Battery Cycle Life/Cost

[Mike Chancey <evtinker@evalbum.com>]

Hi folks,

I have been wrestling with long term battery life issues,  trying to come 
up with an affordable way to get my Civic back into regular daily commuter 
use.  Unfortunately, all I am really discovering is this was the kind of 
analysis I probably should have done before building the car in the first 
place.  Here is what I came up with:

My commute is roughly 25 miles round trip, but charging is usually 
available at work.  On weekends I probably put about 20 miles on each day, 
mostly in little 5 or 10 mile hops, so we are looking at about 165 miles 
per week, or about 8580 miles per year.  Assuming I recharge once at work 
and once at home every workday, and at least two charges per day on 
weekends, that is about 728 cycles per year.  Considering the current power 
consumption of my Civic, roughly 350 watt hours/ mile due to hills and 
heater use, the 12.5 miles of each workday commute leg uses about 28% of a 
charge.  So, the battery pack is asked to deliver about 728 cycles at 28% 
depth of discharge.  The question is, how many cycles can I expect the 
batteries to deliver before failure?

Currently, I have the Civic equipped with a set of 13 used Dynasty group 31 
AGMs (12 volt 100AH).  Due to numerous failures, I have discovered a $20 
used battery probably really isn't cheap after all.  Before I even consider 
investing in 13 new ones though, I feel it might be wise to look at the 
chances of them lasting 1 year under my service demands.  I suspect there 
is little chance of getting a year out of them at all.  If they did last 1 
year, at roughly $1700 a set, my 8600 miles is going to cost about 20 cents 
per mile just for battery replacement cost.  Ouch.  Another alternative 
must be found.

It just might be possible to rebuild the Civic to use 18 8-volt Trojan 
T-875 flooded batteries, though this would really ruin some of the car's 
best features.  Now we are looking at a battery cost of about $1200 per 
set, but due to the greater capacity, the cycle depth would only be about 
20%.  For those who have used the T-875, what is the likely cycle life at 
20% discharge?  I am guessing it might be possible to get at least 2 years 
out of them at this shallow cycle, 1456 cycles, or maybe that is too 
optimistic.  If they would last that long, then the battery cost would be 
down to about 7 cents per mile, quite a bit more reasonable.

Taking this further, dropping down to 18 T-105s would make for pretty lame 
performance, (from 156 volts to 108) but that would increase capacity and 
reduce the cycle depth still further, to around 18%.  Battery pack cost 
would drop to about $900, or 10 cents per mile for 1 year of life, 5 cents 
per mile if they last for 2 years of service.

At this point we are getting so far away from the existing conversion I 
would probably be better off to buy/build/trade for an EV designed for 
these more cost effective battery choices.  A 120-144 volt, T-105 equipped 
pickup would probably be a more appropriate choice for this commute I guess.

So, what kind of words of wisdom can you folks share with me?  Is there a 
better alternative?

Thanks,

Mike Chancey,
'88 Civic EV
Kansas City, Missouri
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