Modern vehicles have more auxiliary units drawing battery power than ever before. In the past, when vehicles were powered off, so were all electrical loads. Now, many of today’s commercial trucks require electricity full time.
Battery management is critical because increased loads, when not managed properly, can draw down batteries, leading to stalls and breakdowns.
Battery Disconnects – How and Why
Battery disconnect switches—also known as battery disconnects, master disconnect switches, or battery disconnect units—are an ideal way to prevent battery drain from parasitic loads. They are designed to isolate most electrical loads (i.e., circuits) from the battery.
If you have specialty vehicles or seasonal vehicles that are sitting parked for extended periods of time, shutting down the master disconnect protects the battery and gives you a better shot at starting the vehicle without having to jump start it.
Because battery disconnects cut off electrical power, they can also help protect against electrical fires and vehicle theft, ensure auxiliary items such as pumps and lights are powered down correctly, and enable reliable shutdown of power during vehicle maintenance.
Manual vs. Electric Battery Disconnects
Battery disconnects come in two main types—manual and electric. Manual disconnects, as the name implies, are switched on and off by hand. Electric disconnects are switched on and off via electric current. There are pros and cons to both types.
Manual disconnects are the most common. These simple On-Off switches are available in single pole (to control a single circuit) or double pole (two circuit) options. Manual disconnects are typically low-cost and are reliable for straightforward battery cutoff and “lock out tag out” for vehicle maintenance. Nearly all manual disconnects are single throw, meaning that there is one “On” position.
Right: A 400A Master Battery Disconnect from Cole Hersee, a Littelfuse brand.
For an excellent overview and comparison of the wide range of manual disconnects available, see Littelfuse’s Manual Disconnect Selector Guide.
Electric disconnects are more complex and expensive than manual but provide greater flexibility in operation and design. They can be positioned far away from the battery, such as inside a cab, by virtue of their lightweight, low-current wiring. Some are available with built-in timers, so that auxiliary functions such as lights can remain on for a certain amount of time before being disconnected. Electric disconnects can also be controlled by a telematics system, enabling a back-office operation to remotely shut down a vehicle.
Right: A 300A Bi-Stable Remote Relay from Cole Hersee, a Littelfuse brand.
Electric disconnects are typically constructed as bi-stable relays, meaning that once they have been powered on (or off), they remain in that state with no further energy required. This enables them to work efficiently.
In addition to disconnects used for battery cutoff, here are a two electric disconnect options to consider for unique situations.
Battery Combiner/Isolators. Many commercial trucks and vehicles use two batteries – one for main starting, and one for auxiliary “house loads” such as lifts, pumps, lights, sirens, radios, TVs, and the like. When the vehicle is running, a combiner/isolator allows both batteries to charge in combination from the alternator. When the engine is off, the isolator cuts off the two batteries from one another so you’re not prematurely draining the starter battery.
Right: A 200A Smart Battery Isolator from Cole Hersee, a Littelfuse brand.
Low Voltage Disconnects. When a vehicle is turned off, parasitic loads continue to drain the battery—especially if you continue to use a radio, headlights, or other auxiliary devices. Low voltage disconnects are solid-state electronic modules that protect the starting power and life of the battery by terminating connection to non-essential components when the voltage drops beyond a critical level.
Right: A FlexMod Low Voltage Disconnect from Cole Hersee, a Littelfuse brand.
Learn More About Battery Disconnects
For more in-depth information on specifying and installing battery disconnect switches, see Safeguarding Current in Today’s Commercial Vehicles and Equipment – a webinar by Littelfuse and Waytek, now available on demand.
In addition, check out these other battery management articles by Waytek:
- A Cooler, Smaller Way to Charge and Isolate an Auxiliary Battery
- How to Choose Between a Relay, Solenoid and Contactor
- A Guide to Battery Management Systems
- Battery Isolators and Automatic Charging Relays
- How to Handle Battery Switching and Circuit Protection at the Same Time
- How to Choose Between Battery Isolators and Separators
- A Guide to Successful Dual Voltage Systems
See all the battery management products available and in stock at Waytek.