Musing on Battery Pack

[Jeffj]

Just some idle musing as I wait for my car to make landfall next week.

The battery pack is 22.4 kWh, of which 18.8 is usable, I believe. So range is always calculated (and measured, I assume) from the 18.8 kWh. Does that mean if the pack could be increased to 30 kWh somehow (future upgrade, obviously, based on cell size reducing or energy density going up, or both or something else), that the effective range would be increased by 42%? That would significantly change the equation on this car, as the EPA numbers would go from 81 (for BEV) to 115 miles, and REx would go from 72 to 102 miles.

I think the psychological acceptance of this car with > 100 mile range would be pretty large.

I can't find any detailed info on the physical size of the battery pack, nor the cells within it. From watching the construction videos, everything is packed in pretty tightly, but I wonder about other physical configurations that were considered during design. For example, what if an extra layer of cells had been laid on top of the current configuration (which appear to be upright)? That would have made the pack thicker by a 3 or 4 inches, but probably could have accommodated the extra 8kWh. Cost, weight, performance, layout, etc. all come into play with this, and I imagine the engineering gets quite intense, but hitting the 100+ mile range would have been huge.

I'm hopeful that 3 or 4 years from now there will be an upgraded pack available that could literally be a plug-compatible replacement with a larger capacity.

 

[wraithnot]

Just some idle musing as I wait for my car to make landfall next week.

The battery pack is 22.4 kWh, of which 18.8 is usable, I believe. So range is always calculated (and measured, I assume) from the 18.8 kWh. Does that mean if the pack could be increased to 30 kWh somehow (future upgrade, obviously, based on cell size reducing or energy density going up, or both or something else), that the effective range would be increased by 42%? That would significantly change the equation on this car, as the EPA numbers would go from 81 (for BEV) to 115 miles, and REx would go from 72 to 102 miles.

I think the psychological acceptance of this car with > 100 mile range would be pretty large.

I can't find any detailed info on the physical size of the battery pack, nor the cells within it. From watching the construction videos, everything is packed in pretty tightly, but I wonder about other physical configurations that were considered during design. For example, what if an extra layer of cells had been laid on top of the current configuration (which appear to be upright)? That would have made the pack thicker by a 3 or 4 inches, but probably could have accommodated the extra 8kWh. Cost, weight, performance, layout, etc. all come into play with this, and I imagine the engineering gets quite intense, but hitting the 100+ mile range would have been huge.

I'm hopeful that 3 or 4 years from now there will be an upgraded pack available that could literally be a plug-compatible replacement with a larger capacity.

The entire battery in the i3 weighs 230 kg (http://www.asymcar.com/graphics/14/i3/bmwi3b.pdf). The difference between the weight of the i3 REx and the i3 BEV is 120 kg. So you could theoretically take an i3 BEV, add a second smaller 120 kg battery pack that increases the capacity by 52%, put it where the REx normally goes, and end up with a BEV that weighs as much as the REx, but has a range of about 109 miles (1.52 times the range of the REx assuming the heavier i3 BEV would have the same efficiency as the i3 REx).

If you're willing to wait for improved battery chemistry, it looks like Samsung is shooting for a 92% improvement improvement by 2019 that would yield an i3 BEV with a range of about 156 miles and a REx with a range of about 138 miles.

"According to Samsung SDI’s technology roadmap, its cells will offer and energy density of around 130 Wh/kg until about 2019, with the advent of an advanced NCM chemistry and energy density of ~250 Wh/kg." (http://www.greencarcongress.com/2014/07/20140715-bmw.html)

Tesla already uses 250 Wh/kg cells, but those cells are less stable and require additional engineering to keep them happy. But the new Kia Soul uses 200 Wh/kg cells and if those could be used in an i3 BEV without any additional changes then you would get an i3 BEV with a range of around 125 miles.

 

[ultraturtle]

Fun question.

The i3 battery system consists of 8 x 2.7 kWh packs that BMW calls "cells" (each containing 12 x 3.75 volt, 60 Ah actual cells) wired in series for a nominal capacity of 21.8 kWh, 360 volts, of which 18.8 kWh are useable (~86%).

The high voltage, series configuration keeps amperage down so thinner, lighter cabling can be used. High voltage has its limits, however, and anything over 400 volts introduces a different set of rules that best be avoided. The i3's voltage range is 259 to 396, and therefore maxed out for a single series using 60 Ah cells. This leaves two ways to increase capacity - greater Ah cells in the existing string, or parallel strings using the same cells. Greater Ah in the same form factor will come with time (lithium Ion batteries double in energy density every 8-10 years), but that does not solve today's problem, so even a slight increase in capacity using 60 Ah cells will drive BMW to parallel strings.

