ACDC is back with more diagnostic help when working on high voltage systems. High voltage (HV) is isolated from the chassis for good reasons. When any high voltage part has a connection to the chassis, that could spell trouble, depending on how much current the connection can carry. The engineers have cleverly designed circuits and strategies that can determine a “lack of isolation” and report that to your scan tool and set a warning light. How does that detection system work? (Note we use ASE L3 terminology as much as we can. If you are not L3 certified, ACDC can help you.)
When faced with a code for a high voltage leak to chassis ground, here is what you need to remember. When the HEV-PHEV-EV (ACDC calls them EMVs, which is all-encompassing and stands for electrified motor vehicles) is in “Power On” mode (Fig. 1), most OEMs will check the battery pack for a HV leak to the chassis. We called this a Type “A”, sometimes known as an “Active Loss of Isolation” (LOI) Detection System.” Once the HV battery has passed an HV leak test, the contactors will close in a certain sequence and the high voltage parts will be powered on in a precise order, unless there are other codes preventing a “READY” light (Fig. 2).
Another detection system, called Type “B,” or a “Passive LOI Detection System” is also used. This system is fast to react. As HV components, cables and more come on line, if there is a short to chassis ground the contactors will open instantly, the HV capacitors will discharge and codes will set and “lock” out the driver’s ability to “READY” the EMV. These codes should be treated with utmost care as something is wrong with the HV safety system.
Type A High Voltage Leak Detection
This “active test” is done before the contactors are closed. When in “Power On” mode most of the time the HV battery isn’t connected to any other HV part. We know of three exceptions: the older Honda IMA systems, the Renault Zoe EV early model, and newer Ford HEVs as the HV battery is connected to the HV system in “Power On” mode. So make sure you’re always protected from high voltage when working with an EMV brand you’re not familiar with. There are no “rules” an OEM must follow, so be careful!
Once the EMV has been set to “Power On,” a 12-volt circuit (Fig. 3) in the “HV battery ECU” will look for a leak in the high voltage battery by comparing two sections of the HV battery for “leakage.” Just like EVAP separates the EVAP system into two sections and then looks for decay with pressure or vacuum to locate a leak, the HV battery ECU uses that thinking. (Fig. 1 shows a simple version of type A detection.)
Fig. 2 Once the HV battery passes an HV leak test, the contactors will close in a certain sequence and the high voltage parts will be powered on in a precise order unless there are other codes preventing a “READY” light.
This circuit can monitor for an HV leak by placing a “voltage divider” between the Positive and Negative posts of the HV battery. That effectively divides the pack in half. If you had a 200-volt pack and measured at the halfway point of the cells arranged in series, you would have 100-volts at each measurement point. Upon restarting the EMV, comparing the two readings, assuming they were the same, one would know that the pack was not compromised. With learned memory, any small changes over time would be accepted as normal and the battery would pass an “HV Leak Test.” Then the contactors would cycle through their closing and testing phase. If the HV battery split voltage readings showed a larger than expected variation between the two sides, a determination could be made that the HV battery has a leak to the chassis. The contactors would not close, so no READY light. A code would be set. (This is a simplified explanation.) You could replace the entire battery pack or you may be able to remove the HV pack, open it up and closely examine it for something that may cause that code. Some packs can be inspected while still in the EMV.
Type B High Voltage Leak Detection
Once the HV battery is connected to the rest of the HV system, another high voltage detection circuit in the ECU takes over. Look at the Type “B” circuit diagram (Fig. 4). It has an “AC source” (No. 5) that sends alternating current to “Capacitor A” (No. 3) and chassis ground (No. 7). The interesting thing about the use of capacitor “A” (usually a DC device) is that it can transfer the AC current through the capacitor without having the high voltage connect to the low voltage of the sensing circuit. Think about a double-headed drum (Fig. 5), like a “Mridangam” from Eastern India. If you pound one end of the drum, the opposite end will vibrate at the same frequency. When the AC current is applied to the low voltage side of Capacitor “A”, an AC sine wave is sent onto the high voltage DC cable (No. 10). That sine wave has an amplitude and a frequency. The frequency never changes, but the amplitude will if there is connection to the chassis. A very small circuit (No. 9) makes an extremely low current connection back to the detection circuit to compare amplitude levels. Those electrical engineers are clever. Depending on how big the leak is (the height of the amplitude), the system can either turn on a warning light and set a code, put the EMV into limp in mode, or if the amplitude is too low, the contactors open and the high voltage is turned off. The EMV will not READY up until that HV leak is fixed.
One more fact to consider: when any high voltage component is mounted in the vehicle, it is always bolted securely to the chassis in more than one place. This is to ensure that a person will not become the ground path if a component has a high voltage leak to the chassis.
Fig. 4: This Type B circuit diagram shows an “AC source” that sends the alternating current to capacitor A and chassis ground.
Visual Inspections Solve Problems
EVs with a bottom-mounted HV battery pack (Fig. 6) are sealed so if driving through standing water, water will not enter the pack. Some plug-in hybrids and most hybrids do not seal the HV pack as they are in the trunk or under a rear seat. Experience has taught us to look for these signs, mostly on hybrids:
• Animal chewing on cables and wiring
• Water in the pack from a leaky hatch seal
• A customer who spilled a liquid
• Overheated NiMH cell leaking electrolyte
• Dirt and debris in pack plus a dirty blower motor
• Someone who tried to “fix” it before
There is more to learn about the construction and function of the many new components in EMVs entering service bays. Stay tuned for the next installment about tools and equipment you will need to service them.
