Marine Battery Switch Basics

Perko Battery Selector SwitchCustomers who are brand new to boating will often ask what that “big red switch” is for and why their boat needs it. This Navigator explains what battery switches are used for and how to know when you might need to buy one with the “AFD” (Alternator Field Disconnect) feature.

Any boat with more than one battery will usually be set up with two “banks” of batteries – one that is used purely to start the engine (the “start” battery bank), and another (often referred to as the “house” bank) to run any items on-board that require power (like refrigeration, lights, radios, etc.). When sitting at anchor, it’s a good idea to keep these two battery banks separate so that if you accidentally drain down the house bank, you will still be able to start your engine and get back to port. Installing a battery switch gives you complete control over which bank is being used and/or charged.

Battery switches usually come with 4 positions:  

  • OFF – Means that when you are running the engine, the alternator is not charging either of your battery banks.  If you are plugged into shore power, this switch is bypassed and all banks will be charged regardless of the battery switch setting.
  • 1 – Is usually assigned to your “start” battery (you’ll need to check your own boat to be sure). Use this position when you are starting your engine and want to give it all the juice a starter battery is meant to give – or to charge your starter battery with your alternator while the engine is running.
  • 2 – Is usually assigned to your “house” bank (again – check your own boat, don’t make any assumptions). Use this position when you are at anchor and need to run something that requires power such as your refrigeration or lights. Also use this position to recharge your house bank with your alternator while the engine is running.
  • ALL (or 1+2) – This position means you are drawing power or charging ALL of your batteries – sounds like the perfect setting, right?

e-Series Battery Switch from Blue Sea SystemsBeware of ALL

As stated, using the ALL position gives you the power of all of your batteries – which might prove handy if you’ve accidently run down your starter battery and need all the help you can get to start your engine.  But what happens if you’re sitting at anchor, with ALL selected – enjoying your stereo, drinking your ice cold beverages and turning on the lights when it gets dark?  While the extra power you get may sound great, you are now running the risk of depleting ALL of your batteries and may no longer be able to start your engine when you’re ready to leave the next day.

Add-A-Battery System from Blue Sea SystemsWhen using the popular OFF - 1 – 2 – ALL battery switch, start your engine with the switch set at position 1 (start battery), and motor with the switch set on 1 to recharge the start battery.  After about 20 minutes, switch to 2 to recharge the house bank for the remainder of your motoring - than you will already be on our house bank when you anchor.

Don’t want the hassle/responsibility of having to remember all the setting changes? We highly recommend that you consider installing a battery switch system that automatically manages this for you - such as a Blue Sea Systems “Automatic Charging Relay” (ACR), or their “Add-A-Battery” system – which can take all the worry and inconvenience out of battery management at a nominal cost. You can learn more about these automatic systems here.

So What’s an AFD?

When your boat is operating under engine, the alternator is used to charge your batteries. In boats without a battery isolator, it is a common practice for the output of the alternator to be led to the load side of the battery selector switch, which allows you to choose which bank you want to charge. This is especially useful if one battery bank is fairly low while the other is still fully charged.

But what happens if the battery switch is accidentally turned to the “OFF” position while the engine is running – say if you want to change from one bank to the other? When you pass by the OFF position (even momentarily) the output path from the alternator is suddenly eliminated. In just a fraction of a second, this causes a voltage spike of several hundred volts which can burn out the voltage regulator and the diodes in the alternator, rendering it useless. That’s why it’s very important to never disconnect the output wire of a running alternator.

An AFD (alternator field disconnect) switch prevents this problem by disconnecting the power to the field windings of the alternator (cutting its output) before the battery switch is allowed to break the connection to the battery. This avoids the voltage spike that causes damage to the alternator diodes and voltage regulator. Conversely, the AFD switch will only reconnect power to the alternator’s field windings after the connection to the battery has first been re-established.  

On boats equipped with a battery isolator, the alternator is always connected to the batteries, so if you have a battery isolator you do not need this feature in your battery switch.

We hope this Navigator has helped you understand what battery switches are used for and whether you need the AFD feature when adding one to your boat. If you have further questions or need clarification, please don’t hesitate to contact our experts at (800) 426-6930.

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