Submitted by Randal Beattie, SAKOR Technologies
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High voltage battery simulator and test systems are essential for conducting testing of the high voltage DC power systems that are such a critical element of electric (EV) and hybrid/electric (HEV) vehicles. To accurately test a high voltage hybrid or electric drivetrain, you need to be able to provide precise, repeatable high-voltage DC power. Standard off-the-shelf power supplies will not work, because they cannot absorb power from the EV/HEV’s regenerative system and may even be damaged or destroyed if used with one.
To fill the need, new technology is being developed specifically to test high-voltage HEV batteries and simulate these batteries in an electric drivetrain environment. At the heart of the system is a fully line-regenerative DC power source, which can provide reliable, repeatable power which in turn allows for repeatable data because the system’s performance is not affected by battery charge state. In addition, this innovative method provides much greater power efficiency and measurably reduces overall operating costs. Such a system can also test batteries by precisely simulating road conditions, so engineers can see how the battery will really respond if put in a vehicle under real-world load conditions.
High-voltage battery and charging system at the heart of hybrid and electric vehicles
The battery is the heart of an EV or HEV, acting as an energy storage device that allows the vehicle to store electrical energy and then use it to drive when needed. In an EV, the battery is analogous to the gas tank – fill it up and it runs until it is empty. With an HEV, the electrical system is used to assist an internal combustion engine.
In a normal condition, an electric motor can be used to drive the vehicle. However, it can also be used to resist vehicle motion during braking. At this point it becomes a generator, generating electricity and pumping it back into the battery to be stored and used to drive the vehicle later. This energy recapture is called dynamic braking. Whether in an EV or HEV application, the battery needs to store the regenerated power so that it can be used later to power the vehicle on demand.
Testing overall electric or hybrid vehicle performance as part of new product engineering requires a system that can be used to test the high-voltage battery system, as well as simulate these high-voltage batteries while conducting EV or HEV driveline and inverter system testing.
Using a battery as the power source for an EV or HEV testing system will not give you the repeatable data that is required. This is because the battery’s charge and discharge state will affect the results. Batteries absorb and generate power at differing rates depending on how fully the battery is charged (its charge state). To use a battery for testing, you must therefore ensure it is always at the identical charge state at the beginning of each test, and charged exactly to the same level every time – or you are likely to get somewhat different results.
What’s required instead is an electronic power supply, running off of the power grid, which can actually provide the very stable DC high voltage capability that the battery would give, and also be able to absorb power like the battery would.
Unfortunately, providing precise, repeatable high-voltage DC power for accurately testing a high-voltage hybrid or electric drivetrain is no easy task. Most standard power supplies are single quadrant units, which means they supply current in one direction – they generate power to drive whatever device they are attached to. What they don’t do is absorb power. Driving power back into this type of AC power supply will likely overload the power supply, and permanently and severely damage it.
New and advanced battery testing technology
In the recent past, battery manufacturers have not needed sophisticated systems to test batteries, but the advent of EV and HEV has led to a greater need for new and advanced battery testing technology. For example, Michigan State University is developing a battery testing laboratory as part of its advanced energy storage technology research.
In response to this critical need, technology is being developed that can be used for high voltage battery testing. The testing system subjects the battery to the same charge/discharge profile as it would encounter in an actual vehicle on an actual road course.
With these systems, during regenerative modes, absorbed power is regenerated back to the AC mains instead of being dissipated as waste heat, which is common practice among previous generation testing systems. This highly efficient method provides much greater power efficiency and measurably reduces overall operating costs.
The technology uses an inverter system to convert high voltage three phase AC power to very stable DC voltages. This is a complex technology, especially if done in a regenerative fashion. To provide a high voltage DC source that can also absorb power, you need high power high frequency switching, using very high quality components. The system also needs to provide very good filtering to provide the stable DC voltage required.
For example, SAKOR has developed a new HEV battery test and simulation system that precisely controls DC voltages up to 1000 volts or more. The system switches seamlessly and precisely from powering to regenerating modes.
In addition to providing precise testing, the technology is extremely (approximately 95 percent) energy efficient. Absorbed power is placed back on the AC mains to be re-used or re-sold to the power company. The system can actually cause the electric meter to run backwards, reducing the electricity bill and wasting little or no heat energy. This results in significant savings in total testing costs.
A typical HV Battery test system would include:
• A high-voltage regenerative DC power supply. If the system is just being used to test a battery, this power supply hooks up directly to the battery being tested.
• A DynoLAB EM test cell supervisory system, attached by Ethernet to the power supply, and serving as a command to the battery testing system. The DynoLAB EM can be configured to perform any charge/discharge for road load cycle required.
• External instrumentation, depending upon what the system is being used for. Examples include a bank of thermocouples and pressure transducers.
In addition, high voltage battery systems may be equipped with a separate smart battery charging device; if so, the DynoLAB EM system can also communicate with and control this device.