anko said:
Okay, I see what you mean. But driving slower for longer periods of time with Save mode would not prevent EV mode from occurring. Driving faster for longer periods of time without Save mode would prevent EV mode from occurring. So, is it speed or Save mode that prevents EV mode? Also, if you allowed the battery to drain normally, to its normal low water mark, would the ICE not also keep running when you dropped below 125? Like we agreed, Save and Normal mode are exactly the same except for the position of the low water mark.
The DoD story is a different thing, of course. But I have never heard that hovering around 30% would be much worse than hovering around 60%. At least, not if you plan on fully draining the battery eventually, before reaching the next charge point anyway.
Sure, *if* you are going to drain the battery to the point of needing the engine to come on, it probably doesn't matter too much (although I would it's still better to use the ICE on the freeway, because draining the battery on the freeway does it at a higher average power level and generates a lot more internal heat). But here's an example. My commute is about 16 miles, and 2/3 of that is freeway. So the idea here is that by going into Save mode, I prevent the battery from going into a high DoD at all. Typically, I can use only about 20-35% of the battery on my 16 mile commute (instead of 50-65% of it). And then I recharge at work. We've seen the effect of DoD on battery life (Table 2 on this page): http://batteryuniversity.com/index.php/learn/article/how_to_prolong_lithium_based_batteries.
The thing is, hovering at 30% isn't that much worse than hovering at 60%. But doing, say, 80% to 30% every day compared to 80% to 55% every day is going to kill your battery a lot faster. About four times as fast actually (you get double the number of CYCLES with half of the DoD, in other words, going from 100% to 50% twice as often as going from 100% to 0% means the battery lasts twice as long. But I'm not merely talking about doing half of the DoD twice as often per day, I'm talking about doing half of the DoD the same number of times per day, so we expect the battery to last 4 times as long, because it's only being used half as much, and the DoD factor doubles its life on top of that) Now if you compare 55% to 30% vs. 80% to 55%, that's where there's not much difference. The absolute SoC doesn't really matter too much, as long as it's below about 4.05V/cell, but the depth of discharge matters a lot.
anko said:
I think you cannot compare a HEV with a PHEV. Because, what an HEV does not have is a relatively cheap supply of grid power that you want to use up before you get home.
Sure, but I guess what I'm trying to say is that PHEV batteries are by far the most abused. PHEV batteries have usage patterns that tend to make batteries die very quickly.
HEVs have the benefit of holding the batteries at a pretty good SoC the vast majority of the time, and even though the instantaneous C rates at which they are charge and discharged are high, this typically does not last very long.
BEVs have very high capacities relative to daily driving. Think a 300+ mile Tesla or a 230 mile Bolt. How many miles does the thing get driven per day? Probably a lot fewer than the capacity of the battery, and hence, DoD is low. Because of this, many BEVs have a feature that allows the user to tell them to only charge to 80% or so, and to only go to the maximum that the BMU will allow before long trips. Also, because BEVs have such large batteries, they go through not too many cycles over their lifetimes. For a BEV with a 300 mile range battery, 100000 miles is only 333.3 cycles.
PHEV drivers on the other hand really tend to beat up their batteries. Because the batteries are smaller than those for BEVs, they not only discharge at high C rates for extended periods of time (like at 2C for if driving on the freeway in our Outlanders using 24 kW without using Save mode, while the huge size of the batteries in BEVs mean that even using 24 kW, you are only discharging the 100 kWh battery in a Tesla P100, for example, at 0.24C.), but they also are barely enough to supply power for our daily drives, and many times, not enough. Therefore, a lot of people drain the batteries to the maximum allowed, then charge them up to full, every single day, and sometimes multiple times per day. The BMU obviously tries to protect the batteries by only allowing discharge to 30% and stopping charging at around 85%, but there's really only so much you can do with a usage pattern like that. So by using Save mode on the freeway, I'm also trying to manage degradation in a way that preserves my battery for the long term. Yes, it's probably a little more expensive to use the ICE on the freeway compared with using electricity, but that neglects the cost of the battery itself. As a critical drivetrain component, as well as an expensive component to replace, I'd rather preserve it to do what it does best (powering the car on surface streets), and use the ICE during times it's reasonably efficient to do so. If you calculate the cost of battery replacement from allowing a high DoD every day, I suspect that the calculations would show that it's not worth it to burn electricity on the freeway.
Autobahn, 150 kph, hills, "no lack of power at all"... Anyone like to back that up? My BMW X1 sdrive28i struggles at 130 kph on hills, and that's signficantly lighter than the Outlander phev. Lack of power is almost universal with online reviews:
