Thermal Regulation and Considerations for On-Board Chargers

Thermal Regulation and Considerations for On-Board Chargers

Extreme temperatures and temperature fluctuations significantly affect electronic devices, contributing to a greater chance of failure. As a result, original equipment manufacturers (OEMs) must carefully consider the safe operating and storage temperatures for the on-board chargers in motive applications. 

Equally important, OEMs must also determine which cooling method they’ll use to shed heat generated during charging and how mounting configuration affects thermal regulation. 

The Two Major Temperature Considerations for On-Board Chargers

The two temperature-related scenarios that pose the greatest risk to on-board chargers are being subjected to extremes for prolonged periods and experiencing severe fluctuations.

Extreme Temperatures

Extreme temperatures—hot or cold—carry the potential to damage chargers, their components, and batteries, or limit their effectiveness. For example, charge-transfer resistance increases the colder it becomes, and charger components will begin to break down due to high heat produced.

A good standard to adhere to for battery chargers is safe operating and storage down to -40 oC (-40 oF) at the coldest. On the other end of the spectrum, chargers should withstand up to 65 oC (149 oF) during operation and 85 oC (185 oF) in storage.

Transient Temperatures

Transient temperatures are fluctuations between hot and cold. These swings in temperature place significant stress on circuit boards and components. This is primarily because the different materials used in their construction—like copper, fiberglass, and soldering material—all have different thermal coefficients for expansion and contraction. Therefore, the materials experience additional mechanical load in these conditions, particularly where joined together.

When chargers experience transient temperatures, they may undergo:

• Thermal shock – Certain applications for mobile machinery will require moving between significant temperature differences. For example, if a lift truck is used to retrieve items in cold storage and bring them out to a loading bay registering 30 oC (86 oF), that drastic and rapid change could lead to circuit boards damaging the solder joints that connect their components.
  • The risk of thermal shock necessitates charger design, engineering, and manufacturing partners that will ensure quality production—especially at scale.

• Thermal cycling – Cycling leads to thermal fatigue over time but is unavoidable, as it occurs every time a charger is connected to a power source. However, if chargers are used frequently (e.g., multiple times per day), this accelerates fatigue and must be accounted for in design and engineering. Normal thermal cycling for chargers involves the following:
  • When beginning a charge, the charger matches the ambient temperature of its environment.
  • The charger continually heats up until reaching its normal operating temperature, which takes roughly 30 minutes.
  • After charging, the device will cool off.
• Humidity and moisture – Temperature fluctuations increase the presence of humidity (upon heat increases) and moisture or condensation (upon heat decreases). As liquids can be problematic for charger components, it’s essential that chargers be equipped with a vapor-exchange membrane to keep pressure equalized and liquids from entering.

Thermal Regulation and Cooling Methods for On-Board Charging

OEMs considering which cooling method will best maintain optimal temperatures for on-board chargers generally have three options:

• Passive – Most suitable for applications less than 1 kW
• Fan – Most suitable for applications between 1 and 6 kW
• Liquid – Most suitable for applications greater than 6 kW

Each method starts by transferring heat from semiconductor components to a heat sink via thermal reduction. How heat is then shed depends on the method chosen.

In some cases, charger power requirements will be the sole determinant. However, each cooling method comes with its own benefits and tradeoffs to consider as well.

Passive Cooling

Passive cooling relies on natural airflow and finned construction to disperse heat without any forced air or other assistance. It’s based on the simple principle that heat rises upward. Cold air is drawn into the side of the heat sink between the fins and, as it warms up, rises up from between them to carry the excess heat away.

Because there are no other mechanical, fluid, or powered systems involved with this cooling method, it’s:

• Less costly
• Quiet
• Simple and reliable (e.g., no moving parts)

However, passive cooling’s effectiveness depends on having a large heat sink with many fins to facilitate that heat transfer. The larger units may be more difficult to mount—and they must be mounted correctly. If the fins are not oriented facing upward (or angled upward), the heat will be trapped rather than dissipate.

Fan Cooling

As the name suggests, fan cooling involves a powered fan that directs air over the heat sink to provide better, faster dispersal. Because it provides more effective thermal regulation, fan cooled chargers can be smaller than passive options, and orientation doesn’t matter so long as there’s a vent or sufficient space to direct the hot air away from the charger.

But fan cooling does add some complexities and considerations:

• It adds costs, makes noise, and increases the amount of moving parts that might eventually require maintenance or replacement.
• Fans—particularly if plastic—must be protected for them to operate as intended. Ground and liquid sprays (e.g., gravel, water) can damage them or decrease their effectiveness. Most OEMs know to safeguard on-board chargers from environmental conditions, but not all of them make sure to shield, waterproof, and protect fans.
• Mounting and the proximity of other systems and components substantially affect fan efficacy. This is because fans draw air in through one side of the charger’s enclosure and direct it out the three remaining sides (not counting the mounting or the connector sides). So fans need to maintain an air gap of about 40mm (~1.5”) away from potential blockages. 

Liquid Cooling

Although the most expensive, liquid cooling methods provide the most effective and space-conscious solution for regulating charger temperatures. Because air gaps and specific orientations are unnecessary, liquid cooling units can also be mounted in virtually any configuration that works for a given machine—even directly stacked atop each other.

However, liquid cooling is the most expensive method for thermal regulation. And it generally requires that the mobile machine already operates a liquid cooling system (e.g., radiator and pump for water or coolant) that the charger’s can plumb into.

Ensure Charger Operating Temperatures are Just Right with Delta-Q 

Excessive heat or cold—or rapid fluctuation between the two—poses a threat to battery chargers, their reliability, and their lifespan. With the right cooling method, however, OEMs ensure their mobile machines easily shed heat and maintain optimal operating temperature.

To optimize charging and battery capabilities, OEMs need a partner they can trust. Collaborate with the experts at Delta-Q Technologies, and we’ll break down the various considerations regarding cooling methods, from air gaps and air flow to costs and configurations. 

Sources: 

IEEE. The effect of temperature on the reliability of electronic components. https://ieeexplore.ieee.org/document/6740182/

Progress in Natural Science: Materials International. Temperature effect and thermal impact in lithium-ion batteries: A review. https://www.sciencedirect.com/science/article/pii/S1002007118307536