SanDust said:
AndyH said:
[Good quality lithium will give a good 1500-2000 cycles before it degrades to around 80%. A123, Phoenix Silicon International are two in the LiFePO4 world, and E-Moli is one in the LiNiMnO world.
Discharge cycles and age are two different issues. He's talking about age. You can put a laptop battery on the shelf and at the end of four years it may be dead with zero cycles on it. That's why Tesla will sell you a replacement battery at the time of the sale -- you'll need it. Obviously the large format cells being used in EVs will be different. But they'll be tricky because the discharge rates will probably be well in excess of 1C but they the number of cycles may be very low.
Nissan's battery warranty suggests that it believes it cells will degrade a bit less than 5% year, assuming at most 1000 cycles and no extremely hot temperatures.
I agree completely that longevity has an age component and have talked about it on the forum - absolutely! But I don't think Steve is asking what happens to his battery if he leaves his car parked in the garage for 8 years.
Folks with cell phone and laptop and Tesla experience are not a good source of comparison info for us for a number of reasons. One of which is that lithium cobalt (also commonly labeled lithium ion or lithium polymer) has close to the shortest cycle and calendar life in the lithium battery world. In addition, for these devices batteries are built for light weight and longer run-time - so pack life is compromised from the start. 500 cycles is a long life for a raw cell. Due to design and user abuse (many abuse the battery without knowing it), 500 cycles is little more than a dream.
Let's look at just laptops... The battery is snapped into the case next to the 'space heater' that is the CPU and video card. Strike one for the battery: too much heat. Most users leave the battery in the laptop and keep it plugged into the mains. Strike two for the battery: Lithium does not like to be constantly on charge. Strike three is the design bounds of light pack working hard.
None of this has anything to do with the Leaf. The LiMn (and very, very likely LiNiMn) cells are built for 1500-2000 cycle lives and we're keeping the pack cool and running it much more conservatively.
Caution...tedious geek stuff follows...if you have a headache or are prone to them, click another thread now...
The main ways to kill a cell include: Too much heat (more than 65°C damages the electrolyte; lower heat can shorten life), Charging or discharging at too high a rate (too many Amps in or out), Cell voltage too high, cell voltage too low.
The Leaf's systems monitor pack temperature (4 sensors), cell voltages, and current in and out. We cannot drive or charge the car if the pack temperature is too high or too low. We cannot drive or charge if any cell voltage is too high or low. We cannot drive or charge if current in or out is too high. The car protects us from the 'gross' killers.
Packs will degrade on the shelf. We don't know the storage degradation rate for the Leaf's pack.
Packs will degrade if charged too quickly all the time.
- L1 - about 4A into the pack (~C/20) - not a factor for over-current or overheat.
- L2 - about 9A into the pack (~C/9) - not a factor for over-current or overheat.
- L3 - about 60A-120A into the pack (~1.2-2.4C) - not a factor for over-current though greater than 1C; can be a factor for increased pack heating
Pack will degrade if discharged too quickly all the time.
- The motor/inverter is rated at 80Kw peak. Nominal pack is ~355V. Nominal inverter current might max at about 225A. Most of the time it will be much less than that - likely 40 or 50A.
- We know we have 24kWh of usable energy. There are strong indications that the pack is at least 10% and more likely 20% oversized. We know from AESC that the user capacity of each cell is 33Ah and we know that we have two cells in parallel in the pack - 66Ah. To allow for the extra capacity, each cell needs to be in the 41Ah area. Two in parallel is 82Ah. This will be our 1C number.
- Good quality LiNiMnO cells are capable of 14C surges and 3C charges, though 1C/1C gives longest life.
- Our estimated Leaf pack's 82Ah 'cell' should give the longest life if we charge/discharge at or below 82A. Our 'race car' take off that draws a full 225A is 2.7C. The more normal 50A cruise is less than 1C.
The battery's gonna degrade some if we never drive it. It's gonna degrade more if we abuse it. If we follow Nissan's instructions and keep L3 charging to a minimum, only charge to 80% if that works for us, don't keep topping-off the charge, and don't use this for racing, we'll minimize the rate of decay from 'moving violations' and get the most life from the pack we can.
