Fast Tip…by Dana Sims..Battery Switching

FAST Tip – – Battery Switching – – Dana Sims
F.A.S.T = For A Safer Trip FT15-03

ASI Keelboats have multiple batteries and a battery switch (1, 2, ALL, OFF).

The battery switch should never be moved to or through the OFF position while the engine is running. Doing so could cause damage to the diodes in the alternator and result in an expensive repair.

Why do boats have a battery switch and how do I use it?

Having a battery switch allows you to control battery usage, so that you do not drain both of the batteries down and strand yourself without enough power to start the engine. For this philosophy to work, you must have the switch in either position 1 or position 2 when operating on battery power alone (not connected to shore/dock power or with the engine running). This battery in use (1 or 2) becomes your “House” battery and all power consumed, while anchored for instance, will be from that battery, leaving your other battery in reserve to serve as the engine “Starting” battery. The important thing is not to leave the switch in the ALL position when anchored overnight or when batteries are not being charged on shore power.

To balance the battery usage and battery life, it is recommended that you alternate which battery is used as the “House” battery. Battery 1 on odd calendar days, battery 2 on even calendar days, for instance.
For starting and charging, you can use both batteries (ALL switch position). The engine should be run at least 2 hours per day to fully charge both batteries (or the boat connected to shore power). For the good of the engine, run the engine at 1500 RPM or higher, while motoring, for instance.

What if I have totally drained the House Battery?
If your “House” battery is severely drained or dead, you might want to start on only the “Starting” battery. In this battery situation, in the ALL position, the “Starting” battery will be drained by trying to charge the depleted “House” battery in addition to starting a cold engine. After the engine warms up, you can stop the engine, change the battery switch to “ALL”, and restart the engine to complete battery charging.
1. These instructions are written and applicable to all boats with a battery switch.

2. Most newer battery selector switches have “make-before-break” contacts so that you can move from position 1 to 2 or to ALL without damaging the alternator. ASI has older model boats, (Overture is over 30 years old), and so it is ASI’s practice that you do not move the switch to any of the positions 1-2 or ALL while the engine is running. Never move a battery switch to or through the OFF position – no matter what you have read or heard. Equipment damage may result.

3. Older boats, like ours, only charge the battery according to the switch position.

4. Newer boats may have circuits which charge both batteries regardless of switch position (1, 2, ALL); allow safe switching while the engine is running; and regulate battery charge rates independently to each battery. Individual boat manuals should
be consulted. Multi-engine boats (catamarans) are managed differently, due to having multiple engines and sometimes more batteries.

5. For safety, some things are wired to drain batteries regardless of switch position (ex. bilge pumps, anchor lights)

6. Some boats have different types of batteries for “House” and “Starting”. One battery is always “House”, the other always “Starting”. ASI boats use deep cycle marine batteries for both and can alternate between batteries.

7. It is always best to leave the dock with fully charged batteries.

Next week’s FAST Tip will address battery charging, testing, and system health.

Thank you to Jack Townsend for input on this topic.

Any question, comments, or suggested topics can be sent to

“FAST Tips are written by ASI members for the ASI membership, and apply to the operation of ASI vessels. While the contributing members may not be experts in a particular sailing field they are experienced members who have developed some “best practices” and wish to share them with their fellow members

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