Shrink the 12V Battery by Half and Maintain Cold Crank?

Advances in safety features, comfort and infotainment systems in automobiles are escalating power requirements, challenging power systems engineers to meet the growing power demands with restrictions in weight and space. Over the last decade, the typical 12V power requirements have increased from 1.2kW to 4kW continuous load, which has a compounded effect on the 12V lead-acid battery, which must be sized for double the required power because of their 50% depth of discharge (DoD) restriction.

Why use a 12V lead-acid battery at all? Well, PHEVs and MHEVs cannot completely eliminate the 12V battery as it remains the only choice for cold-crank starting. However, sizing the 12V battery correctly to meet all the quiescent current (always-on) use cases as well as cold crank is a complex calculation.

Optimizing the 12V battery for new power demands

Most OEMs are redesigning their power delivery architectures for both BEV and ICE vehicles to support the demands on mild-hybrid (MHEV) and plug-in hybrid (PHEV) categories. They are working feverishly to contribute to the adoption of electric vehicles for consumers who are reticent about limited BEV range and charging network availability and compatibility issues. Today, the PHEV is a practical option delivering the benefits of electrified traction without the range anxiety.

Power system designers have to consider worst-case scenarios and all corner cases, which include –40 to +50°C environments. Starting the engine at –40°C is one of the most difficult use cases and is referred to as cold cranking.

Batteries are specified with their cold current supply in amperes at –40°C with a term known as ‘cold current amperage’ or CCA. Batteries, due to their chemistry, inherently resist charging and discharging at low temperature and the CCA rating helps system engineers decide whether the chosen battery will meet the system demand or not. At these low temperatures, the typical traction lithium-ion batteries perform poorly due to their chemistry. They cannot provide this rapid peak current (75A/30µs) via the DC-DC converter to support the engine cranking requirements. The lithium-ion cathode chemistries are optimized for packaging efficiency and maximizing BEV range, not cold cranking.

An added advantage of the 12V battery is that it provides the required capacitance for absorbing any transients from the low voltage bus. These transients are typically generated from the electrical starter generator motor.

Packaging challenges for the 12V battery

Typically, the starter batteries are packaged under the front passenger seat as the engine bay temperatures are not suitable for the batteries to be packaged. However, finding package space for the PHEV battery often takes priority to maximize the EV range. Furthermore, the growing power requirements for the 12V battery make packaging even more difficult. An automobile typically requiring an H6-sized battery now requires an H8 AGM battery (Figure 1), requiring relocation to the rear passenger compartment. The packaging, coupled with the added distance of routing 50 mm² cables (up to three times further) increases the total system cost and adds weight.

Power delivery architectures and component packaging on vehicle platforms are typically designed to reduce system complexity across all powertrains (ICE, MHEV, PHEV) and therefore the increased system costs carry over from PHEVs to ICE models as well.

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Table 1: This represents the sizing, dimensions and typical naming conventions for 12V Starter Batteries. As more OEMs increase 12V battery capacity (from H4 to H8), the added size and weight serve to limit vehicle range. With Vicor modules, 12V battery sizes can be reduced by 2/3^rd by weight and almost ½ by volume. (Data sourced from Automotive DIN standards)

Minimizing the 12V battery to maximize the power delivery network

The typical loads for a 12V battery have two primary purposes – one is to start the engine and the other is for standby operation or ‘quiescent current.’ With the growing number of electric customer comfort features, and the number of ‘always-on’ features, the demand of the standby current and the number of features that need to be always-on has been increasing exponentially. In the typical use case, most car OEMs design the standby time for 50 days. This usually amounts to 14 – 16mA current drain which over 50 days is equivalent to 16A·h of total capacity of the battery pack.

The infotainment, telematics and vehicle approach unlocking features, further add up to 6A·h of battery capacity (as they are not always on). The always-on loads have always increased the cycling on the lead-acid batteries which have led to high warranty spends arising out of flat (depleted below 50% DoD) lead-acid batteries.

Alternatively, the always-on energy can be supplied from the traction Li-ion battery using Vicor fixed-ratio bus converter modules to transfer the energy efficiently to a load that is always on or cycles on and off, allowing better control and better efficiency. Further, the Vicor Sine Amplitude Converter (SAC™) topology used in bus converters enables even faster transient responses than a 12V lead-acid battery (Figure 2).

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Table 2:  Vicor modules have transient performance much faster than 12V batteries and can easily process 700 amperes per millisecond

The SAC topology can process thousands of amperes from a high-voltage battery to the load without any dips and bench testing shows response times three times faster than a typical 12V battery (Figure 3).

A key advantage of fixed-ratio converters, like SAC, is the ratio or K factor, which is the ratio of primary to secondary turns, that has a squaring effect on its effective output capacitance.

The SAC topology has a squaring effect on its effective output capacitance.

For example, for a 48-to-12V converter the K factor is ¼ which means the effective secondary capacitance is four squared, or 16 times, the primary capacitance. 

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Figure 1: Vicor modules have excellent transient response and can be treated as a ‘virtual battery’

A lighter, smaller virtual battery for standby power and routine starting functions

A Vicor high-voltage isolated (BCM®) (Figure D) and low-voltage non-isolated (NBM™) bus converter modules, provide optimal solutions for a “virtual battery” that replicates the essential properties of the battery while reducing size, weight and temperature limitations.

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Figure 2: Vicor modules are the smallest and lightest power modules. The image shows a K 1/16 BCM that packages 2kW of power in the palm of your hand

If the standby loads are powered by the Vicor BCMs or NBMs along with the majority of starting conditions, the sizing of the 12V lead-acid battery can be significantly reduced down to H4 or even lower sizes for only cold crank, further improving the system weight. This further improves the packaging options to fit under the front passenger seats, reducing associated harnessing.

Being closer to the load reduces parasitic inductances and series resistances that come with a high-amperage power system. Vicor offers packaging advantages which allow it to be closer to the load. This eliminates any internal series inductance at the input or output and can easily process 700,000 amperes per second or 700 amperes per milliseconds as they show excellent transient response (Figure 3).

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Figure 3: Vicor modules coupled with 12V starter battery helps to shrink the 12V battery size significantly without losing the critical -40 degrees C cold-cranking capability. It also allows the batteries to be packaged in favorable locations, while achieving a simpler and common electrical architecture across MHEVs & PHEV. Lastly, it helps to achieve ASIL D power system ready for Level 3 ADAS features

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Table 3: Vicor modules not only help to improve the material cost but also overall system costs while simplifying the electrical architecture, vehicle efficiency and improved performance all without losing any functionalities. Effectively car companies can realize savings up to €94 per vehicle

Vicor power modules enable new ways to optimize EV power systems

With the ever-increasing complexity of the growing comfort and connected features in combination with multiple powertrains on a single platform, OEMs will benefit from the simplification of the power distribution architecture while improving the reliability.

As automotive PDNs strive for greater power design efficiency, Vicor power modules offer innovative opportunities for engineers to achieve their goals. There is a variety of benefits for PHEVs to adopt a power module PDN (Table F). Leveraging power modules and reducing the 12V battery, enables a reduction in power system design size by 66%, reduces weight by more than 50% and increase efficiency over 15%.  Effectively car companies can realize savings up to €94 per vehicle.

With modular power, engineers can minimize the 12V battery, achieve a common power architecture across ICE, MHEV & PHEVs and meet additional feature requirements. Designing with power modules reduces complexity and weight, increase reliability and improves system efficiency – all of which contribute to improved system costs and increased EV range.

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