Battery Management Systems

This topic isn’t actually a Hybrid Vehicle topic but it’s worth spending a little time on it.  If you pay attention to some of the latest trends under the hood of conventional engines you’ll no doubt come across the concept of battery management systems.  So what is it all about?  Well, I’m glad you asked.

Your conventional electrical system on cars doesn’t have any way to determine the state of health of your battery.  You know this if you’ve ever been stranded somewhere wondering why your starter makes that dreadful “Click” when you turn the key.  Yes, you have indicators on your instrument cluster that will tell you your charging system is not operating correctly but there’s really nothing there that tells you your battery is on it’s last legs.  Battery Management systems are typically consist of the following.

• A current sensor on your negative battery cable
• A communication between this module and your engine computer
• The ability to shut down (or reduce) electrical loads throughout the vehicle
• The ability to shut off the alternator on command

So how does it all work?  Well, the state of health of the battery is determined by the module on the negative post.  It does this by knowing key parameters of the battery that are provided by the battery manufacturer and measuring the current, voltage, and temperature of the battery.  This information is fed back to your engines computer which already has access to other key pieces of information like engine RPM and an idea of what electrical loads are running at the time.  With this information all in one place the computer can make some decisions regarding how it should operate.

For example, during a starting event, the main computer can shut off the alternator and other loads making the high current draw event of the starter a little easier on the starter.  The computer could also determine that your battery is in such a state that we can begin a long charge cycle.  This might happen if the temperature is favorable for the battery accepting a charge and it seems that the RPM’s are pretty stable (like on the highway).

Another example is that long term parking at the airport.  You may not realize that your car’s electrical system goes to sleep after about 30 minutes but certain events can wake it up again (like other peoples key FOBs!).  What this means to you is that larger than expected currents are flowing from your battery causing it to drain faster than the engineers would like.  You could end up returning from your trip to a dead battery.  The Battery Management system can deal with this by sensing the battery state and shutting everything down.  This proactive measure keeps the battery from getting into a deep discharge state which even if you are not stranded, can shorten the life of your battery considerably.

Another benefit of Battery Management systems is that they can improve your fuel economy.  It does this by managing when your alternator is turned on.  Under certain drive cycles (depending on the state of health of your battery) you can just run on battery power alone.  Your alternator can be turned off which significantly reduces the load on the engine.  It can be turned off because the battery is being monitored to make sure it doesn’t discharge too much.

Well, that’s it for now.  If you haven’t heard about these systems yet, I’m sure you will be soon.  It seems that any technology that reduces those MPG’s is going to be advertised or otherwise made known so that you know the automakers are doing everything they can to get green.

Regenerative Braking

So when your neighbor comes over to check out your new hybrid car – tell him about the brakes! 

While it’s true that most cars have a complex braking system especially when you consider anti-lock systems, they just aren’t as cool as the regenerative braking systems on hybrid electric vehicles.  Why regenerative braking?  Well, it really comes down to a balance between electrical power required for all vehicle systems and the power that the battery can supply over time.   When your hybrid is operating purely on battery power you are taking away the reserves of your battery.  It needs to be replaced eventually.  This battery charging process occurs during the drive cycles when the engine is running at cruising speeds.  The engine turns the generator and it supplies electrical power back to the battery.

So what does this have to do with your brakes?  Well, the engineers that have developed this complex system have determined that the time charging the battery in the engine driven drive cycles may be too infrequent for some drivers.   Under this scenario, you would take a little bit out of the battery every day without putting enough back.  It is sort of like making a small withdrawal out of you savings account everyday.  It goes unnoticeable for quite awhile and then suddenly you’re broke.  You would be left wondering why your engine runs all the time and your fuel economy took a nose dive.  The engine would have to run constantly to get the battery back to a full state of charge and then the cycle would start all over again.  To prevent this from happening, other sources of energy needed to be found. 

Fortunately, braking offers a solution.  In your normal vehicle, brakes use a hydraulic system to put pressure on the brake pads to clamp against your rotors to stop.  Well, hybrids can do the same thing but it also has another system it can use – the generator.  The generator is what powers your wheels when you’re operating under electric power.  One of the characteristics of electric motors is of course when you apply a voltage to them they turn.  In our hybrid, it powers us down the road.  One of the other characteristics is that if I turn the generator mechanically it will generate a voltage.  It can operate in both directions.  In our hybrid, when we want to slow down, we switch the generator so that the wheels drive it and it produces power that is fed back to the battery.  The mechanical friction of the generator begins to slow us down.  If we need a sudden stop that the generator can’t provide, we still have the normal braking system.  Every little bit of energy can return back to the battery is that much less you have to provide during the engine running scenario.  That helps us with our fuel economy as well because when you have to charge the battery during the engine cycles, it requires more work for the engine which requires more fuel.  So when you’re neighbor wants to check out your new hybrid, give him an earful on how your regenerative braking system works.

Rich Dennison