[SGVLUG] OT: Hybrid efficiency (was:New Linux Lug)

[Jeremy Leader]

Emerson, Tom wrote, in part:
>> [David:]  Charging batteries is only 36% efficient in the Prius II.
>> 
> 
> While I have the first generation prius, I *suspect* there is an
> artificial cap placed on charging.  I've heard rumors that the system
> tries to keep the battery at a slightly-less-than-full charge, and I
> tend to believe those rumors based on what I've seen: the "state of
> charge" portion of the display typically reads 3/4 charged
> -almost-all-the-time- for me, and only dips below that for an
> extended uphill run.  When "coasting" down Angeles Crest from the
> closest "peak" [essentially the vista point/ranger station/vetter (?)
> approximately 12 (linear) miles from La Canada], I'll get the display
> to read "100%" about the time I reach the country club (the final set
> of turns when going downhill).  This drops down to 3/4 within 1 mile
> of "freeway driving" on level ground.
> 
> Conversely, it isn't until I reach the second peak & ranger station
> (near palmdale) that I can actually drop the charge down to 1/4 or
> even "fully discharged", but this "recovers" back to 3/4 charge
> within a mile or two of "downhill" driving/coasting.
> 
> Long story short, I believe the system *could* recover more energy,
> but doesn't.  Why it doesn't do this is more likely due to political
> or marketing reasons than technical, but this borders on conspiracy
> theories...

I suspect the system is designed to avoid going "to the limit" whenever
possible.  That is, you don't want the battery to be discharged when
you happen to need a little more energy (e.g. to accelerate harder than
the gas engine alone can manage), and likewise it would be nice to
avoid having the battery fully charged when you'd like to do regenerative
braking.  So the control system probably attempts to leave a little room
at either end of the charging curve.

If the battery is at 100%, the system will choose (if possible) to use
more battery power (and less gas power) than it normally would, while if
the battery is near 0%, the system will choose (if possible) to use more
gas power than it normally would, to get some charge back into the
batteries.

There are tradeoffs between greater reserve acceleration capacity (and
also greater range after running out of gas), versus the inefficiency
of storing energy in the battery.

-- 
Jeremy Leader
jleader at alumni.caltech.edu
leaderj at yahoo-inc.com (work)