HV Battery, BMS, and Charging

The BMS is the brain of the battery pack. It monitors every individual cell voltage and temperature throughout the entire pack — hundreds of cells in a typical EV battery. It manages cell balancing by transferring small amounts of energy between cells to keep them at equal states of charge. It calculates state of charge — how full the battery is right now — and state of health — how much total capacity the battery retains compared to when it was new. The BMS controls the battery thermal management system to keep cells in their optimal temperature window. It also enforces safety limits. If a cell voltage gets too high during charging, the BMS reduces or stops charging. If a cell voltage gets too low during driving, it limits power output. If temperature exceeds safe limits, it reduces current. BMS faults appear as reduced range, charging limitations, reduced power warnings, limp mode, or warning lights. Scan the BMS module first on any range or performance complaint.

State of Charge is how much energy is in the battery right now — like a fuel gauge. State of Health is the battery's current total capacity as a percentage of its original rated capacity. All lithium batteries degrade with use, time, heat exposure, and charging habits. A battery at 80 percent SOH delivers 80 percent of its original range. Range reduction complaints must be evaluated against current SOH data from the scan tool before any other diagnosis. If SOH is appropriate for the mileage and age, the battery is performing normally — the customer's range expectation needs to be recalibrated, not the battery. Most manufacturers warranty the battery to 70 or 80 percent SOH for 8 years or 100,000 miles.

Level 1 — standard 120V AC household outlet through the portable cord set that comes with the vehicle. Delivers 3 to 5 miles of range per hour of charging. Fine for overnight top-offs on plug-in hybrids. Impractical as the only charging source for a full EV with a large battery. Level 2 — dedicated 240V AC circuit, same voltage as a clothes dryer. Delivers 10 to 30 miles of range per hour depending on the onboard charger capacity. This is the most common home charging setup and the standard public charging station. Level 3 DC Fast Charge — the station converts AC to DC and delivers high-voltage DC directly to the battery, bypassing the onboard charger entirely. Delivers 100 to 200 miles of range in 20 to 30 minutes on compatible vehicles. Fast charging generates significant heat in the battery. The BMS actively manages cooling during fast charge sessions and may reduce charging speed to protect the cells.

North America is converging on the NACS (Tesla) connector for both AC and DC charging. CCS (Combined Charging System) uses the J1772 AC connector with two additional DC pins below it. CHAdeMO is a separate DC connector used primarily by older Nissan Leafs. When diagnosing charging faults, the connector type matters because each uses different communication protocols between the vehicle and the station.