have to go back and tweak the numbers more realistically when you realize that you need to tow a sub-station behind you on a barge to supply your electrical requirements.
Use whichever number is the highest for all future calculations. Let’s call this the <i>Total Daily Load</i>.
 
=== Example Calculation ===
 
This example yields a <i>Daily Charging Period</i> of 0.6 hours or 36 minutes. A spreadsheet is provided: [[Media:HousebankCalculation.xlsx|MS Excel format]]
 
<table width="50% border="1">
<caption>House Bank Calculation - Example</caption>
<tr>
<th>Line</th>
<th>Item</th>
<th>Amount</th>
<th>Calculation</th>
<th>Comments</th>
</tr>
<tr>
<td>A</td>
<td>Total Daily Load AC (DC AH)</td>
<td>200</td>
<td></td>
<td>Normalize to 12 or 24 VDC (AC watts/DC voltage)</td>
</tr>
<tr>
<td>B</td>
<td>Total Daily Load DC AH</td>
<td>50</td>
<td></td>
<td>Normalize to 12 or 24 VDC (DC watts/DC voltage)</td>
</tr>
<tr>
<td>C</td>
<td>Total Daily Load AH</td>
<td>250</td>
<td>= A + B</td>
<td></td>
</tr>
<tr>
<td>D</td>
<td>Charging Interval (days) </td>
<td>1</td>
<td></td>
<td></td>
</tr>
<tr>
<td>E</td>
<td>Battery Drain Between Charges AH</td>
<td>250</td>
<td>= C * D</td>
<td>Amount to recharge</td>
</tr>
<tr>
<td>F</td>
<td>Battery Efficiency Factor</td>
<td>1.1</td>
<td></td>
<td>Typically 90%</td>
</tr>
<tr>
<td>G</td>
<td>Charging (Safety) Factor %</td>
<td>400</td>
<td></td>
<td>Use 350-400+</td>
</tr>
<tr>
<td>H</td>
<td>House Bank Required AH</td>
<td>1100</td>
<td>= E * F * G</td>
<td></td>
</tr>
<tr>
<td>I</td>
<td>Battery AH</td>
<td>275</td>
<td></td>
<td>Use the AH rating of selected battery</td>
</tr>
<tr>
<td>J</td>
<td>Number of 8D Batteries</td>
<td>4</td>
<td>= H / I</td>
<td></td>
</tr>
<tr>
<td>K</td>
<td>Battery Capacity</td>
<td>1100</td>
<td>= I * J</td>
<td>Reality check in case H and K are not equal. </td>
</tr>
<tr>
<td>L</td>
<td>Charging Factor %</td>
<td>33</td>
<td></td>
<td>25% is the norm for flooded cell; 40% for gel cell; 50+% for AGM</td>
</tr>
<tr>
<td>M</td>
<td> Basic Charging Rate AH</td>
<td>363</td>
<td>= K * L</td>
<td> </td>
</tr>
<tr>
<td>N</td>
<td>Fixed DC Load AH</td>
<td>5</td>
<td></td>
<td></td>
</tr>
<tr>
<td>O</td>
<td>Fixed AC Load AH</td>
<td>50</td>
<td></td>
<td>Normalize to 12 or 24 VDC (AC watts/DC voltage)</td>
</tr>
<tr>
<td>P</td>
<td>Other DC Load AH</td>
<td>0</td>
<td></td>
<td>Load while charging</td>
</tr>
<tr>
<td>Q</td>
<td>Required Charging Capacity AH</td>
<td>418</td>
<td>= M + N + O + P</td>
<td> </td>
</tr>
<tr>
<td>R</td>
<td>Time to Charge Hours</td>
<td>0.6</td>
<td>= E / M</td>
<td></td>
</tr>
</table>
== Capacity of House Bank 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 A flooded cell bank will take 55 min the longest to charge, a gel cell 34 min 60% of the time and an AGM cell 28 min50% of the time. 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  == Charging Systems == There can be several times largermultiple charging sources with automatic switching: * Shore power charger* Alternators* Trickle charger See [[ACSecondarySystems]] and [[DCPrimarySystem#Charging_Systems]].
[[Category:BatteriesPrimary]]
[[Category:ElectricalPrimarySupply]]