AC vs DC Which is more dangerous?

 

AC Vs DC

AC vs DC Danger:

• Both are dangerous: Both AC and DC can be lethal. The severity depends on voltage, current, duration of contact, and the path the current takes through the body. It's a misconception that DC is inherently "safer."

• Specific dangers of AC: AC's changing polarity can cause sustained muscle contractions, making it difficult to let go of the source. This prolonged exposure increases the risk. As you mentioned, the peak voltage of AC is higher than its RMS value (e.g., 120V RMS has a peak of around 170V), which can be significant.

• Specific dangers of DC: While less likely to cause fibrillation, DC can still cause severe burns due to its constant current flow. It can also cause a single, powerful muscle contraction that can throw someone away from the source (which can be both a blessing and a curse).

Current Levels and Effects:

• 0-5mA: Generally, a tingling sensation.
• Around 10mA: "Let-go current" - the point where muscle contractions make it difficult to release the source.
• Above 25mA: Risk of serious injury and death increases significantly. This is where ventricular fibrillation becomes a serious risk with AC.

Note:  The Let Go Current :  AC is around 10mA to 20mA 

                                                 DC be around 60mA to 75mA, 

Exceeding this threshold can cause severe muscle contractions and make it difficult to let go.

The severity of the shock depends on factors like voltage, current, Resistance and duration of contact.

Key Takeaway:

While the body's impedance plays a role, the primary reason AC is often considered more dangerous is its frequency, which can disrupt the heart's rhythm. 

DC or AC till some lower voltages can be touched by both the hands but if you cross certain voltage levels specifically above 40V then both DC & AC are dangerous.

Means AC Voltage of 50V could be equal to 120V DC, Above which both AC and DC are leathel.

 However, both AC and DC are potentially lethal, and safety precautions should always be taken with any electrical source.

BMS Daisy Chain Fault in an EV Lithium Battery

BMS DaisyChain Fault in EV Battery Fault Code

A BMS (Battery Management System) Daisy Chain Fault occurs when the communication link between multiple modules inside a lithium-ion EV battery pack is disrupted.

This fault can cause battery performance issues, failure to monitor individual cells, or even a complete shutdown of the vehicle’s power system.

What is a Daisy Chain?

A daisy chain is a wiring scheme in which multiple devices are connected in a sequence, or a chain.The daisy chain configuration is a common way to connect multiple BMS modules in a series communication network. 

Each module receives data from the previous one and passes it to the next, forming a continuous chain of data transmission. This method reduces the number of communication wires but makes the system vulnerable to faults.

Each BMS module collects voltage, current, and temperature data from its assigned battery cells.

The data is transmitted from one module to the next in the daisy chain using a communication protocol for example., SPI(Serial Peripheral Interface), UART(Universal Asynchronous Receiver-Transmitter), or CAN(Controller Area Network).

The final module in the chain sends all the collected information to the main BMS controller.The BMS controller makes real-time decisions about charging, discharging, thermal management etc.

Possible Causes of Daisy Chain Fault:

• Loose or No Connection – A damaged wiring harness or loose connector between BMS modules can break communication.

• Faulty BMS – A defective or malfunctioning BMS due defective microcontrollers,sensors can disrupt the communication chain.

• Firmware or Software Issue – Incorrect firmware updates or corrupted software can lead to communication errors.

• Damaged Busbars or Harnesses – If the signal routing on the battery pack is broken, communication can fail.

Effects of a Daisy Chain Fault:

• Battery Critical Warning Light on Dashboard.
• BMS Fails to Communicate with the Vehicle Control Unit (VCU)
• Cells Not Being Monitored Properly
• Battery Shutdown or Reduced Performance Mode or Limphome mode Alert

Troubleshooting Steps:

1. Check Wiring and Connectors for physical damage, Fix or replace loose or corroded connections.
2. Firmware reflash or reset the BMS system, Even after reflashing or resetting the BMS doesnt work, then replace with new BMS.
3. Check for any open busbars,If the battery pack has damaged busbars, repair or replace them.

Reasons for Sudden SOC jump in your EV Vehicle

 

SOC Jump in EV Vehicle Cluster

• Battery Management System (BMS) Recalibration:

The BMS is the "brain" of your EV's battery. It constantly monitors various parameters like voltage, current, temperature, and the state of individual cells within the battery pack. One of its crucial functions is to estimate and display the State of Charge (SOC). Over time, minor inaccuracies can creep into the SOC calculation. To maintain accuracy, the BMS periodically performs a recalibration.

How?: During recalibration, the BMS might make adjustments to its internal algorithms based on the latest readings. This can sometimes result in a sudden jump in the displayed SOC, especially near the extremes of the charge range (close to 0% or 100%). Think of it like the BMS "double-checking" its calculations and making a correction.

Is it normal? Yes, occasional recalibration is a normal and healthy part of battery management. It ensures the SOC reading remains as accurate as possible.

• Software Update:

EV manufacturers regularly release software updates to improve various aspects of the vehicle, including the battery management system.

How?: A software update might change the way the BMS calculates or displays the SOC. It could also introduce new algorithms for estimating SOC or refine existing ones. These changes can sometimes lead to a noticeable jump in the SOC display.

Is it normal? Generally, yes. Software updates are designed to improve performance and accuracy. A change in SOC display after an update isn't usually a cause for concern, as long as the overall battery performance remains normal.

• Charging Behavior:

If you consistently charge your EV to a certain level (e.g., 80%) and then suddenly decide to charge it to 100%, the BMS needs to adjust to this change.

How?: The BMS learns your charging habits. If you always stop at 80%, it might have optimized its SOC estimation for that range. When you charge to 100% for the first time in a while, the BMS might need to recalibrate its understanding of the full battery capacity, leading to a jump in the SOC display.

Is it normal? Yes, this is a normal response to a change in charging habits. It's the BMS adapting to the new charging pattern.

• Temperature Changes:

Battery performance is affected by temperature. Extreme heat or cold can impact the battery's chemical reactions and its ability to hold a charge.

How?: The BMS takes temperature into account when calculating SOC. If the temperature changes significantly (e.g., you park your car in the sun on a hot day), the BMS might adjust the SOC reading to reflect the temperature's impact on the battery. This adjustment can sometimes appear as a sudden jump.

Is it normal? Fluctuations in SOC due to temperature are possible, but they are usually gradual. A very sudden jump due to temperature alone is less likely.

• Battery Balancing:

An EV battery pack consists of many individual cells. For optimal performance and longevity, these cells need to be balanced, meaning they should have roughly the same charge level. The BMS performs this balancing act.

How?: During the balancing process, the BMS might redistribute charge among the cells. This can sometimes cause small fluctuations in the overall SOC, which might be perceived as a jump.

Is it normal? Yes, battery balancing is a necessary process. Minor SOC fluctuations during balancing are usually not a problem.

Summary: 

A sudden jump in SOC isn't always a reason to panic. It can often be attributed to normal BMS operations like recalibration, software updates, changes in charging habits, or temperature variations. 

However, it's always a good idea to monitor the situation. If the jumps are frequent, large, or accompanied by other issues like reduced range, charging problems, or error messages, then it's best to consult with your EV's service centre. They can diagnose the issue and ensure your battery is healthy.

Difference between CP Vs PP in EV Vehicles

CP vs PP in electric Vehicles

Control Pilot (CP)

The CP manages the actual charging process by establishing communication between the EV and the EVSE. It ensures that charging starts and stops safely, and it controls the amount of current delivered to the vehicle.

How it works:

Voltage Levels: The EVSE generates a square wave signal on the CP line. Different voltage levels of this signal represent different charging states:

• State A (Disconnected): +12V - No connection
• State B (Connected, EV Ready): +9V - Cable connected, EV ready to charge
• State C (Charging): +6V - Charging in progress
• State D (Charging with Ventilation): +3V - Charging with ventilation required (* NA in India*)
• State E (Fault): 0V or other abnormal voltage - Fault detected

Pulse Width Modulation (PWM): When in State C (Charging), the CP signal uses PWM to communicate the maximum available charging current from the EVSE to the EV. The duty cycle of the PWM signal (the proportion of time the signal is high) corresponds to the available current.

 

Key Functions:

• Connection confirmation
• EVSE current capacity advertisement
• Charging start and stop control
• Fault detection
• Ventilation request (if needed)

Proximity Pilot (PP)

The PP is primarily about connection detection and safety interlocks. It ensures that the charging cable is physically connected to the vehicle before charging can begin and prevents hazardous situations like driving off while still plugged in.

How it works:

Resistance: The EV applies a specific resistance across the PP pin and the Protective Earth (PE) pin. This resistance value is determined by a resistor in the charging cable.

Current Rating: The resistance value corresponds to the maximum current the cable can safely handle. This helps prevent overloading the cable.

Detection: The EVSE (charging station) detects this resistance. If the resistance is within the expected range, it confirms a valid connection.

Connector Locking (Type 1): In Type 1 connectors (common in North America), the PP also controls a mechanical locking mechanism that secures the connector to the vehicle during charging. This prevents accidental unplugging.

Connector Locking (Type 2): In Type 2 connectors (common in Europe and increasingly in India), the locking mechanism is separate, but the PP still provides the connection confirmation.

Key Functions:

• Cable connection detection
• Cable current rating identification
• Preventing drive-away during charging (connector locking)

Relationship between CP and PP

The PP and CP work together in a sequence:

Connection: The charging cable is plugged into the EV. The PP detects the connection and the cable's current rating.

Readiness: If the PP signal is valid, the EV signals its readiness to charge via the CP (State B).

Current Information: The EVSE uses the CP signal (PWM in State C) to inform the available charging current.

Charging: The EV's OBC (On-Board Charger) starts drawing current based on the advertised value.

Monitoring: Both CP and PP are continuously monitored during charging. If the PP signal is interrupted (e.g., the cable is unplugged), charging stops immediately. If the CP detects a fault, charging is also interrupted.

                                  

CP vs PP

Under Progress