Electric Vehicle Multilevel Inverters

One inverter for every two batteries. The benefits of which are higher efficiency, longer motor life, better safety, hybrid pack capability, and much more.

Inverters for electric vehicles (EVs) are typically heavy, expensive, inconveniently-shaped behemoths meant to squeeze under the hood, often requiring wiring to each individual battery, or worse, an entirely separate system with electronics on each battery.

Here at Green Roof Solar, we believe in revolutionary concepts, and we came up with the one-inverter-per-battery idea. A prototype of this concept was even tested in a GM EV1 (see graph). Diagrams follow below.

Traditional multilevel inverters are operated from isolated sources which, through transformers, typically originate from the same source. Other types have only a few levels which are not galvanically isolated. A limited number of benefits of the multilevel inverter can be realized unless the power source is comprised of independent batteries (or pairs of batteries, for lithium chemistries). In this case, no transformers are required, and with six-volt batteries, no pulse width modulation (PWM) is necessary.

The benefits of this topology are many. First, PWM is typically stressful for motors, creating winding shorts and early bearing failure in severe cases. It also creates a large amount of interference, often rendering a radio in an EV useless. Avoiding PWM increases efficiency and escapes the latter issues. Second, charge between batteries can be actively balanced while driving or recharging. With proper software configuration this allows for hybrid packs, such as lithium-ion mixed with lead-acid, or even mixed capacity batteries of the same chemistry.

Safety is a huge concern for electric vehicles, and the problem with permanently-connected batteries is that a high voltage is always available somewhere. The frame of the vehicle gives a path for conduction, and if a battery is punctured against it in an accident, this not only represents a fire hazard but a shock hazard as well. The Green Roof Solar, LLC multilevel inverter does not suffer from this problem. The highest voltage available in an accident, or whenever the vehicle is not moving, is a mere six to twelve volts. This is not enough to cause a shock hazard, and even during a short, the problem is confined to one battery as current cannot flow to those around it.

Our multilevel inverter design has only thin printed circuit boards mounted on each battery, which typically would be necessary in a non-multilevel design, and two contactors, seven relays, and a master circuit board inside a small control box under the hood. This opens up the hood for use as a storage area or allows room for more batteries. Additionally, batteries can be added or subtracted from the system and are automatically accounted for by the control box upon vehicle re-initialization. A trailer with fully charged batteries can be put in series with the vehicle’s original battery pack, and even if the on-board pack is nearly dead, the vehicle can be operated normally as long as the trailer holds charge. The trailer can also recharge the on-board pack while driving in a completely regulated manner.