It would make sense for the next i series vehicle out of the factory to be a stretched version on the basic i3 (i5 perhaps?), using as many common components as possible in order to keep costs down. I would assume that stretch not only gives more legroom in the rear seat area and a larger cargo volume, but enough room to add another row of battery "cells", upping the capacity 25% to 27.25 kWh, of which 23.5 kWh would be useable. At first glance, one might think a 25% increase in battery capacity would result in a 25% increase in range, from 81 to 101 miles, but not so fast.

Increasing battery capacity alone would add 110 pounds (2 "cells" at 55 lb each) to the vehicle, but this neglects the additional volume to accommodate the battery system, the additional cooling lines, the additional structure, the additional copper to handle the increased current imposed by parallel battery strings, and the additional weight that all of this adds.

Let's toss out the weight increase due to the added battery and support systems for now, and look at the efficiency penalty of simply stretching the car to accommodate the extra volume. The BMW 335i and 535i make for sound examples as they use the same power plant and also serve as a likely stretch for the i3. The extra 10.6 in length costs 362 lbs, or 10% of the 335i's weight. Using the rule of thumb that a given percentage increase in weight results in half that percentage increase in fuel consumption, a similar 10% increase in weight due to stretching it should result in a 5% consumption increase over the current i3.

That 25% increase in battery capacity would likely result in something less than a 19% increase in range, to something under 96 miles.

The additional weight of additional battery capacity significantly impacts efficiency, and therefore range. See http://insideevs.com/efficiency-range-cant-advanced-technology-can-help/

 

[TomMoloughney]

Fun question.

The i3 battery system consists of 8 x 2.7 kWh packs that BMW calls "cells" (each containing 12 x 3.75 volt, 60 Ah actual cells) wired in series for a nominal capacity of 21.8 kWh, 360 volts, of which 18.8 kWh are useable (~86%).

The high voltage, series configuration keeps amperage down so thinner, lighter cabling can be used. High voltage has its limits, however, and anything over 400 volts introduces a different set of rules that best be avoided. The i3's voltage range is 259 to 396, and therefore maxed out for a single series using 60 Ah cells. This leaves two ways to increase capacity - greater Ah cells in the existing string, or parallel strings using the same cells. Greater Ah in the same form factor will come with time (lithium Ion batteries double in energy density every 8-10 years), but that does not solve today's problem, so even a slight increase in capacity using 60 Ah cells will drive BMW to parallel strings.

It would make sense for the next i series vehicle out of the factory to be a stretched version on the basic i3 (i5 perhaps?), using as many common components as possible in order to keep costs down. I would assume that stretch not only gives more legroom in the rear seat area and a larger cargo volume, but enough room to add another row of battery "cells", upping the capacity 25% to 27.25 kWh, of which 23.5 kWh would be useable. At first glance, one might think a 25% increase in battery capacity would result in a 25% increase in range, from 81 to 101 miles, but not so fast.

Increasing battery capacity alone would add 110 pounds (2 "cells" at 55 lb each) to the vehicle, but this neglects the additional volume to accommodate the battery system, the additional cooling lines, the additional structure, the additional copper to handle the increased current imposed by parallel battery strings, and the additional weight that all of this adds.

Let's toss out the weight increase due to the added battery and support systems for now, and look at the efficiency penalty of simply stretching the car to accommodate the extra volume. The BMW 335i and 535i make for sound examples as they use the same power plant and also serve as a likely stretch for the i3. The extra 10.6 in length costs 362 lbs, or 10% of the 335i's weight. Using the rule of thumb that a given percentage increase in weight results in half that percentage increase in fuel consumption, a similar 10% increase in weight due to stretching it should result in a 5% consumption increase over the current i3.

That 25% increase in battery capacity would likely result in something less than a 19% increase in range, to something under 96 miles.

The additional weight of additional battery capacity significantly impacts efficiency, and therefore range. See http://insideevs.com/efficiency-range-cant-advanced-technology-can-help/

+1

 

[DAZ]

WOW, thanks for the education. More than just adding more batteries for more range. Satisfied with my i3 BEV as for now.

 

[Jeffj]

Really appreciate the detailed responses here!

 

[alohart]

The entire battery in the i3 weighs 230 kg (http://www.asymcar.com/graphics/14/i3/bmwi3b.pdf). The difference between the weight of the i3 REx and the i3 BEV is 120 kg. So you could theoretically take an i3 BEV, add a second smaller 120 kg battery pack that increases the capacity by 52%, put it where the REx normally goes, and end up with a BEV that weighs as much as the REx, but has a range of about 109 miles (1.52 times the range of the REx assuming the heavier i3 BEV would have the same efficiency as the i3 REx).

I have mused about making such a second smaller battery pack a removable REx. Dragging around an extra 120 kg of battery pack during those times when its addition range is not needed would decrease the efficiency and performance of an i3. If this REx battery pack could be engineered so that it would be owner-installable/uninstallable, that would be a very attractive option. One would need a small rollable lift like those used to install/uninstall a car engine and an easier-to-open rear access panel so that an i3 owner could open the access panel, roll her REx battery pack into the cargo area, and lower the battery pack into place.

