What is a hybrid battery pack?

Every hybrid has two batteries. The small 12-volt battery runs the lights, radio and computers, just like in any car. The big one, the high-voltage traction battery, stores energy for the electric motor that helps move the car.

Where the energy goes

When you brake or coast, the motor works as a generator and pushes energy back into the traction battery. When you accelerate, the battery feeds that energy back to the motor so the gas engine works less. That back-and-forth is what saves fuel.

How big is "high voltage"?

Hybrid packs run at hundreds of volts. One Hyundai/Kia pack serviced by EVTU was about 270 V (EST, 9 modules at about 30 V each). That is well beyond a safe level for direct contact.

Battery management

A battery management system (BMS) watches the pack's voltages and temperatures. If something looks wrong, such as voltages that are too low, it can refuse to charge the pack to protect it. That is often why a hybrid that sat for a long time will not start or charge.