Emergency fast charging for evacuations?

[OldEVDude]

I'm just wondering about emergency evacuations (hurricanes, etc.). Normally, charging slows down as the battery SOC increases. Is this because of temperature issues or is it to protect the battery?

The reason I'm asking is that in case of emergency evacuations could Tesla do an emergency update to allow for faster charging throughout the SOC as a one-off? My thinking is that a one time fast charge shouldn't hurt the battery and would relieve crowding at supercharger stations. I recall that in the past Tesla has pushed out updates that would increase the range by about 10% for evacuation.

Of course, if limiting charge rate is due to temperature issues than this would be a moot point.

 

[RTPEV]

I think most people are under the impression that the battery management system (BMS) actively throttles the charging process to "protect" the battery and that is the reason for the tapering at the top end of the charge.

This is not actually what is happening. Rather, the charge process goes into what's called constant voltage (CV) mode where the charging station holds the charger voltage at a set voltage and current flows into the battery in an Ohm's Law like fashion based on the difference between the charger voltage and the pack voltage. I.e. the battery takes as much charge as it's going to take, and as the voltage difference gets lower (as the pack fills up), less and less current is going to flow into the battery, resulting in less and less power. It really comes down to the physics of the battery.

Of course there are steps that could be taken by the BMS to potentially speed up charging. First, it could switch from constant current (CC) to CV at a later time, possibly tapering the amount of current being called for. Hyundai/Kia and I think Porsche vehicles do this to stay in CC mode longer. And I suppose that it could call for a higher CV voltage, possibly tapering down the voltage, but this is risky because (a) the way the BMS determines that the battery is "full" is when the amount of current flowing into the battery at a specific CV drops below a certain amount, so adjusting the CV voltage would throw this off and you risk overcharging the battery and (b) since you are holding the voltage at a certain level, you wouldn't be able to get a good read on the pack SOC until the point the current drops...you would have to rely on dropping the CV voltage when you reached intermediate current events.

Plus, with all of that, I doubt it's going to make a huge difference in time anyway, and during an evacuation event, with full Superchargers, the limiting factor is going to be the power coming into the site anyway. Increasing the amount of power vehicles are getting in the last 20% of the charge isn't really going to happen because the whole site is going to be underpowered anyway, so the station may not have more to give.

So what should we do during evacuations?

Well, first off, preparedness on the part of people will go a long way (at least for predictable evacuations, such as hurricanes). We know about potential hurricanes days in advance. There is no reason that people should not have their cars already at a high SOC in advance of the storm. This should generally get people 250 miles or so away from the area of concern, which is mostly good enough to get away from areas with widespread power outages, or at least to safety. And the good news is that electricity is very easy to "transport" as compared to gas. Lines at gas stations and completely running out of supply are typical in advance of big storms (and after, by the way!), but electricity can be imported from very far away. Yes, the grid will see a temporary strain in the lead up, but it should be able to be handled.

The one possible exception is with certain hurricanes such as Hurricane Irma that came straight up Florida. For residents in southern Florida, they would have to travel the entire length of Florida to escape, which is well beyond the range of the car. Okay, so here we have a real problem.

In that event, what I see Tesla (and hopefully other CPOs) doing is rolling out their mobile charging stations. Again, these types of events are quite predictable and hopefully they see the need to have these things charged and ready to deploy on trucks to affected areas. The rest of the time they can be used to relieve congestion on busy corridors at holiday times. Eventually this will be a must. Hopefully they see the need and prepare for this and not just create one or two flatbed demonstration vehicles that never get mass produced.

A less predictable need for evacuations is wildfires. Residents may not have enough advance warning the prepare (at least at the start of the wildfire), but the distance needed to travel during these kinds of evacuations is much less, and the resultant power outages are not so widespread. Yes, it may result in huge lines at charging stations surrounding the affected area.

 

[Sophias_dad]

No, Tesla would never do that.

A) an emergency update isn't necessarily fast at all. Could easily take many hours or a week to get.
B) It slows because the battery shouldn't be charged faster than that. Forcing more charge into it may cause catastrophic failure, and instead of the car taking 10 minutes less to charge, it bursts into flames and shuts down the entire site for hours or days. Even if it doesn't burst into flames it would likely reduce the longevity of the battery which will quickly become Tesla's problem.

To reduce crowding in case of evacuation, Tesla could probably set a hard cutoff at 75% charge(or whatever) from the Supercharger side and thereby force people to move, presumably fanning out across the country to less crowded superchargers.

Unless the emergency evacuation is a total surprise, all Tesla's should be able to drive 200 miles at a minimum, and I'd hope that would get them out of trouble. If that's not enough, 15 minutes at a supercharger is good for another 100-150 or something.

 

[dafish]

@RTPEV

excellent post. Props.

 

[Earl]

I think you're getting lost in the details and missing the forest for the trees. The reason really is about protecting the battery. The only place you're wrong is:

I think most people are under the impression that the battery management system (BMS) actively throttles the charging process to "protect" the battery and that is the reason for the tapering at the top end of the charge.

This is not actually what is happening.

The rest is correct but the reason for the CC and CV, when they switch, and their current, voltage, and temperature limits are all to protect the battery.

I totally concur that a full Tesla range is probably good enough to escape from a disaster area, at least to a place with less damage. Nearly any full Tesla can go 300 miles at a slow pace (like an evacuation jam). South Florida may be an exception. If you're concerned, bring your generator with you but remember that gasoline is very dangerous stuff.

 

[Big Earl]

Evacuations usually take place in advance of a storm like a hurricane. The grid doesn't usually have problems until a storm hits - hopefully after evacuations have been completed. Even if you could speed up the charging, a full site will be limited by the AC input from its utility transformer, which is around 80-90 kW per stall for most V3 sites. I can't think of any scenarios where faster charging is going to accomplish anything in a real-world evacuation scenario.

Evacuations in fossil-fueled cars are problematic because the traffic rush depletes gas station inventories, which rely on the trucking supply chain to replenish. This supply chain is susceptible to interruption due to the locations of fuel distribution hubs, the evacuations themselves and traffic.

In my opinion, you're better off in a Tesla.

 

[RTPEV]

I think you're getting lost in the details and missing the forest for the trees. The reason really is about protecting the battery. The only place you're wrong is:


The rest is correct but the reason for the CC and CV, when they switch, and their current, voltage, and temperature limits are all to protect the battery.

I totally concur that a full Tesla range is probably good enough to escape from a disaster area, at least to a place with less damage. Nearly any full Tesla can go 300 miles at a slow pace (like an evacuation jam). South Florida may be an exception. If you're concerned, bring your generator with you but remember that gasoline is very dangerous stuff.

Let me highlight the key word in what I said that I think you missed:

I think most people are under the impression that the battery management system (BMS) actively throttles the charging process to "protect" the battery and that is the reason for the tapering at the top end of the charge.

Of course the point at which the switchover from CC to CV and the associated current and voltages at which those phases operate are chosen to optimize several different properties of the battery (charge speed, longevity, etc.), but the point is they are not actively adjusted based on environmental conditions (although as I pointed out, some manufacturers are actually exploring or even doing this in production).