[manmal]

LFP batteries can do 3k-10k cycles. Assuming a range of 500km per cycle out of an 100 kWh pack, that's 1.5 million km of total range (assuming 3k cycles). Only a small number of people will ever get that much out of a car - maybe 40% that, if maintained very very well. But we're talking Prius/Mercedes level endurance here, most cars won't make it that long. So in most cases, 70+% of the usable cycles in an LFP pack won't be done by the EV it was first installed in. Knowing that, selling cycles is actually a great way to offset purchase costs.

[throwaway894345]

Your range figures seem pretty crazy high. Pretty sure most EV batteries are smaller than 100k--my Model Y is only 75k for instance, and I'm not getting anywhere close to 500km per cycle because (1) my Model Y doesn't actually get 500km on a full charge and more importantly (2) like most EV owners, I don't charge to 100% and run my battery down to 0% but rather something closer to 80%/20%. So for my Model Y scenario, it's closer to half of your total range figure.

Also, I'm not sure what components would wear out in an EV before the battery such that it would total the car out. If the EV batteries in totaled cars can be repurposed for grid use, then great, but I would expect them to get recycled and put back into cars unless strapping them to the grid is cheaper than recycling (seems maybe plausible?).

[coryrc]

When speaking of battery cycles and lifetime, people almost always mean "full cycle equivalent". So when you say "I only go 80%->20%" then that would be considered 0.6 cycles counted against the lifetime.

[manmal]

I don't, LFP batteries are quite resilient. 80% -> 20% is a usability nightmare nobody with a $50k vehicle should accept. My Ioniq (first gen) doesn't even have an LFP battery and is optimized for 100% -> 13%, maybe even 100% -> 10% (turtle mode starts at 5%). Still at 100% SOH after three years.

Way more important for battery life is the charging rate (again, LFP are more resilient there too). Which wouldn't be an issue for grid use.

[coryrc]

I think you do. If you drove 10 miles and recharged, you wouldn't consider that a "cycle". You could do that 100k times (probably more), but no one would say the battery cycle life is 100k.

[manmal]

I mean I didn’t count in 0.6 cycles per discharge. My battery does 0.9-0.95 cycles usually.

[throwaway894345]

Is that how it works? Can I just charge every night, even if I've only driven a few miles, with no more wear on my battery than if I did full cycle charges? My vague impression is "no", but I really have no idea.

[coryrc]

It is, with lithium, basically no different to go from 80%->20%->80% once or from 80%->75%->80% twelve times.

The only complicating issue is calendar life of the battery when it is above 80%. Over time a battery loses capacity just because it exists, which I'm calling calendar life. The closer your battery is to 100% the calendar life decreases at a roughly quadratic curve (a battery at 80% has about 1/8 the calendar life loss of one at 100%). AKA you lose 8x more capacity per year at 100% than 80%. Temperature has a similar effect above room temperature or so.

So if your frequent charges keep the battery above 80%, that would reduce calendar life (increase capacity loss per year) on its own. LFP has far greater calendar life than lipo, but also cycle life too, so I think it's just as important to keep your EV at 80% or below, whenever convenient, regardless of chemistry, unless your usage will cause cycle life to end the battery usefulness before calendar life is significant; i.e. multiple full cycles per day. But also at multiple full cycles per day you probably won't spend much time above 80% even when charging to 100%.

Ah, to summarize, I'll repeat my simple advice: I think it's important to keep your EV at 80% or below, whenever convenient.

[Koemul]

If you keep the charge between 20 - 80%, your battery will last for far more cycles than when charging 0 - 100%. So you'll only get 60% of the kWh per cycle, but the battery will do 3-6 times as many cycles.

In fact, the battery management system won't even let you fully charge or discharge the battery for exactly this reason. When it shows 100%, there will still be 1-2 kWh empty and the same for a zero percent charge.

For example, a Toyota Yaris use a tiny (0.7 kWh) lithium ion battery and it gets charged/discharged constantly while breaking/accelerating, but it still last a long time because the charge is kept at about 50%.

[manmal]

There are 100kWh EVs out there, and they will quickly increase in number. For them, 500km is a rather conservative estimate.

> I would expect them to get recycled and put back into cars

In that case, you'd also get something in return for not having used those cycles. It's a matter of choice then - do I rent out my battery during use, or do I sell it after 8 years.

[throwaway894345]

I don't doubt they exist, I doubt that they're common enough to treat as the general case.

> In that case, you'd also get something in return for not having used those cycles. It's a matter of choice then - do I rent out my battery during use, or do I sell it after 8 years.

Yeah, but which is more economical is the salient question.

[manmal]

I agree, that’s an interesting question. You‘d need to factor in a fair bit of uncertainty due to inflation, future battery prices etc