EVDL Administrator via EV wrote:
I seem to recall that charge efficiency of batteries is very close to 100%
when they are below 80% SOC. I'm quite sure this is true for lead, and
probably for other types too.
The coulombmetric efficiency is very close to 100% (amphours out /
amphours in). But the internal resistance of the battery means that the
*voltage* in and out are different. Higher voltage when charging, lower
voltage when discharging.
Example: Start with a battery at 12.5v (about 50% state of charge).
Charge it with 10 amps for 1 hour (10 amphours in). Now discharge it at
10 amps for 1 hour. It will be right back at 49-50% state of charge.
Essentially 100% coulombmetric efficiency.
But during charge, the voltage went up to 13.5v (average over the
charging time). During discharge, it sagged to 11.5v (average). Thus 13v
x 10ah = 135 watthours in. 11.5v x 10ah = 115 watthours out. So energy
efficiency is 11.5/13.5 = 85%.
Regen is probably not worth it for folks who use series motors. It calls
for too much additional hardware and you're probably better off spending
that money on more battery capacity.
I agree when you have a run-of-the-mill series motor, where brush arcing
gets bad with regen. But there are some series motors with interpoles.
This solves the arcing problem and makes regen more practical for the motor.
Then, you need a series motor controller that does regen. Again, most do
not. But there are a few.
However, it's relatively easier and not all that costly to add regen to
sepex and induction motor controllers, so there it's easier to justify.
And, you can also add a separate generator, just for regen. The
quick-n-dirty way in a conversion is to keep the stock alternator, and
spin it with a belt from the traction motor. Regen power charges the 12v
battery. You can switch the alternator's field from normal to "full
power" with the brake light switch to provide mild regen braking. Or
rewind the alternator for a higher voltage to get much stronger regen
into the whole pack.
It also has some often forgotten benefits, particularly for lead battery
users. By keeping the battery at a higher SOC during a given use, it can
reduce battery depreciation. Also, periodic high current charging (higher
than most overnight chargers can produce) is healthy for lead batteries, as
long as they're below 80% SOC.
Yes, I agree. Other battery chemistries as well seem to benefit from
occasional brief charging from regen.
And, I find a car is a bit more pleasant to drive with regen.
--
A designer knows he has achieved perfection not when there is
nothing left to add, but when there is nothing left to take away.
-- Antoine de Saint Exupery
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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