12V System Voltage Limits

The 12V system voltage limits are quite well defined by the automotive industry as there is a need for all of the components to survive.

Note: the 6V cold engine start limit only requires key ecu’s to operate

Based on this LV 124 sets out Vmin and Vmax limits for different classes of function.

6V16VFor functions that must retain their performance during the engine starting event.
8V16VFunctions that do not have to retain their performance during engine starting event. The Vmin value could be breached if the engine is cold, hence, if possible the function should operate at 6V or not have to retain performance under engine start.
9V16VFunctions that must retain their performance when the engine is not running.
9.8V16VFor functions that must retain their performance when the engine is running.

The introduction of stop/start into vehicles has pushed the operation of the battery further as it needs to support the car operation when the engine is off at traffic lights. Plus the transition from engine running to engine off to engine cranking and running needs to be seamless for the user. A sudden dropout in the 12V supply and the entertainment system and perhaps the phone call you are making will end.

This post has been built based on the support and sponsorship from: About:EnergyAVANT Future MobilityQuarto Technical ServicesTAE Power Solutions and The Limiting Factor. 

This demonstrates the requirement for a systems approach to battery pack design, understand the components and the 12V System Voltage Limits.

History – the max regulated voltages of the pre-alternator era, namely using dynamo’s, if using a Lucas charging system could be either 16Volts for the Two Bobbin RB102/6 Compensated Voltage Regulator or 15.5V for the three Bobbin RB340 Current & Voltage Regulator. And the Lucas Dynamo’s of that era could produce 22Amps all day long!

Kevin Vickers


  1. LV 124 and LV 148 Overview, WKS Informatik
  2. How your car starts, autobatteries.com
  3. Introduction to LV124 Compliant Products and Services, espec

Mastering 12V Lithium Iron Phosphate (LiFePO4) Batteries

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