Michael Ross wrote:
Do you think if LiFePO4 are BB'd and limited to operating between 20% and
80%, or 10% and 90% SOC that they will have degraded cycle life?
Lithium cells have been around for almost 20 years now. If you look,
there are lots of examples that can tell us how long they lasted, and
how did usage affect their life.
Laptops are a classic example. They almost all use Li-Co or Li-Mn
formulations, because it's cheap and provides the highest energy storage
per pound. We find that:
- cycle life is fairly short (a few hundred deep discharge cycles)
- calendar life is fairly short (a few years)
- higher temperatures drastically shorten life
and, most laptops tend to run their batteries hot
which is a major contributor of their short life
- deep discharges shorten life
so the less deeply you run it down, the longer it lasts
some systems prevent deep discharges; their cells last longer
- overcharging shortens life
many cheap chargers chronically overcharge, because it's a way
to increase operating time per charge in the short run
but it shortens life in the long run
some systems limit charging, to extend life but shorten run time
- a BMS system is mandatory (or you get fires!)
Note that many of the current auto company produced EVs are using Li-Mn
based cells. That implies that their life will follow a similar pattern
(marketing claims notwithstanding). They are depending on limiting depth
of discharge and charging, and better temperature control to compensate.
Will their efforts be enough? Time will tell.
Li-Fe based lithiums are newer, and there is less field data on their
life. They cost more and store less energy per pound, and so are less
popular in consumer products (where price and long running time matters
more than life). But safety testing has shown them to be safer. They
have the potential for longer life (have done so under ideal controlled
conditions in the lab).
- consumer products that have used Li-Fe cells (like cordless
drills) don't seem to have any longer life
- they still "die young" from deep discharge cycles, or chronic
overcharging, or high temperatures
- so, their life is dependent on initial quality control, limiting
depth of discharge, charging, and temperature
- a BMS is not as important to prevent fires; but it is necessary
to avoid abusive events that would seriously shorten life
They don't have any fancy elements in them (Co, Ni, Mg) Which helps cost
a lot. They are safer (higher burn temperature).
Well, this is true for pretty much all rechargeable batteries. The
higher the energy storage, the more fragile they are, and the more
dangerous their failure modes are.
I read that some OEM EVs with Li Ion packs limited there SOC range between
25% and 75% - only 50% of capacity actually used. That makes sense if you
are willing to over design that much.
My understanding is that they are doing this to prevent early battery
failures, and the resulting high warranty costs and bad publicity.
You can charge faster if you never go near 100%, and very long life
is attractive. I think the S Model is limited in SOC range too.
Yes, everything I've read says they are doing this.
There is a *long* history of companies making exaggerated claims about
their EV's range. They depended on overcharging, and deep discharging to
get that range. The result was short battery life. This cost them dearly
in sales and reputation.
It does appear that the auto companies have learned from this, and have
quit exaggerating their claims quite so much. But that could be due to
fear of warranty claims, lawsuits, or government regulations.
I have a salvaged pack of 40Ah Thundersky cells for my little pedal assist
EV, and I am thinking of operating this way; so I am interested in
opinions. I am willing to do the work to BB them and set up a cut off
system. It is a good learning experience.
I am thinking of using 17 cells instead of 16 and charging with 60V and not
worry about a charging cut off. Just a discharge cut off.
Ultimately, the only way you'll know is to TEST FOR YOURSELF. No one
will go to the trouble and expense to test, and then give all the data
away for their competitors to take advantage of.
It's not that hard to set up a BMS for 17 cells. Watch what they do, and
control your driving and charging to limit the stress on them. Just
about *everything* lasts longer if not overstressed.
--
There is hardly anything in the world that someone cannot make a
little worse and sell a little cheaper. Those who consider price
alone are that person's lawful prey. -- John Ruskin
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/projects.htm
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