Along with BatteryType and BatteriesLayout, the main considerations in the design of the house bank are:

• Daily load
• Capacity
• Charging capacity of the alternator
• Trickle charging

Total Daily Load

Load is measured in total daily amp-hours (AHAmpere-hours (A*H)), 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 DCDirect current and ACAlternating current 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 VVolt. For the AC start with watts and divide by the VDCVolts direct current 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.ggram., 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.

All estimating processes must be subject to a reality check. It’sSecond better to get each number as exact as possible, then add a fudge factor to the total, rather than fudge numbers individually.

In addition, you need to calculate the peak and surge requirements. To estimate the peak demand, determine which of the largest appliances will be used simultaneously. To estimate the surge demand, determine the surge on start up of large AC motors. (DC motors do not have a surge.)

Use whichever number is the highest for all future calculations. Let’s call this the Total Daily Load.

Example Calculation

This example yields a Daily Charging Period of 0.6 hours or 36 minutes. AAmpere (amp), SI unit of electrical current spreadsheet is provided: MS Excel format

Capacity of House Bank Required

When you have determined the Total Daily Load in AH, multiply it by the desired Charging Interval in days to determine the Battery Drain Between Charges.

Battery Drain Between Charges = [Total Daily Load] * [Charging Interval]

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 House Bank Required. 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 Battery Drain Between Charges by 333% and throw in a 15% fudge factor for good measure, i.e., multiply the Battery Drain Between Charges 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 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 House Bank Required.

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 ftFoot, 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 Required Charging Capacity so you can size the alternator.

The Charging Factor 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 Basic Charging AH, multiply the House Bank Required in AH by the Charging Factor. To this, add the battery load while charging, i.e., the Fixed DC Load, Fixed AC Load and Other DC Loads. This total gives you the Required Charging Capacity AH. The larger this is, the bigger and more expensive the alternator required.

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 Daily Charging Period, divide the Battery Drain Between Charges by the Required Charging Capacity (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.