Questions about the Prius Braking System

How should I brake to recover the most energy to the battery?

In any car, the most economical way to brake is to avoid doing it at all.  Braking takes energy that you have put into the car's motion by burning fuel and throws it away as heat.  Unfortunately, you can't always find a straight, level, traffic-free road to your destination, so braking becomes necessary to cope with road conditions.  In a conventional car, anticipating the need to slow down is one of the most effective ways to improve fuel economy.  The sooner you get your foot off the accelerator, the sooner you stop burning that fuel!

The same rule for braking applies to the Prius.  Anticipate the need to slow down so that you need to brake as little as possible.  Get your foot off the accelerator as soon as you see that you may not be able to maintain speed.  Coasting allows the car's energy to do useful work - moving you along the road where you wanted to go anyway.  But, when you do need to brake, the Prius gives you one more tool to conserve energy - regenerative braking.

When you brake, the Prius computers configure MG2 as a generator and use it to charge the battery.  The torque needed to spin MG2 as a generator slows the car.  However, there is a limit to the rate at which the battery can safely accept energy from MG2.  Therefore, there is a limit to how much braking torque regenerative braking can give you.  If you ask the braking system to slow you down faster than the maximum regenerative braking torque allows, the friction brakes will also come on.  Now you are throwing away energy.  The trick, then, is to ask only for the braking force that regenerative braking can supply.  This maximizes the amount of energy recovered and stored in the battery.  Safetly must always come first, of course, so you must plan ahead to make this possible.  Again, anticipate the need to slow and get off the accelerator.  Begin braking early and brake gently.

If you have a Prius Mini-Scanner (or some other way of watching the battery current), you can see exactly what force to apply to the pedal to brake reasonably quickly but not waste energy in the friction brakes.  You will find that, however hard you brake, the current charging the battery doesn't rise much above 55 amps.  This is as much as the battery can safely accept.  By glancing at the battery current as you brake, you can learn what pedal pressure results in this current.  If you only use this pressure, the friction brakes will not come on and as much of the car's energy as possible will be recovered.  Always remember that safety comes first and don't take your eyes off the road for more than a moment.

At highway speed you only need to brake very lightly to get 55 amps of charge.  This is because of Newton's Laws of Motion - force equals mass times rate of change of speed and work (power) equals force times speed.  What this boils down to is that when you're going fast, reducing the car's energy of motion at a certain rate (by generating a certain amount of electrical power in MG2) results in a relatively small braking force.  As the car slows, you may see the charge current drop a little and you can increase pedal pressure slightly.  When you get near to stopping, the current will fall rapidly and you can brake much harder.  In fact, you may feel the car brake harder without changing pedal pressure, which is a little disconcerting ar first.  Although this is difficult to describe, you can see this effect clearly with a Mini-Scanner and after a while applying the right pressure at the right speed becomes instinctive.

I should point out that braking gently at first, then a little harder as you slow and finally quite hard is not what your passengers or other road users are likely to expect.  Passengers may think you're not going to stop in time.  Drivers behind you may be misled by your light braking and end up too close as you increase braking force.  Be sensible when you use advanced techniques to optimize economy in the Prius and keep your passenger's comfort and other road users tempers in mind.


All material Copyright  2004 Graham Davies.  No liability accepted.