Battery Disconnects 101

One of the most common questions we field is,  “Which battery disconnect do I need?” In order to determine that, you need to know how you intended to use it. There are two main purposes:

  1. Disconnect the battery when storing the vehicle. This convenience allows you to store the vehicle for an extended period of time without worry of the battery running flat due to parasitic draw of the accessories . ECUs, BCMs, PCMs, radios, clocks, etc. all draw a very small amount of current with the vehicle sitting to retain settings or data.
  2. Pass tech at your local track. The NHRA and most other sanctioning bodies mandate a Battery Disconnect Switch that is clearly labeled and externally accessible for any vehicle with a battery that has been relocated to the rear of the vehicle. This affords the safety crew the ability to shut off a running engine and disconnect power to the vehicle’s accessories in the event of a crash, rollover, etc.

Vehicle Storage

This is simple enough. There are two ways of approaching this. Both are simple and the outcome is the same.

Standard Battery Disconnect Switch

Turn it OFF and you’ve disconnected power to the vehicle. The switch can be wired to interrupt either the positive or negative battery cable; it matters not, as the function is the same.

The Moroso 74101 Battery Disconnect Switch is the correct switch for this application.

Battery Disconnect Terminal

Simply turn the knob and the battery is disconnected. These are simple to install, inexpensive, and available for side or top post batteries: no cutting required. These are designed to interrupt the negative battery cable.

The Moroso Side / Top Post Battery Negative Disconnect is correct for this application.

Pass Tech at Your Local Track

You’re a racer and you want to pass tech. No problem, we can help! The first consideration is to know exactly how the rules read in regard to acceptable locations for the Battery Disconnect Switch. This can vary by the sanctioning body so you’re advised to invest in a rule book. Acceptable location(s) of the switch itself will be spelled out clearly in the rule book. It’s very important to know this beforehand so that there are no surprises when you show up to race!

To determine which switch is correct for your application, we need to know whether you have a one-wire alternator or one with a traditionally wired regulator.

One-wire Alternator Installations

If you have a one-wire alternator, the Moroso 74101 Battery Disconnect Switch is the correct switch for this application.

It is wired per the following diagram:

Battery Disconnect for One-Wire Alternator

To take the guesswork out of this installation, we’ve developed a full line of Master ON/OFF Add-on Kits.  These are available for both our Standard and Deluxe Battery Relocation Kits and are offered in 4, 2 and 1/0 AWG sizes for alternators of all sizes. At CE Auto Electric Supply, you can purchase everything you need:

  • Battery Relocation Kit (choose Standard or Deluxe)
  • Master ON/OFF Add-On Kit (match to Standard or Deluxe Battery Relocation Kit)
  • Moroso 74101 Battery Disconnect Switch

Simply visit the Battery Relocation Kits page to do so.

Traditionally Wired Regulator Installations

If you have an alternator with a traditionally wired regulator, also referred to as a three-wire, you can also use the above diagram and parts and it will work perfectly. If you’re weight-conscious and want to eliminate running a large charge lead the length of the vehicle, you have a second option; use the Moroso 74102 Battery Disconnect Switch and we can build you an Exciter Interrupt Harness (typically $29.95 or so depending on the length) of the length you require. The Moroso 74102 is actually a pair of switches in a single housing where a single switch motion closes/opens both switches. One of the switches is high-current (interrupts the battery cable) and the other is low-current (interrupts the exciter circuit of the alternator).

It is wired per the following diagram:

Battery Disconnect for Traditional Regulator

Again, at CE Auto Electric Supply can you purchase everything you need:

  • Battery Relocation Kit (choose Standard or Deluxe)
  • Exciter Interrupt Harness
  • Moroso 74102 Battery Disconnect Switch

Simply visit the Battery Relocation Kits page to do so. You’ll have to call or email us to order the Exciter Interrupt Harness as we custom build these based on the length you require. Before doing so, it’s wise to choose a mounting location for the Battery Disconnect Switch and then measure from it to the regulator harness of the alternator.

For either of the Moroso Battery Disconnect Switches, you have the flexibility of mounting the switch externally (for example: to the bumper or near the license plate) or internally (closer to the battery). If you choose to locate the switch internally, we also offer the Moroso Push/Pull Battery Disconnect Mounting Kit to simplify this.

Finally, we stock a nice variety of Battery Boxes and Battery Trays should you also require either.

So there you have it; now you know. At CE Auto Electric Supply, we’re here to help!

Update – 06/27/18

Recently, a customer asked, “Can I use a disconnect switch on my vehicle?  It has Electronic Fuel Injection.”

The answer is YES!

For the purpose of this discussion, there are two types of electronically fuel injected vehicles – OEM equipped, and Aftermarket.  Either way, interrupting battery power to the ECU(s) can cause the system(s) to have to relearn idle characteristics, begin running emissions tests, etc. upon restart.  All undesirable side effects.

For vehicles equipped with an Aftermarket EFI system, simply connecting the power and ground leads directly to the battery terminals as directed by the manufacturer is the key.  It’s just that simple.

For vehicles equipped with an OEM designed system, it’s not as cut and dry as there can be multiple ECUs / PCMs/ BCMs that require uninterrupted power to retain settings, data, etc. when the vehicle is not in use.  In that case, we recommend that you hire the service of an ASE Certified mechanic fluent in that make of vehicle to help you with the installation.

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