Questions about the ID.4 battery capacity and cold weather

evianfinn said:

@EVone this statement is not correct, but I think the problem with your advice here is that it's based on your priorities which may not be the same as others.

First, about usable battery. 77/82 is 93.9%, or 94%. So, the buffer that VW puts aside on the battery is 6%. However, that 6% buffer is not all applied to the top end. There is also a part of that applies to the bottom end. Only VW knows if this is split 50/50 or otherwise, but it doesn't matter. What matters is that the buffer is likely less than 5%, and does not represent the full buffer necessary to take care of a battery if one's priority is maximizing the battery's useful life.

If I understand your priorities, they are to minimize your range anxiety and maximizing drive distance from each charge cycle by keeping it topped up every night and charging to 100% or so while driving. Of course, that's just fine for your priorities, but advising everyone else, especially people who are only starting to learn about the battery technology (or don't care, they just want some basic rules to follow to take care of their expensive purchase) to just charge to 100% every time is misleading at best.

There are two reasons that VW and just about everyone else experienced with the chemistry of this battery technology recommend daily charging to 80% and only occasional charging to 100%.

First, regularly limiting charging to 80% significantly reduces stress on the battery cells, thereby extending the useful life of the battery. There are nice graphs showing how a battery with this chemistry that is constantly charged to 100% simply does not maintain as much of its capacity over hundreds or thousands of charge cycles. A full charge to 100% (usable) does in fact cause tiny incremental damage to the battery that, over time, results in an accelerated reduction of overall charging capacity. You may be fine with this, perhaps because you're content being protected by the warranty or have a lease vehicle and the car isn't your problem after 4 years, so maximizing lifespan doesn't matter to you. But others might have different priorities. If an owner's priorities include maximizing the useful life of the battery high up in their list, then they should absolutely avoid charging beyond 80% on a daily basis and only occasionally charge to 100%. When is it ok to go beyond 80%? Well, whenever it's more important than long term life of the battery, but the baseline answer is whenever you're expecting to travel a longer distance than can be comfortably driven on 80% charge. So, 90 mile round trip daily work commute plus a visit to a friend in the afternoon? Start that day with 80%. But a 400 mile drive for a week and trip to another city? Then it's absolutely worth the full 100% charge to maximize distance on the first leg of the trip. What's the upper end of this recommendation? There is no clear answer, because every charge above 80% or so is more stressful on the battery so the more of them you do, the more you impact the battery's useful life (although, the "stress" is much more harmful when the battery is left at a very high percentage for extended periods of time, like multiple days, so charging overnight to 100% and immediately driving down to 80% or lower as one would do on a road trip is much less damaging than always charging to 100% and leaving it that way whenever not in use). But to me it seems that a reasonable person would try to limit stress on the battery whenever tht doesn't cause them stress about their driving plans.

The second reason experts recommend stopping charging at 80% has to do with the charging curve. Simply, the fastest charging happens between about 10% and 80%. So, if an owner wants to spend the least amount of time charging the battery on a road trip, they should absolutely stop charging as soon as the charging speed drops off, which is specifically mapped by VW to happen at around 80%. Do you need to jump up from your rest stop Big Mac meal as soon as you get the alert you've reached 80%? No, you're still charging and since you're not ready to go, you're only incrementally "wasting time". Also, since you'll be immediately pulling energy from the battery as soon as you jump in and get back on the road, there's no reason to be concerned about the battery life. But if you wait the additional 30 minutes or more to get that final 20% charged up, you're choosing to use what is essentially the same amount of time it takes to charge 70% of the battery capacity if you were doing so between 10-80% SOC. So, again, it's OK to charge to 100% if a maximum leg distance is your priority but it's not recommended because it's simply not most efficient use of time while traveling.

One small note. While it's true that following these recommendations plays to VW's advantage in terms of its warranty liabilities, I doubt they would make this their priority versus offering maximum trip leg range if the arguments I make above weren't true. The simple fact is that the warranty is meant to protect owners from serious failures in the battery technology, but unnecessarily winding up with a 10-15% reduction in overall battery capacity after 200k miles is very likely not in most long term owners' best interest, so developing good habits from the beginning of our new way of life with electric cars is just a smart thing to do.

I hope this clears up the discussion above and also helps people better understand that the choices they make are subjective and based on their own priorities and their own circumstances, rather than a hard and fast rule across the board.

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