Using Storage Mode every day

[smorgasbord]

The graph shows the deeper the discharge, the lower the number of cycles.

Isn't the graph predicated upon fully charging the battery and then discharging to the the specified DOD? In other words, charging to 100%, then driving to 90% versus charging to 100% then driving to 80%, etc. If so, that's not what happens when someone charges to 50% and drives to 40%.

That graph is all over the web, but I can't find where it originated. I found this informative link, though. It has that same graph and says about it:

The above graph was constructed for a Lead acid battery, but with different scaling factors, it is typical for all cell chemistries including Lithium-ion. This is because battery life depends on the total energy throughput that the active chemicals can tolerate.

They compare some examples from the graph:

the total energy throughput is fixed so that one cycle of 100% DOD is roughly equivalent to 2 cycles at 50% DOD and 10 cycles at 10% DOD and 100 cycles at 1% DOD.

Furthermore, at this link, they say:

A more representative measure of battery life is the Lifetime Energy Throughput. This is the total amount of energy in Watthours which can be put into and taken out of a battery over all the cycles in its lifetime before its capacity reduces to 80% of its initial capacity when new. It depends on the cell chemistry and the operating conditions. Unfortunately this measure is not yet in common use by cell manufacturers and has not yet been adopted as a battery industry standard. Until it comes into general use it will not be possible to use it to compare the performance of cells from different manufacturers in this way but, when available, at least it provides a more useful guide to applications engineers for estimating the useful life of batteries used in their designs.

If that's true, then drawing from 50% to 40% isn't worse than drawing from 80% to 70%, since both draw the same Watthours.

But, then again, we know this isn't the only factor. Charging level is also important. There this graph:

It show that reducing the charging level (done by reducing the charging cutoff voltage) extends the number of charging cycles that can be obtained. Tesla talks about this on their blog. I think that's what people are hoping to leverage by not fully charging the battery (or not even charging to Tesla's Standard Mode 87% charge).

However, we know that pulling lots of power from a battery with a low charge is also bad for the battery. I believe that's one reason Performance Mode charge charges the battery to a higher level (another is simply to help the range out). So, if you're doing the Storage charge all the time thing, you might want to drive in Max Range mode to avoid over taxing a low charged battery.

I agree with Scott451 that we should do what Tesla recommends and not try to game the system leveraging factors we don't fully understand and have not tested. There are interactions that aren't covered by published theory and I believe one is more likely to make things worse than better.

 

[S-2000 Roadster]

However, we know that pulling lots of power from a battery with a low charge is also bad for the battery. I believe that's one reason Performance Mode charge charges the battery to a higher level (another is simply to help the range out). So, if you're doing the Storage charge all the time thing, you might want to drive in Max Range mode to avoid over taxing a low charged battery.

This is exactly what I anticipated as a concern. Since the Roadster continues to drive in Storage Mode after you unplug, it would seem that the firmware should operate in Max Range mode at that time. Considering how awkward the Storage Mode is, though, I wouldn't depend on Tesla having programmed for this usage scenario.

I agree with Scott451 that we should do what Tesla recommends and not try to game the system leveraging factors we don't fully understand and have not tested. There are interactions that aren't covered by published theory and I believe one is more likely to make things worse than better.

That's basically always the advice that I follow: Do what the OEM recommends.
However, if I'm going to push the limits of anything, abusing Storage Mode is probably where I would be.

 

[scott451]

I remember reading that Li-ion batteries have a certain number of charging cycles, regardless of whether you count a cycle as a single 100% discharge and 100% recharge or as two 50% discharge and 50% recharge sessions. At first I thought that this graph might disprove that tidbit of Li-ion folklore, but then I realized that it all depends upon how you define "number of cycles." Does that graph count a 50% charge as a full cycle? Does it count a 10% recharge as a full cycle? I would expect the graph to be decreasing, as shown.

Yes, 10% discharge and then 10% recharge is counted as one cycle, BUT the total energy out of the battery is more with the shallower discharges. From How to Prolong Lithium-based Batteries - Battery University :

if 100% discharge gives 500 cycles then we would expect a 50% discharge to give 1000 cycles, but it doesn't, it give us 1500 cycles, 25% more lifetime energy out of the battery.

In other words, do you have a link to the study that produced that chart?

I thought it came from here: How to Prolong Lithium-based Batteries - Battery University But I can't seem to find the page it was on. The description on the site seems to me to be consistant with LiIon DOD curves in the graph.

...drawing from 50% to 40% isn't worse than drawing from 80% to 70%, since both draw the same Watthours.

I don't think you can draw that conclusion. Here's why: For a given LiIon battery, the battery with the lower SOC, has a lower voltage and a higher internal resistance. So for the same power output, the lower SOC battery will need to supply more current than the 80% soc battery. The higher current combined with the higher internal resistance, means that the more heat is generated in the battery. You can see this on your on roadster. Notice that the battery heats up much faster at lower states of charge.

 

[S-2000 Roadster]

Thanks for the link!

if 100% discharge gives 500 cycles then we would expect a 50% discharge to give 1000 cycles, but it doesn't, it give us 1500 cycles, 25% more lifetime energy out of the battery.

I've been working on adding another dimension to my graph that shows the percent increase in cycles based on size of discharge. The original graph (and my comparison graphs) are difficult to interpret because the 1/x aspect of the curve dominates. I think it will be much more revealing if that aspect is factored out. I'll post a graphic if I get around to making one.

For a given LiIon battery, the battery with the lower SOC, has a lower voltage and a higher internal resistance. So for the same power output, the lower SOC battery will need to supply more current than the 80% soc battery. The higher current combined with the higher internal resistance, means that the more heat is generated in the battery. You can see this on your on roadster. Notice that the battery heats up much faster at lower states of charge.

Thanks again. This is certainly something that I can verify by looking at the VehicleLogs.

In any event, my weekday experiment with Storage Mode is over. I want to take a nice, long drive tomorrow in this nice weather, and I'm going to need everything that Standard Mode offers, perhaps even Range Mode.