[ausev] Open Garage this Sat... 9am - 1pm

Thu Aug 13 16:08:23 GMT 2009

Great stuff Brian.
This is good info to know as it’s the true picture of the efficiency of the
system.
We often only focus on DC efficiency, i.e. efficiency of the car that moves
the vehicle a certain distance for a given amount of available energy.  For
example to estimate the range of a small vehicle with 10kWh of usable
energy, one could assume an average of 250Wh/mile efficiency (or "fuel
economy") which would give it a 40 mile range (+/-).  But we don't often
talk about the system efficiency, which includes battery efficiency and
charger efficiency and any parasitic loads on the car while charging
(cooling pumps for example).  I think the Tesla claims a 350Wh/mile
efficiency number taking into account all of these things.  Flooded lead
acids are less efficient than li-ion, for one reason, because they require
an overcharge for balancing which eats into efficiency.

So by that metric, your fuel economy/efficiency has been:
Feb 389 wh/mile
March 455 
April 543 
May 592 
June 1230 
July 874 

We can assume that the efficiency of the charger is relatively the same
throughout the year, but battery round trip efficiency does change with
temperature, cycle life, and age.  It appears your batteries are now using
twice the energy from the wall to give you the same mileage.  Because the
chemistry of the battery is changing for reasons previously listed, one
would need a charging system that would adapt as well.  Using the simple
"charge to X volts" doesn't really work (from an efficiency point of view)
after the battery has been sufficiently used.  I bet some amount of
overcharge could be eliminated by a charging algorithm that stopped charging
due to different criteria than just voltage and range would still not be
diminished.

Don't get me wrong, I think the PFC charger is an amazing piece of
equipment, very versatile and powerful.  It's just hard to have a the
knowledge of how best to charge a battery as it ages/cycles/temperature as
even the manufacturers are still learning.  As you can imagine, not much
research was performed to determine optimal end of life charging criteria
for golf cart batteries.  Now that EVs are being developed by OEMs, that
work is being done and hopefully those lessons, algorithms, and equipment
will trickle down for the hobbyist.

Brian, what batteries are you going to replace them with?  Same ones or
something different?
How many cycles are on them?  
What is the average depth of discharge?
Perhaps you could discharge one at 25A and compare it to published data to
see where it falls on the graph.
Marc Kohler   

-----Original Message-----
From: ausev-bounces at austinev.org [mailto:ausev-bounces at austinev.org] On
Behalf Of Brian Lasseter
Sent: Thursday, August 13, 2009 10:00 AM
To: AustinEV News Announcements and General Discussion
Subject: [ausev] Open Garage this Sat... 9am - 1pm

I have used and abused my batteries every day for the past 15
months... and it is time for some new batteries.

So, I will be holding an open garage this Sat for anyone who wants to
help me remove batteries, anyone who wants to inspect my car closely,
or even anyone who wants to just sit in a chair and laugh as I lift
70lb batteries.   :-)    All comers welcome.

@ Lasso's House  :  (512) 736-1677
Saturday, August 15th  :  9am - 1pm
11402 B Ptarmigan Cove
Austin, TX  78758

--

If you are wondering WHY I am replacing the pack, I seem to have been
slowly melting a battery post over the last few weeks, and it finally
started dripping lead last weekend.  Really though, the whole pack is
becoming unworkable.  The range is still a good 30 miles, but the heat
generated keeps my pack at a fairly constant 140°F - 150°F which is
making it difficult for even the water miser caps to retain water.
I've also noticed that the heat is really killing my electrical
efficiency, and preventing me from getting anywhere near the correct
charging voltage.  At 140°F charging temperature, then 174vdc would be
the manufacturer's recommended charging voltage... but that was rarely
reached.

For anyone interested in numbers... here are my efficiency numbers for
the last few months of driving, and the total over the life of the
battery pack.  Efficiency has units of miles/kwh.  I'm not sure if
anyone else has any insight as to whether 6206 miles or 3882kwh is a
good run for a lead battery pack?

               kwh end miles end               kwh     miles
efficiency

       Jan     15456   149605
       Feb     15669   150152          213     547             2.568075117
       Mar     15869   150592          200     440             2.2
       Apr     16045   150916          176     324             1.840909091
       May     16242   151249          197     333             1.69035533
       June    16579   151523          337     274             0.81305638
       July    17080   152096          501     573             1.143712575

               13270   145953
       Total   17152   152159          3882    6206            1.598660484

-- 
TTFN,
Brian "Lasso" Lasseter

  ·  (512)736-1677  ·  AIM:digininja  ·  ICQ:2238123  ·  MSN:azoreg  ·
"No Sane man will dance."   -Cicero (106-43 B.C.)

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