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LG Chem Resu 6.4Ex vs Tesla Powerwall
For Tesla the average ambient temperature over the system’s life should be 30°C or less. LG battery warranty is voided if the battery operates below 0ºC or over 40ºC.LiFePO4 : Traditional batteries start strong, and then the voltage slowly goes down as the battery discharges. The graph below shows discharge curves for Lithium vs. common sealed lead acid (SLA) batteries. Notice that the LiFePO4 does not lose significant voltage until 90% of its capacity has been used. Lithium batteries maintain their full voltage almost to the very end. NiCad batteries can go dead in a month with self discharge at rate of about 1% per day, LiFePO4 has a typical self discharge rate of 5% per month.
Traction Batteries for EV : A typical family car would need a battery capacity of about 40 KWh to provide a one way range of 200 miles and a 40 KWh Lead Acid battery weighs 1.5 tons. It goes without saying that low cost, long life (more than 1000 cycles), low self discharge rates (less than 5% per month) and low maintenance are basic requirements for all applications. Traction batteries generally operate in very harsh operating environments and must withstand wide temperature ranges ( -30°C to +65°C) as well as shock, vibration and abuse. Protection circuits are also essential for batteries using non-Lead Acid chemistries. Traction batteries are very expensive and like all batteries they deteriorate during their lifetime. Customers expect a minimum level of performance even at the end of the battery’s life, so the buyer is likely to specify the expected performance at the end of life (EOL) rather than the beginning of life (BOL). Under normal circumstances for EV applications the EOL capacity is specified as not less than 80% of BOL capacity. The diagram below compares the battery power and capacity requirements for a vehicle of the same size and weight when configured as an EV, an HEV or a PHEV. Battery designs may be optimised for power or for capacity (energy content) but not both. EV Batteries Are More Awesome Than We Thought : I found a document from a car manufacturer (Smart Electric Drive), I don’t have permission to reproduce it, but I can give you the highlights. 1# At DOD 80% – 9000 cycles at 1C (extrapolated), 4000 cycles 2C, 3000 cycles at 3C. Another interesting point? At 2% DoD cycling, at insanely high rates, the cells lasted 3.5 million cycles! That’s insane! Guess we don’t have to worry about regenerative braking wear!
What about the pre-eminent electric car: The Model S? Well, that’s a bit of a trick. You see, the exact cells that Tesla uses are probably unique to them. But we can try to make some educated guesses based on similar Panasonic cells! And it looks like it’s about 300 cycles to 80% capacity, at 3C, for these particular cells. Gee, that seems low, doesn’t it? But let’s really look at this further. How could Tesla use such cells, and not have cars going flat in just a few years? We know that Teslas in the wild seem to be holding up really well! There are a few main reasons for this: 1.Depth of Discharge, 2.Rate of Discharge, 3.Temperature Management, 4.We might not be looking at a valid datasheet. Lets look at each of those a bit closer:
Average depth of discharge in a Tesla is going to be very, VERY low compared to any other electric car. If we look at the average American commute distance of about 15 miles, that would result in a DoD of around 11%. That’s tiny, and will greatly expand cycle life. As the graph above shows for a similar lithium-ion chemistry, cycle life can be greatly expanded by lower depth of discharge. So, while a Nissan Leaf or a BMW i3 needs to have relatively tough cells to deal with high DoD cycling, a Tesla can use much less expensive cells.
Rate of discharge in a Tesla yields similar gains. Most of these cell tests are performed at relatively high rates of discharge. This is how they are used in consumer electronics, but this is NOT how they are used in cars. At a 1C discharge rate, the P85 would be using 85 kW or 115 hp, and the battery would run flat in an hour. But we know that in the real world, the battery discharges over 5 or more hours (265 miles, 60 mph… do the math). That puts the worst case average discharge rate at .2C or less. Far less for your in town driving. This will also help extend cycle life.
Temperature management is also key to battery life. This is why most electric cars have chosen to have a full, water-based thermal management system. This system is to keep the battery cool in the summer, and warm in the winter. The impact is huge. At 45 degrees C, battery life is cut in half compared to 25 degrees C! It’s interesting to note that the only car that has had battery life issues that we know of is the Nissan Leaf with its passive cooling system.
And finally, you have to question if Tesla is actually using the cells from that datasheet. Tesla is buying enough cells to warrant special treatment here. It’s ripping through hundreds of thousands of cells a week. I’m sure Panasonic is willing to work with Tesla to hit whatever goals it needs. But even if Panasonic isn’t, and these are the cells Tesla is using, I fully expect that they’d be able to stand up for 10+ years and retain 80% capacity, so long as they’re looked after!
Audi launching second-life program for depleted EV batteries : The current crop of batteries made by Audi are designed to last for at least eight years or 150,000 km, with the average EV owner doing approximately 18,000 km annually. Pretty soon, the “obsolete” batteries will be useful again as Audi wants to install them in the electrical grid to work with solar technology as a way to harvest power during daylight and store it to discharge the energy in the night.