A better battery swap - just 1/4 of the battery?

A better battery swap - just 1/4 of the battery?

There's a lot of talk about battery swaps and why bother when a supercharge only takes 30 minutes or quicker. There's also talk of Metal-Air batteries with slow charge times but extended range.

If you put that all together, it makes for an interesting option.

Lots of assumptions here… but… What if the car comes with 3/4 of a LiON battery pack, and 1/4 metal-air battery. The metal-air might double the car's range, but rarely get used - and when it is used an overnight charge would top it up, just not a supercharger. A battery swap then would purely focus on replacing a smaller metal-air battery. The car might even be smart enough to book the battery swap ahead of arrival, and with that knowledge deplete the metal-air battery (charge the LiON battery) before arriving at the swap station, so that the car gets a quicker supercharge of the LiON at the same time as the metal-air is being swapped over.

The smaller size could also mean the same swap batteries for Model S and for Model E cars. It also allows for new battery configurations - the smallest configuration could be a 40Kwh LiON battery and a placeholder for a metal-air battery (of the same weight), so when you plan a long trip you swap in a metal-air battery when you leave etc.

Would that make more sense?

Timo | February 24, 2014

IMO battery swap doesn't make sense. Period.

Why bother putting any effort in already obsolete concept?

holidayday | February 24, 2014

"Why bother putting any effort in already obsolete concept?"

Tax Credits.

You know, until charging happens fast enough that *that* earns extra tax credits.

jkn | February 24, 2014


Theoretically Li-air battery can have up to 11 kWh/kg. That is close to gasoline. 85 kWh Li-air battery would weight 8 kg. Even if only 1/4 of that is achievable in practice, battery swap would be possible without robots (two 16 kg 43 kWh modules).

It is probably not reasonable to reduce size of Li-ion battery, because Li-air has poor power density.

Even without Li-air battery swap might become profitable. I wrote about it in thread battery-swap-numbers started by SamoSam. Now Google finds it (The business case for Tesla has been contested., but it is not in this forum any more! I even found page 2 from Googles cache.

Anyway I can repost what I wrote:

Largest obstacle for battery swap is superchargers. I don't believe supercharging speed to increase much. Even for taxis current supercharging is fast enough. Taxi company needs to install supercharger so that drivers can charge while waiting next customer or having a break. I believe supercharging electronics is not terrible expensive. Costs come form connection to network and place.

If a swap station has stock of 10 batteries, it must have 10-stall Supercharger electronics to charge those. Such swap station might handle 100 swaps in day.
15 $ / swap -> 1500 $ / day -> 500000 $ / year

For this car batteries must be leased. If not, then each car must have it original battery back and stock of 100 batteries is needed. Assuming cars return in 24 h.

With leased battery car would be cheaper and number of potential buyers would be higher. Of course leasing cost would be rather high. Perhaps:

Battery lease 200 $ / month
+ 0.33 $ / kWh charged to battery, more for first and last 10 %,
more if supercharger is used.

Leasing income for Tesla:
Assuming 200000 miles driven in 8 years with 300 Wh/mile
8*12*200 $ + 200000*0.3*0.33 $ =
19200 $ + 19800.0 $ = 39000 $

For a car costs would be:
cheaper price -30000 $
monthly 200 $ lease
0.33 $ / kWh = 28 $ full charge (85 kWh)
15 $ / swap
Supercharger fee? (Supercharging is free for S & X, so new name needed.)
If in swap station driver accepts 80, 75, 70,... kWh battery, costs will be lower. This way batteries can be used really long time.

I don't know costs for Tesla. So these numbers might need some adjusting.

Timo | February 25, 2014

@jkn; "Largest obstacle for battery swap is superchargers. I don't believe supercharging speed to increase much"

That's actually point of my message. If you *can* increase charging time to close to practical battery swap speed (starting from decision to drive to battery swap station to resume normal operation) with less hassle with battery inventory etc., then whole concept of battery swap is obsolete before it was born.

Timo | February 25, 2014

"increase charging speed" or "decrease charging time". Obviously.

grega | February 25, 2014

The premise of what I was saying though was that the Li-Air batteries are supposed to have 5-15 times the storage capacity, but not be good at recharging (or slow?). This provides the potential of having a Li-ON battery for common usage - instead of 85kWh it might be 60kWh - with a "loadable" battery for longer distance.

There are actually some similarities to the 'extended range' electric car concept, with a secondary battery to provide extended range, swap out, and perhaps overnight charging. If the Li-Air batteries are much lighter than Li-On then the potential increases for easier for swap outs of just that battery component.

If they hold 10 times the charge - then you split the volume of an 85kWh battery into a 64kWh LiOn plus a 210kWh Li-Air battery (that can be swapped out). An >250kWh battery where only 1/4 can be supercharged… makes the battery swap concept attractive and valuable. Even on a road trip, that's a lot of range.

Lots of unknowns there, just wondering if that's a potential future.

Brian H | February 25, 2014

Many look on the system as a back-door way to upgrade. Swap from a 60 to an 85, and keep the battery, paying the difference.

jkn | February 25, 2014


Yes but I don't believe charging time will be reduced much. More energy into battery is more important and you cannot have both. Now owners have allowed superchargers hoping to get customers. If charging time is reduced to minutes, that will fail.

Brian H | February 25, 2014

Maybe they get a cut of the rental fee?

grega | February 25, 2014

Do you think having a 'back door way to upgrade' from 60 to 85 (or beyond) is actually a bad thing? Assuming it's paid for in some way of course (purchase, rental, whatever).

Brian H | February 26, 2014

The system Elon specified at the reveal was a bill to the swapper for the assessed difference in capacity and remaining lifespan. If you don't like the deal, return the battery for your own.

Timo | February 26, 2014

@jkn, It makes much more sense than battery swap. Same space, nearly same equipment, no battery inventory hassle and less maintenance.

Battery swap would only make sense if you do it by hand, in home, slowly and you own the battery swapped in and out. Which doesn't really make sense. So it doesn't make sense even in situation where it should make sense.

jkn | March 2, 2014


For most people supercharge makes more sense than swap, even with current supercharge speed. But there are people who want swap. So it might make sense when there are 10* or 100* more Teslas on the road. It is worth of studying and testing.

Perhaps Tesla will some year make 120 kWh battery. It could accept 180 kW charging with total charging time same as 85 kWh with 120 kW now. Realistic range for 85 kWh is about 400 km. For 120 kWh it would be 600 km. This would reduce charging time for long distance traveler, because of less charging stops.

Alternative would be perhaps 85 kWh with shorter charging time, but in my opinion this is not right way to go.

grega | March 2, 2014

I agree that as it stands for most people a supercharge makes a lot more sense than a battery swap. Essentially they are being asked "would you like to wait 5 minutes and spend $80, or wait 30 minutes. (And when you swap the battery back, is that another $80?).

The premise of my question was Li Air batteries hybrid batteries - way too many assumptions I'm making of course, but the basics of a smaller (1/4) size swap, lighter battery technology, potentially common battery with Model E, that gives you an extra 600 miles, that can't fast charge - that seems to have far greater benefits to a swap. The extra range makes sense whether the Air battery is combined with the rest of the battery, or a separate pack - but the separate pack would seem easier to swap.

The unasked question is "what makes a battery swap worthwhile".

* If someone needs a maximum charge then the supercharge takes longer.
* Swapping/trading up to a 100kWh battery (or whatever) for a road trip has extra benefits.
* If you have a 2nd person, don't need a break, and your combined time is worth paying for a quick swap.
* To preserve low miles on your own battery while doing a LONG road trip.

Brian H | March 2, 2014

Your convenience is the company's hassle with zinc-air. Until reprocessed (fresh powdered aluminum refill) they are dead once used up, and must be physically transported to a processor. Expensive.

Timo | March 2, 2014

@jkn; But there are people who want swap

No, there are people that want full battery faster than current SC can. If you can make battery full in approx same time with charging with nearly same equipment and less hassle to company, then this swap-business goes away. That's why it is obsolete before it was born.

Like I said 6+C makes 10minute charge possible. Even if that's not possible now it will be soon and that would kill the battery swap business. You don't want to put any effort in business that's out of business in 5-10 years timeframe, pretty much before you have done any kind of network to support the stuff.

grega | March 2, 2014

Brian, I only meant in reference to the current research into rechargeable metal-air batteries. Sodium/Zinc/Lithium-Air... whatever the future brings.

I wasn't very clear. And as I understand more the number of years needed before metal-air works effectively/properly.... I understand more the futility of my question.

jkn | March 3, 2014


6C is not impossible, but I don't believe it can be done without sacrificing range. I don't know how much, but I believe difference is significant. Longer range is more important, so 6C charge is not going to happen. 6C * 85 kWh = 510 kW. That requires heavy cables. If something goes wrong, fire could start very quickly, something might even explode. A stick of dynamite (0.19 kg) contains roughly 1 MJ of energy. 510 kW is one stick of dynamite every two 2 seconds. (Gasoline contains much more energy than battery and releases it faster.)

120 kW could charge 85 kWh in 43 minutes. Rate is lowered to protect battery. 6C would have same problem.


Metal air battery might have so high energy density that swap can be done manually. Reducing size of lithium ion battery to 3/4 also reduces max power and max acceleration. Of course cells can be redesigned for higher power density. Metal air battery might be chargable in factory only. In that case Li-ion battery should not be reduced.

Many unknowns. Nothing should be rejected now.

Timo | March 3, 2014

I bet those cables would still be lighter than whole battery in battery swap station. That's my suggestion, use robotics just like swapping battery, but instead of swapping just connect very heavy duty cables.

About that 6C being compromise between energy and power I don't believe it will stay that way. I have seen some really insane C-rates in lab results (like over 10kC).

Batteries are not that heavy anymore that just by "sacrificing range" /volume or /weight is not out of question. 85kWh is just 350kg in pure batteries (3.1Ah Panasonic standard battery). Tech is starting to be good enough that cost is only factor that really matters.

Brian H | March 3, 2014

The internal cables (inside the car) must be equally heavy. Aside from the weight increase, serious internal structural redesign is necessary, for safety if nothing else.

jkn | March 3, 2014


C rating without energy density is useless number.

I personally would choose more kWh instead of faster supercharging, assuming I would live in area with superchargers. I would also choose less heavy car instead of faster supercharging. I need a break after driving 400 km. Having two drivers in the car won't help.

Timo | March 3, 2014

That C-rating was just general improvement over currently used techs in some battery type. Can't remember which type though (other than Li-ion).

Would you choose battery swap instead of faster charging, that's the question, not how much kWh the battery has. Not really anymore, and even less in near future.

jkn | March 4, 2014

Perhaps an example might explain why C rating without energy density is pure marketing nonsense designed to fool ignorant investors: I'll by 85 kWh battery back, relabel it as 40 kWh back and modify electronics so that only 10 - 50 kWh of original battery capacity is available. Of course it still can be charged with 120 kW. So it is 3C battery back. Amazing new tech!

I would choose longer range or lighter car instead of faster charging. Battery swap I would use only rarely. I would, if I'm late from some important meeting and battery swap is only way to there in time.

Important question is: How much energy density is reduced by faster charging capacity? Since this interest you please find values of C rating with energy density.

After that comes safety. Even 120 kW is large amount of power. If something goes wrong, safety system must cut power before things catch fire or explode. You want 500 kW into same size battery. Then safety system must be faster. Risks increase.

Timo | March 4, 2014

No, that would not be a C-rating. C-rate would still be ~1.5. C-rate is unit used in battery science, not marketing nonsense. Saying that it is higher than it is would be lying and paper would not pass peer review (I hope). That was ten thousand C, I'm pretty sure that no matter how low the original specific energy or high c-rating the original battery had that was quite an improvement.

500kW is not that much higher than what car already has (it draws 300+kW from battery when accelerating). Risk increases, true, but not much (basically from insignificant to still insignificant). One reason why I would like to use robotics would be ability to use two cables at once, one for positive and one for negative, and remove human error factor from equation.

jkn | March 5, 2014

I hope scientific paper would not pass peer review without critical information.
My example is misleading. It does not lie, because specific energy is not mentioned. If I have developed my own cell, peers cannot know what its specific energy is unless I tell. In peer review my paper should be rejected because specific energy is not given.

Using robot to connect power cables protects humans. But does robot always notice broken or dirty hardware? Tesla increases charging power carefully. 90 kW, 120 kw, 135 kw, 150 kw? Small steps. They don't want to take risks.

Most charging is done at home, superchargers are used only when needed. Home charging takes perhaps 30 s human time. Supercharging 30 min. (Assuming no need to service driver.) If 90 % of chargings are done at home, average charging time is 3 min. About refueling time for an ICE car. If driving time to and from fuel pump is included, average charging takes less time than refueling.

I found faster solution for you:

Brian H | March 5, 2014

The form factor is necessarily large and blocky. Not compatible with Tesla's "skateboard", IMO.

Timo | March 5, 2014

Using robot to connect power cables protects humans. But does robot always notice broken or dirty hardware?

Just as well as in swapping batteries. If the connection isn't proper then don't release power. Communication between charger and car.