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Backup batteries sometimes referred to as auxiliary or supplemental batteries, serve an important role in maintaining power for certain vehicle systems even while the 12-volt battery power is cut. The backup batteries can aid in emergency situations by maintaining power for features like GM OnStar, Toyota Safety Connect System, and FCA Assist to name a few.
These batteries are not to be confused with vehicles that have more than one battery, like many hybrids. These types of backup batteries are often part of another module or part. Typically, the backup battery cannot be recharged and must be replaced if depleted. Which is why it is important to know whether the vehicle being repaired has a backup battery for a certain system and how to properly disconnect the main 12 V batteries prior to repairs. Battery disconnect procedures can be found within the vehicle specific service manual.
For example, per the 2025 Chevrolet Equinox service manual, for vehicles equipped with a battery back-up:
"Do not disconnect the main vehicle battery or remove the OnStar® fuse with the ignition key in any position other than OFF. Disconnecting power to the OnStar® module in any way while the ignition is ON or with retained accessory power activated may cause activation of the OnStar® Back-Up Battery. This action is per design as the back-up battery is designed to provide power to the telematics communication interface control module so an emergency notification call can be made after an event where the main battery is disabled. Once the Back-Up Battery is activated it will stay on until the power is restored back to the telematics communication interface control module. The telematics communication interface control module naturally chooses the main supply voltage as it’s default supply, but if the main supply is removed or lost for any reason the OnStar® module will use the Back-Up Battery as a power supply as long as the default supply can not be detected. The back-up battery is not rechargeable and once discharged below 9.5 volts the back-up battery must be replaced."
This is just one example, the proper procedures to disconnect a battery can vary for each vehicle. Always follow OEM procedures and warnings to ensure a complete, safe, and quality repair.
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Article validated in 2025