RegGuheert said:
SanDust said:
If a deep DOD were more problematic than a full charge, the engineers could have designed the Volt to charge to a 90% SOC and allowed it to discharge only to a 30%. They didn't.
Of course they didn't! The Chevy Volt has such a small amount of energy available to the user that it must be kept charged to the max that GM allows all the time. Unfortunately it is not clear that 85% or even 80% is much better for storage than 94%. Some studies I have seen show it as being worse.
This response sidestepped SanDust's point altogether that engineers didn't design the Volt to charge at higher SOC and to stop the discharge at higher SOC because they didn't deem it necessary. The Volt doesn't have "such a small amount of energy" like RegGuheert said. The Volt has plenty of energy for the majority of trips most people would make in a day. It's irrelevant that it must be kept charged to the max from the perspective of Sandust's point that GM engineers didn't need to choose a higher SOC at the top and higher SOC at the bottom. I feel like RegGuheert used a diversionary response here to not have to address SanDust's very valid point, that GM engineers didn't see the need to do what RegGuheert advertised as important, because to GM engineers, it's not as important as claimed by RegGuheert.
RegGuheert said:
SanDust said:
They chose to more or less evenly apportion the unused battery space to the top and bottom of the SOC. Nissan made a similar choice. It recommends that you charge to a 70% SOC (80% of 90%), but allows you to fully discharge to 10%. It could have recommended fully charging and recommended either avoiding deep discharges or electrically preventing it. But it didn't, choosing to emphasize avoiding charging at the top end rather than discharging at the bottom.
Nissan does not offer a 70% SOC charge option. When you choose 80% SOC, you get 80% SOC, or very close to that. We keep our LEAF charge to a lower SOC until the car is needed.
But it did. The LEAF has 24 kwh but only 21 kwh is used. So if you take (21khw/24kwh)*.8, you get 70% from 24kwh.
RegGuheert said:
SanDust said:
When you take these engineering decisions together, it seems that both companies are indicating that fully charging is at least as damaging to cell life as fully discharging. And your DOD numbers only capture the latter and not the former.
It's not true. Charging to the high SOCs is not as damaging unless you charge it up there and leave it there like you would with the Chevy Volt. We don't charge to full until just before we leave.
One can argue just the same that if one charges up right after one reaches the low SOC, then it's not as damaging either. Why wouldn't the same logic apply there? Has there been any published paper to prove otherwise? And usually by the time one reaches the low SOC, one already ends up at home in the garage charging up right away again.
What RegGuheert is implying here is that reaching a deep SOC causes irreversible damage even if you recharge it right away to take it out of that mode asap, while topping off at high SOC does not cause irreversible damage if you use it up right away. While I don't argue with the later, I don't necessarily agree with RegGuheert's point about the earlier, especially if it's not a VERY DEEP discharge. Remember that you're NOT ALLOWED to cause a very deep discharge on the Volt battery anyway, because the ICE will take over as soon as the MILD discharge point (20%) is reached. On the other hand, the LEAF is allowed to drive all the way to Turtle mode, way past LBW and VLBW, to the point where limp mode occurs. Surely this is much deeper than the 20%. So while the Volt is protected against very deep discharge, the LEAF is not.
We can beat this horse to death, but the whole point is that the Volt, because it has the ICE and TMS, has these luxuries in place to allow it to be engineered to have all possible protections in place, from too high a charge to too low a discharge, from too high a temperature to too low a temperature. Meanwhile, the LEAF, because it doesn't have an ICE and no TMS, has to be compromised in order to provide as much range as necessary to owners. The compromise allows from 100% charging to Turtle level discharging. And the lack of TMS allows as much as 7 to 8 temperature bars operation. QC, which is allowed (a necessary thing to sell the car), can exacerbate the deterioration of the battery if used in hot weather or right after heavy driving.
Sure, a careful LEAF owner can make sure to avoid all those conditions to help prolong their battery life as much as possible. But without all the protections that has been put in place on the Volt to avoid owners' abuse, the LEAF is much more vulnerable to owners' abuse. And sometimes you can't even blame the unsuspecting owners for "abusing" it because Nissan creates false marketing impression to advertise the LEAF's capability to its limits (claiming 100 mile range for example), without doing much to prevent owners from pushing the LEAF's battery to its limits. The fact that Nissan is still selling LEAFs in AZ like nothing ever happened underlines this point for sure.