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HouseBank

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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 ==
 
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.
There are several approaches to determine the <i>House Bank Required</i>. A common one is to size the bank so that it cycles between 50% and 80% charged. Using this approach, you would simply multiply the <i>Battery Drain Between Charges</i> by 333% and throw in a 15% fudge factor for good measure, i.e., multiply the <i>Battery Drain Between Charges</i> by 350% to determine the size of the house bank.
 
However, batteries are constrained by their discharge/charge rate. For example, flooded-cell batteries cannot discharge at a rate more than 25% of their capacity. A better way is to base the size on the discharge/charge rate of the selected [[BatteryType|battery type]]. For a flooded cell, you would apply a factor of 400% to determine the total capacity required.
 
For gel and AGM cells, you could go as low as 300%; although in all cases more battery is better than less. The resultant is the <i>House Bank Required</i>.
 
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]</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). A flooded cell bank will take the longest to charge, a gel cell 60% of the time and an AGM cell 50% of the time. Obviously a gel or AGM is the way to go, provided you can manage the larger alternator and charger system.
 
== Charging Systems ==
 
There can be multiple charging sources with automatic switching:
 
* Shore power charger
* Alternators
* Trickle charger
 
See [[ACSecondarySystems]] and [[DCPrimarySystem#Charging_Systems]].
 
[[Category:BatteriesPrimary]]
[[Category:ElectricalPrimarySupply]]