A look at hybrid and EV battery service

Jan. 1, 2019
Learning how to test and service the HV battery is becoming as important as knowing how to test 12v systems

A few months ago, General Motors (GM) donated a large high voltage (HV) Lithium-Ion (Li-Ion) battery section from a 2018 Chevrolet Bolt EV to our automotive department for training purposes. This battery section is just one of five sections inside the Chevrolet Bolt EV battery. This battery section represents the latest reliable Li-Ion battery technology used in GM electric vehicles.

Located next to the Bolt EV battery section in my classroom is a small HV Nickel-Metal Hydride (NiMh) battery section from a 1998 Toyota RAV4 EV. This battery section is just one of 24 sections inside the Toyota RAV4 EV battery. This battery section represented the relatively new high-tech NiMh battery technology available for use in Toyota electric vehicles in the mid 1990s.

Although these two battery sections use different chemistries and are over 20 years apart in age, the complete battery assemblies in which they are placed have the same basic layout and components. Many EV manufacturers allow service of the components inside the battery housing. Many times, it is much less expensive to replace a battery component than it is to replace the entire battery. In this article, we will look at the individual Hybrid and EV battery components, and highlight the ones that are serviceable.

Safety first

There are many important safety requirements you must follow to protect yourself and others from the potential of high voltage shock, electrolyte spills, and fire. This article is not intended to cover the laws or requirements for working around high voltage batteries and high voltage systems. If you are not familiar with those requirements, you should acquire HV automotive safety training before working on these systems.

The United States Occupational Safety and Health Administration (OSHA) sets regulations to help protect you in your workplace. For the purposes of this article, there are two very important high voltage safety requirements of which I want to remind you:

  1. When are Insulative Gloves required? OSHA Standard 1910.269 references the National Fire Protection Association (NFPA) Standard 70E. My summary of this standard: Personal Protective Equipment (PPE) is required when voltage levels are higher than 50V Alternating Current (AC) or 100V Direct Current (DC). Keep in mind that many vehicle manufacturers require PPE above 50V AC or DC for service work on their systems.
  2. How to Protect Insulative Gloves. OSHA Standard 1910.137 is specific to Electrical Protective Equipment. My summary of this standard: Leather protector gloves shall be worn over insulating gloves, except under limited-use conditions, when small equipment and parts manipulation necessitate unusually high finger dexterity. Insulating gloves that have been used without protector gloves may not be reused until they have been recertified by a testing facility.

Just last week, I saw an advertisement for professional automotive HV training where a technician was shown wearing insulative gloves only. I have also participated in factory training classes where the use of the required outer protector gloves was never discussed or emphasized. This is a dangerous and illegal practice that needs to stop before someone gets hurt.

Battery sizes

You may have noticed that high voltage batteries come in a variety of physical sizes and power ratings. Most modern High Voltage (HV) automotive batteries can be categorized into three major groups:

  1. Small size Hybrid Electric Vehicle (HEV) batteries. These are typically rated at 1.5 kWh at 300V or less. These batteries are typically air cooled/heated with a blower fan. Example vehicles: Toyota Prius, Ford Fusion, Honda Civic.
  2. Medium size Plug-In Hybrid Electric Vehicle (PHEV) batteries. These are typically rated at 18 kWh at 390V or less. (A typical driver can drive 3 miles per kWh). These batteries are air cooled/heated with blower fans or liquid cooled/heated (Volt). Example vehicles: Chevrolet Volt, Ford Fusion Energi, Honda Accord, Toyota Prius Prime.
  3. Large size Battery Electric Vehicles (BEV) batteries. These are typically rated at 100 kWh at 390V or less. These batteries are typically liquid cooled, refrigerated (BMW), or not cooled at all (Nissan). Example vehicles: Tesla Models S, X, and 3. Chevrolet Bolt EV, BMW i3, Nissan Leaf.

