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Old 05-12-2003
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Battery Bank Design

 
Keeping a complex electrical system supplied with power requires sound design.
 
One of the first decisions to make when planning a marine electrical power system is how many battery banks to have. A battery bank can be a single battery or multiple batteries connected together to create essentially one large battery of the required voltage and amp-hour capacity. Battery banks can be used for supplying general house loads (or a specific house load such as a large windlass), for starting engines (whether main auxiliary, gen-set, or DC diesel charger), or for a combination of house loads and engine starting. In some ways the battery configuration and capacity are the most important electrical power decision to make, and a wise choice can help guarantee a steady supply of electrical power as well as a system that is simple to operate and maintain.

 
The standard battery switch makes the system totally manual.
 

I firmly believe that the standard battery setup found on most cruising sailboats, one with two equal battery banks and a "1-2-Both-Off" main battery switch, evolved from a lack of understanding of electricity and how a direct current (DC) electrical power system and its various components are supposed to operate. In the past, many sailors with a two-house bank system used one of their banks until the lights dimmed, then switched to the other bank. We now know that by the time incandescent lights are dimming in a DC system, the battery bank supplying them is near the lower end of its capacity. The result of repeated deep discharge of this nature is dramatically reduced battery life.

Let's examine a two-house bank system and see how it can be improved. In a typical two-house bank system the loads and charging sources are connected through the common terminal of the main battery switch. Electrical current flow to and from the battery bank is determined by the position of the switch—bank 1, bank 2, or both banks. With this system the operator must remember to position the switch according to the state of charge of the various banks, and must know which bank was charged or used to supply loads the day before. Since in the past most boats weren't equipped with an accurate (i.e. digital) battery monitor to show state of charge, the lights and other appliances were often employed as the "monitor."

 
Adding isolating diodes automates the charging system but not the power usage.
 
Some sailors have improved upon this system by connecting the alternator output to isolating diodes so the alternator's current flows directly to the two battery banks as needed. An even better arrangement is to use a battery combiner, or link, that joins the battery banks together temporarily under the influence of a charging source. When using isolating diodes or a combiner, the main battery switch is used only for choosing which bank supplies the loads, since the charging current always goes to the same place. This approach is heading in the right direction, but it can still be greatly improved by having only one battery bank for general house loads, a separate bank for a specific house load (if there is a compelling reason to have one), and designated starting batteries as needed.

 
Heart Interface's Echo Charge automatically charges start batteries.
 

In the type of electrical power system described above, all charging sources and loads are connected directly to the house bank, thus eliminating the need for a "1-2-Both-Off" battery switch altogether. Instead, a simple "On-Off" switch with an appropriate current rating can serve as a house bank disconnect, useful when leaving the boat or servicing the electrical system. In addition, each engine on board has a designated engine starting battery with no house loads of any kind connected to it. The engine starting battery(s) is kept topped up by the electrical energy in the main house bank through an electronic charging device such as the echo-charge from Heart Interface. The key to this type of approach is an accurate system monitor that displays current flowing into or out of the house bank, bank voltage, and amp hours removed from the house bank. System monitors typically have a host of useful functions, but their main benefit is that they allow the operator to know precisely when it's time to recharge so as to ensure longest battery life.

 
State-of-the-art battery combiners automate the entire system.
 

If you have an existing two-house-bank battery system and feel it's not feasible to make a change, a two-bank system monitor will help you get the most out of your present setup for a minimal investment. It will clearly indicate when it's time to switch banks. If you do decide that it's practical to make a change, you'll appreciate the simplicity and ease of maintenance of a single-house bank system. To correctly size a house bank refer to my article Sizing Batteries
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