Efficient Electric Generation

Your question is more complicated than you probably realize.

It's been determined that the most efficent 'charge rate' occurs with about 60 amp braking...this is a moderate steady regenerative brake pressure. By most efficient we're talking about the amount of energy from a given speed that ends up going to the battery.

Now, that's all well and good. But most efficient charge rate isn't the same as most efficient braking for conservation of energy/fuel economy.

Think of it this way....You're 2 miles from a stop sign going 60mph and are trying to decide how you want to approach the stop.

At 1 1/2 mile away you can decide to go into a glide...easing your foot off the gas so that the ICE stops burning fuel and only minimal charge is being drained from the battery. You gradually slow over the 1 1/2 miles coming quietly to a stop at/near the stop sign. You regenerated nothing...in fact you drained some charge from the battery during the glide. But also you burned no fuel for a full 1 1/2 miles. So a minor net energy used for that distance.
OR
At 1 mile you decide to just coast...foot off gas and brake. Thus you had to burn gas until the 1 mile mark (net energy loss). Coasting regenerates at a rate of ~12 amps. So over that distance you regenerate back a portion of the gas you burned to the 1 mile mark... Net you're still behind what you would have been w/ scenario #1.
OR
At 1/2 mile from the stop you go into a nice 60amp brake to a stop. You charge the battery up 2 full bars and that looks nice on the display. But you had to burn gas for a full mile more than scenario #1 and a 1/2 mile more than scenario #2. Clearly your net energy use is more for this scenario even though the battery SOC is higher than either of the 2 above.

Now, reality intervenes....if you're on a road with other cars scenario 1 is completely impractical. Scenario 2 is likely very rude if not impractical. Scenario 3 might even piss off some people who'd rather you stay at 60mph until 100 ft before the stop sign.

The beauty is that the energy difference of the 3 scenarios is fairly small and only when you get into the hypermiling situation like some of us fanatics it just doesn't make a major difference...as long as you anticipate stops and traffic and try to avoid abrupt/late braking and aggressive starts you're going to do ok.

 

<div class='quotetop'>QUOTE(sub3marathonman @ Mar 1 2007, 12:50 PM) [snapback]398597[/snapback]</div>

The point of my example was the theoretical 'glide to a stop' to make a point. Clearly that's pretty difficult to do in real life and you would either have to apply brakes if your glide is going to carry you too far or you'd have to add power (either electric or gas) to carry you the full distance.

And the ideal condition would still be scenario 1...that is a perfectly timed glide to a stop...it's just not practical in 99.9% of situations. So I, too, tend to employ something more like what you suggest...get into a glide that helps maintain speed as far away from my intended stop as possible then brake when I can regen maximally depending upon my speed and distance from the stop. This causes the least frustration to traffic behind me well still achieving an efficient energy use state.

 

<div class='quotetop'>QUOTE(cubbyg @ Mar 1 2007, 11:08 PM) [snapback]398904[/snapback]</div>

It's kind of relative and, again, varies w/ conditions.

What I've found most recently is the following:
From a cold start (car off power up to READY) the car will burn much more richly than once through the warm-up stages. The car "likes" to run at 1275 RPM during this stage 1 warm-up. If you can very gently accelerate during this time you'll stay at a minimal fuel flow and minimal battery drain...do that for about 30-60 seconds and you'll get into stage 2 where fuel flow reduces to a more efficient rate for a given degree of acceleration.

Now, if you're already warmed up and you're starting from a stop then a moderate degree of acceleration seems most efficient. If you can keep the ICE RPM b/w ~1700-2300 this seems to be the best acceleration for fuel use range... See Hobbit's work on this HERE