Internal components

Service Disconnect Lever or Service Plug Grip: Before exploring or servicing internal battery components, you need to realize that there are live high voltage circuits under the battery cover. Removing a service disconnect lever or service plug grip accomplishes two things:

  1. It creates an open series circuit inside the battery, which protects you from high voltages as long as the battery cover is installed. Once you remove the cover, you are in the danger zone and must use PPE.
  2. It divides the battery into separate, smaller, safer, lower voltage sections. For example, removing the service disconnect lever of a fully charged 2017 Chevrolet Bolt EV 390V battery separates the battery internally into two much safer 195V sections. Another example is removing the service plug grip of a 2010 Toyota Prius 201.6V battery separates the battery internally into one 64.8V section and one 136.8V section.

I have learned that the higher voltage batteries (400V to 800V) of the near future will have multiple service disconnect levers to divide the battery into multiple, safer, lower voltage sections for service work.

As shown in Figure 4, the larger service plugs and levers contain a HV fuse. In an accident, if there is a short circuit of one of the HV wires, the HV fuse is designed to open and protect the battery from damage. Batteries utilizing the smaller service plugs and levers have the HV fuse located inside the battery housing instead.

Battery tray

The lower battery tray, or carrier, on a PHEV or EV is typically located under the vehicle. It houses all of the battery internal components. If damaged, the tray is typically serviceable, but it is a lot of delicate precision work. The tray, or carrier, on many HEVs is located inside the vehicle and is not typically serviceable without replacing the entire battery module.

Air cooled battery modules or stacks

The battery stack shown in Figure 5 contains 28 3.6V Li-Ion cell packs wired in series with each other for a total of 103.6V. There are two cell stacks wired in series in this battery for a total of 207.2V. Each cell stack is serviceable as an assembly. Each cell stack contains three temperature sensors and 28 cell voltage monitoring circuits.

The serviceable battery smart unit (computer) monitors those sensors and commands changes in the serviceable battery blower fan speed to maintain the proper battery temperature. Additionally, there are serviceable cooling ducts and battery vents in case a battery overheats and starts gassing. Automatic voltage balancing of a replacement cell stack with the existing cell stack(s) can take up to three days. The vehicle can be driven while automatic balancing is taking place.

Liquid cooled battery modules

The liquid cooled battery shown in Figure 7 contains 96 3.7V Li-Ion cell packs wired in series with each other for a total of 355.2V. The 96 cell packs are divided into three serviceable battery sections. Voltage balancing of a replacement battery module with the existing battery module(s) requires use of a special HV battery Charging, De-Powering, and Balancing machine. An example of one machine used by GM and Toyota is the Midtronics GRX-5100 EV/HEV Battery Service Tool.

Shipping a battery

Shipping a new or replacement battery module requires de-powering the battery module down to the nominal (normal or un-charged) voltage level. The Midtronics GRX-5100 EV/HEV Battery Service Tool (Figure 6) is designed to do that on many battery modules. Shipping also requires special hazardous material shipping labels and packaging.

Disposing of a battery

Disposing of a used or damaged battery module requires completely de-powering the module. Complete de-powering can be done with the special Midtronics machine or with a 1 percent salt-water solution bath of the module. Salt water solution discharging can take several days. De-Powering the battery module makes it safe to be near without the need for PPE.

Battery cooling system

Each battery section has serviceable coolant hoses running between them to keep the battery cooled or heated to the proper temperature during operation. Some liquid cooled batteries have serviceable cooling plates, heat transfer pads, and cooling manifolds. The battery tray on liquid cooled batteries typically contains an inspection plug to inspect for coolant leaks without the need to remove the battery from the vehicle. If you remove the plug and coolant comes out, you have an internal coolant leak and will need to pressure test or vacuum test the internal battery cooling system to locate it.

Additionally, each section has serviceable temperature sensors to monitor the temperature of the end cells. The battery has a serviceable built-in battery coolant heater and serviceable battery heater temperature sensor. The serviceable battery computer monitors those sensors, controls the heater, and requests the desired coolant pump speed.

Some battery trays contain serviceable electric radiant heaters positioned below or in-between the battery stacks or sections. BMW uses a large refrigeration unit (evaporator) and a thermostatic expansion valve in their i3 battery tray to cool the battery sections. BMW only sells battery components for their i3, no complete batteries are available.

