4 ms·

Does anyone know what is the cycle limit on these Powerpack station cells?

deleted 10y ago
[deleted]
owenversteeg 10y ago
I've got a good idea what the cycle limit is at normal discharge rates for these batteries, but the tricky thing in the calculation here is that the discharge rate is around 0.6C. That's very low (batteries of a similar size can put out fifty times that comfortably.) Discharge rate affects the usable cycles. So with an 80% cutoff 10C discharge you might get 300 cycles, but with a 1C discharge you might get 500 and a 0.5C discharge you might get 600. Given the very low discharge rate, I'm guessing these will be ran for around 500 to 1000 cycles, but that's just a rough estimate and the real number could lie anywhere between 300 and 3000. Temperature, pressure, charge rate, etc. all affect this.
- tshannon 10y ago
  So what does that translate to in years of operation? Does Tesla basically get a nice recurring income stream with battery replacements every 5 - 10 years?
- hollerith 10y ago
  "the discharge rate is around 0.6C" I looked up this "C rating". 0.6C means that the battery will be completely discharged in 1/0.6 hours. Well, more precisely, it means that it is producing 0.6 * X amps where X is the battery's capacity in amp-hours. Since discharging a battery at a rate of 0.6 * X amps usually produces less heat than discharging it at X amps, the battery is (usually) slightly more efficient, which means it will usually continue to provide power for slightly more than 0.6 hours. Hopefully, the person I am replying to will correct me if I have made an error here.
  - owenversteeg 10y ago
    Yep, all good except I think you meant to say "1/0.6 hours" in that second-to-last sentence. Another thing worth keeping in mind is that discharging (most) Li-ion batteries below a certain voltage is harmful, and thus you can only usually "get" 80% or so of the energy out of the battery. In simple terms, C rating is how fast you're draining the battery. Most batteries are rated for between 1 and 10C. This is the runtime you'll usually get from C ratings: 0.1C/10.5hr; 0.2C/5hr; 0.3C/3hr; 1C/55min; 1.5C/35min; 2C/25min. You'll notice how this doesn't track the expected values perfectly; for example, at 0.1C you get 5% more runtime but at 2C you get almost 20% less. This is because at 0.1C heat is negligible, but at 2C you're discharging almost 30 watts per cell. A good rule of thumb when running over 0.2C is to subtract five minutes from however long the cell "should" be able to run. You can probably now see how discharging at over 10C is tricky. Discharging at a lower current produces less heat and increases the cycle lifetime, as well as a few other things (e.x. decreasing the risk of explosion.) Since Tesla is discharging these at 0.6C, they will get more cycles out of them than if they discharged them at 1C or 10C, for example. The number of cycles is very important here because these will be used at least one cycle per day, and given that most batteries have a lifetime of 200-500 cycles this is a maximum of 200-500 days. Given that there are 16-20 million dollars worth of batteries here, that represents a significant yearly cost.
    - greglindahl 10y ago
      How do you know about Tesla's cell cooling system, and how do you know how much the utility is planning on using the battery? Or do these two things not matter?

deleted 10y ago

[deleted]

I've got a good idea what the cycle limit is at normal discharge rates for these batteries, but the tricky thing in the calculation here is that the discharge rate is around 0.6C. That's very low (batteries of a similar size can put out fifty times that comfortably.) Discharge rate affects the usable cycles. So with an 80% cutoff 10C discharge you might get 300 cycles, but with a 1C discharge you might get 500 and a 0.5C discharge you might get 600. Given the very low discharge rate, I'm guessing these will be ran for around 500 to 1000 cycles, but that's just a rough estimate and the real number could lie anywhere between 300 and 3000. Temperature, pressure, charge rate, etc. all affect this.

tshannon 10y ago
So what does that translate to in years of operation? Does Tesla basically get a nice recurring income stream with battery replacements every 5 - 10 years?
hollerith 10y ago
"the discharge rate is around 0.6C" I looked up this "C rating". 0.6C means that the battery will be completely discharged in 1/0.6 hours. Well, more precisely, it means that it is producing 0.6 * X amps where X is the battery's capacity in amp-hours. Since discharging a battery at a rate of 0.6 * X amps usually produces less heat than discharging it at X amps, the battery is (usually) slightly more efficient, which means it will usually continue to provide power for slightly more than 0.6 hours. Hopefully, the person I am replying to will correct me if I have made an error here.
- owenversteeg 10y ago
  Yep, all good except I think you meant to say "1/0.6 hours" in that second-to-last sentence. Another thing worth keeping in mind is that discharging (most) Li-ion batteries below a certain voltage is harmful, and thus you can only usually "get" 80% or so of the energy out of the battery. In simple terms, C rating is how fast you're draining the battery. Most batteries are rated for between 1 and 10C. This is the runtime you'll usually get from C ratings: 0.1C/10.5hr; 0.2C/5hr; 0.3C/3hr; 1C/55min; 1.5C/35min; 2C/25min. You'll notice how this doesn't track the expected values perfectly; for example, at 0.1C you get 5% more runtime but at 2C you get almost 20% less. This is because at 0.1C heat is negligible, but at 2C you're discharging almost 30 watts per cell. A good rule of thumb when running over 0.2C is to subtract five minutes from however long the cell "should" be able to run. You can probably now see how discharging at over 10C is tricky. Discharging at a lower current produces less heat and increases the cycle lifetime, as well as a few other things (e.x. decreasing the risk of explosion.) Since Tesla is discharging these at 0.6C, they will get more cycles out of them than if they discharged them at 1C or 10C, for example. The number of cycles is very important here because these will be used at least one cycle per day, and given that most batteries have a lifetime of 200-500 cycles this is a maximum of 200-500 days. Given that there are 16-20 million dollars worth of batteries here, that represents a significant yearly cost.
  - greglindahl 10y ago
    How do you know about Tesla's cell cooling system, and how do you know how much the utility is planning on using the battery? Or do these two things not matter?