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Originally Posted by MetroMPG
Interesting. Thanks for the details.
You're smart to have invested in battery monitoring (Pak Trakr). Same idea as the LED pack monitor we stuck in the ForkenSwift recently, but fancier. And with memory!
I'd be very interested to know your efficiency - kWh consumption per mile. Are you keeping a log?
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I am currently keeping a paper log and I have a Kill-A-Watt on the extension cord that feeds it while it rests in its crib at night. As things evolve I am going to integrate a GPS tracker into the data stream that comes from the PakTrakr. The odometer is notoriously inaccurate so it is that or pay nearly $300 to get the speedometer calibrated.
In round numbers it takes about 4.7kWh to bring the batteries to a full charge when they are broken in. If the car is driven on the straight and level at about 30 mph in temperate weather it will go about 25 miles on a fully conditioned set of fresh batteries. Starting and stopping, hills, cold weather and higher speeds will reduce this figure.
An ideal calculation is 4.7kWh/25= .188kWh/mile. Since the Kill-a-Watt records every watt-hour going in it should give a fairly reliable accounting of my electrical usage. Right now after just three or four days of history it is estimating a yearly cost of about $85.
However, there is a rather nasty secret hiding behind this number called per mile battery cost (PMBC). The replacement cost of the Xebra’s current set of batteries is about $2200. If my average annual usage is 3,000 miles (probably high as I live in a small rural community in south east Iowa) then if the pack fails after 1 year my PMBC will be 73 cents/mile. Clearly electricity cost is negligible if the batteries fail early.
From my reading early failure or significant degradation is a real possibility if the pack isn’t managed carefully. This means not unduly stressing the pack with discharges greater than 50%, monitoring individual battery voltages and balancing the pack so that all of the batteries share the load equally, using available technical means to fight sulfation and keeping the pack fully charged.
Cold weather is also a challenge that must be met since the chemical processes that underlie battery performance are degraded at low temperatures. Iowa is cold and windy in the winter. If I am able to maintain these batteries for 5 years or 15,000 miles I might achieve a PMBC of 14.6 cents/mile. My 1991 Honda Accord gets 20 mpg city without much effort on my part. At $4/gallon for gas that works out to 20 cents/mile and it has a whole raft of creature comforts that the Xebra is lacking.
There is a saying in EV circles that your first set of batteries is sacrificed to the learning curve. Forewarned is forearmed. I’m trying to do this right and I’m willing to invest considerable time and energy in making this experiment a success. But as in all experiments the real results may not be what we wish for.