Each battery section also has serviceable bus bars and/or wire harnesses connecting the sections in series electrically. There are also serviceable low current HV fuses that protect the battery coolant heater, plug-in charger, DC-DC converter, and A/C compressor. There is also a serviceable service disconnect lever socket. Each battery section has its own individual battery cell voltage monitoring circuits in a separate serviceable harness. The battery computer monitors those circuits for bad battery cells and other possible malfunctions.

Battery junction blocks

All HV batteries that I have ever seen or researched contain one or more Battery Junction Blocks or Relay Centers in the lower tray. The purpose of the junction block is to connect and disconnect the HV battery from the rest of the vehicle’s HV systems. The junction block is typically controlled by a computer module inside the battery housing.

The junction block of a HEV typically contains:

  1. A Positive Contactor. Function: to connect the HV battery positive terminal to the positive HV battery cable and the inverter upon initial vehicle power on.
  2. A Pre-charge Contactor. Function: to slowly charge the inverter smoothing capacitor through the pre-charge resistor upon initial vehicle power on. Once charged, almost no current will exist in the circuit and the remaining contactor can close without damaging its contacts. The pre-charge contactor will then open.
  3. A Pre-charge Resistor. Function: to limit the charging current of the smoothing capacitor upon initial vehicle power on.
  4. A Negative Contactor. Function: to connect the HV battery negative terminal to the negative HV battery cable and the inverter after the inverter smoothing capacitor is charged.
  5. A Current Sensor. Function to monitor the operation of the contactors and the amount of current leaving and entering the HV battery.
  6. One or more low current fuses. Function, to protect the HV DC-DC converter.

Of course, there are several individual Diagnostic Trouble Codes (DTC)s for each of the junction block components. Some vehicle manufacturers allow individual component replacement while others require replacement of the entire battery junction block for any failure.

In addition to the components listed above for the HEV, the junction block(s) of a PHEV, or BEV contain a few more components to allow for the HV battery to be charged by a plug-in charger. These additional components include:

  1. A Charging Positive Contactor. Function: to connect the HV charger positive cable to the HV battery positive terminal, the positive HV battery cable, and the inverter while charging.
  2. A Charging Pre-Charge Contactor Function: To slowly charge the inverter smoothing capacitor through the pre-charge resistor while charging the battery. Once the inverter smoothing capacitor is charged, almost no current will exist in the circuit and the remaining contactor can close without damaging its contacts. The pre-charge contactor will then open.
  3. A Charging Negative Contactor. Function: to connect the HV charger negative cable to the HV battery negative terminal, the negative HV battery cable, and the inverter after the inverter smoothing capacitor is charged.
  4. A Contactor Temperature Sensor. Function: to monitor the temperature of the positive contactor terminals while charging the HV battery.

Battery Energy Control Module (BECM)

The computer which monitors the condition of the battery from inside the battery housing is known by different names assigned by their manufacturers. Most of these computers perform the same functions:

  1. Communicate with modules outside the battery for proper battery temperature control and to send fault messages.
  2. Operate the contactors in the junction block at the appropriate times to insure proper vehicle operation and collision activated HV system shutdown.
  3. Monitor the battery temperature sensors to insure proper battery cell temperature.
  4. Monitor the individual battery cell or module voltages to insure proper current in the HV circuit and proper cell operation.
  5. Monitor the battery current sensor(s) to verify proper operation of the contactors.
  6. Perform battery cell balancing on some models.

These computers rarely fail, and most are updatable with a factory scan tool. When replacing a BECM, it is critical that its electrical connections are disconnected and reconnected in a certain order. Be sure to read the service information to prevent additional damage. Sometimes other modules or computers will need to be reprogrammed after replacing the BECM.

As we have seen in this article, there are many serviceable components inside an HV battery pack. Regardless of the vehicle manufacturer, most batteries have the same general components. They might not look the same, but they perform the same function. Every time I open another HV battery, I look for those same components and I am instantly more familiar with that battery. Becoming more familiar with the battery pack components will help you in your diagnostic efforts as these vehicles get older and service work is more common.

Best wishes!

About the Author

John Kelly

John D. Kelly is a professor of automotive technology at Weber State University in Ogden, Utah, and a former technician. He specializes in automatic and manual drivetrain and NVH diagnosis and hybrid and electric vehicle technology.

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