(12-11-2018, 05:15 AM)Korishan Wrote: Yes, this is true, mostly. This is assuming we are using the cells from 4.2V to 2.7V full range. However, most are using the cells from 4.0V to 3.2V, which drastically increases their cycle life. Hence the reason for so many cells in the packs.
Daromer has been running at least 1 string for almost 4 years (if I remember correctly) now. There's others who have ran theirs for 2 or more years as well.
We have to remember that the life expectancy of the cells is dependent on how hard we charge/discharge them, as well as what the working range in voltage is. These two factors helps us to get far more life out of the cells than what the datasheet states.
However, you do have a valid point. This is one reason why I am going to be building my packs with ease of replacement and monitoring so I know which cells need to be replaced. And this will be for new or used cells.
When you say cycle life you always need to mention the depth of discharge but for those type of cells the rule is close to being proportional so say DOD is 50% then you will have 2x the cycles at that DOD but then total energy stored over the life time will be the same if you do 100 cycles at 100% DOD or 200 cycles at 50% DOD or 2000 cycles at 5% DOD
If you barely use the battery then they will last even 10 years the degradation over that period in this case will be mostly related to calendar degradation so cost amortization may be even higher.
This high amortization cost for high energy density cells is also the reason why an EV like Tesla will be more expensive than an equivalent ICE
If you look at the warranty of model 3 long range that has the new cells (supposedly better).
Model 3 has a 192000km or 8 year warranty whatever comes first and is based on a 80kWh high energy density NMC
That model 3 has a range of 500km on a full charge (almost 100% DOD excluding the small top and bottom reserve) so 192000km / 500km = 384 cycles
Now the old cells if you look at Panasonic spec sheet is capable of 500 cycles at 100% DOD and by that time it will only have 70% of original capacity.
The warranty also claims 70% of original capacity in 8 years or 192000km thus it means they took in consideration the worst case that is 384 cycles about 90% DOD as there is some 5% top and bottom limits. People will also not drive like that they will likely charge to 80% and almost never get close to 0% SOC so they should be fine with around 15% or so degradation from cycling and the other 15% degradation for aging assuming around 2% per year calendar degradation that will be typical for EV due to higher battery operating temperature.
That 80kWh battery still cost around $16000 (that excludes the BMS and other stuff around) thus you pay that extra compared to an ICE car and when you consider that extra cost it will be $16000 / 192000km = $0.083/km or put in another way $8.3/100km and that is more than what my inefficient 6 cylinder ICE will require in gasoline cost. That is just the battery amortization cost not counting the electricity to charge the EV. This will be about the same for all EV's Tesla was just an example.
Put in a different way $16000 / 70kWh x 384 cycles = $16000 / 26880kWh = $0.59/kWh and that will be the cost amortization for that battery over the life time of the battery.
If they could offer a better warranty they will do so but they can not as those are the limitations of the specific battery technology. That is more than adequate for the average driver so is a good choice of battery for the specific application but as you seen in my earlier comment LiFePO4 can do much better at just 0.16cent/kWh so LiFePO4 is much better for stationary energy storage.
LiFePO4 will not be so great for an EV as is about 2x lower energy density so you will have a car that costs the same but has half the battery capacity and half the range so people will not by this. BYD uses LiFePO4 in their buses and taxis since those type of vehicles will need to put much more km than a consumer personal car.
For me cost amortization over the life time of the product is very important and I calculate that for anything.
That is how my small energy efficient 65m^2 (700sqft) house cost a total of $42 / month in therms of cost amortization for both electricity and PV electric heating about half of that amount for each and it is the most cost effective solution even when compared to natural gas.
rePackr