Alas, this will probably never happen, so maybe I need to design an i3 BEV hidden storage compartment that I could install in the REx engine void.

 

[mindmachine]

The entire battery in the i3 weighs 230 kg (http://www.asymcar.com/graphics/14/i3/bmwi3b.pdf). The difference between the weight of the i3 REx and the i3 BEV is 120 kg. So you could theoretically take an i3 BEV, add a second smaller 120 kg battery pack that increases the capacity by 52%, put it where the REx normally goes, and end up with a BEV that weighs as much as the REx, but has a range of about 109 miles (1.52 times the range of the REx assuming the heavier i3 BEV would have the same efficiency as the i3 REx).

I have mused about making such a second smaller battery pack a removable REx. Dragging around an extra 120 kg of battery pack during those times when its addition range is not needed would decrease the efficiency and performance of an i3. If this REx battery pack could be engineered so that it would be owner-installable/uninstallable, that would be a very attractive option. One would need a small rollable lift like those used to install/uninstall a car engine and an easier-to-open rear access panel so that an i3 owner could open the access panel, roll her REx battery pack into the cargo area, and lower the battery pack into place.

Alas, this will probably never happen, so maybe I need to design an i3 BEV hidden storage compartment that I could install in the REx engine void.

Think about it rationally/really do you see something like that happening, NOT! The average consumer that would buy an EV even isn't going to want to manipulate around in and out of the car a 200 lb plus battery pack. i mean really?

 

[Jeffj]

The most exciting thing I saw by following some of the links provided was that the Samsung batteries are projected to have large energy density increase in a few years. If that holds, then its certainly possible that BMW will have a battery upgrade available in the same form factor.

Lots of change projected to be happening in 2017-2019.

 

[jadnashuanh]

The BEVs that have a heat pump have equipment in that REx 'void' space people think is all open...it is not, at least not all of it. So, trying to sandwich in more batteries or more storage probably is never going to happen, at least with the current design. Just like adding in the weight from the REx decreases max all battery range, carrying around anything in the vehicle will do the same time. BMW spent years and lots of engineering time trying to keep the car as light and as functional as they could, and probably thousands of cost/benefit decisions had to be made (and cost is twofold: both in function and probably purchase price).

 

[alohart]

The average consumer that would buy an EV even isn't going to want to manipulate around in and out of the car a 200 lb plus battery pack.

If it were engineered so that I could roll a REx battery pack to the back of the car on a small BMW-branded hoist, lower it into place with the hoist, and secure and connect it, I'd go for it. But then I'm probably not the average BMW owner never having owned a BMW or any other car that cost nearly as much as our i3. But I agree that this will never be an option because there'd be several potentially difficult problems to solve other than its weight.

 

[alohart]

The BEVs that have a heat pump have equipment in that REx 'void' space people think is all open...it is not, at least not all of it.

The photos I've seen must not include the heat pump. I'll have to take out my Torx driver, open the rear access panel, and take a look. If nothing else, I'll probably be impressed by all the nice cast aluminum pieces and electronics that might be visible through that access panel.

 

[jadnashuanh]

If you do open it up, post a picture. I've not physically seen it, and just relaying what I'd heard.

 

[wraithnot]

The entire battery in the i3 weighs 230 kg (http://www.asymcar.com/graphics/14/i3/bmwi3b.pdf). The difference between the weight of the i3 REx and the i3 BEV is 120 kg. So you could theoretically take an i3 BEV, add a second smaller 120 kg battery pack that increases the capacity by 52%, put it where the REx normally goes, and end up with a BEV that weighs as much as the REx, but has a range of about 109 miles (1.52 times the range of the REx assuming the heavier i3 BEV would have the same efficiency as the i3 REx).

I have mused about making such a second smaller battery pack a removable REx. Dragging around an extra 120 kg of battery pack during those times when its addition range is not needed would decrease the efficiency and performance of an i3. If this REx battery pack could be engineered so that it would be owner-installable/uninstallable, that would be a very attractive option. One would need a small rollable lift like those used to install/uninstall a car engine and an easier-to-open rear access panel so that an i3 owner could open the access panel, roll her REx battery pack into the cargo area, and lower the battery pack into place.

Alas, this will probably never happen, so maybe I need to design an i3 BEV hidden storage compartment that I could install in the REx engine void.

I don't think most people would want to try and move a 120 Kg range extending battery. But I am aware of one kind of electric vehicle that has a user removable battery. It's an electric scooter that has a removable 1.6 kWh battery that weighs 25 pounds and is designed to be recharged indoors so you don't have to worry about parking your scooter next to an outlet: http://www.wired.com/2014/11/super-practical-electric-scooter-removable-battery-charge-indoors/

Each of these batteries would give an i3 BEV an extra 5 or 6 miles of range. You could have a hatch somewhere that let you add a bank of these batteries before a trip- 10 of these would make a measurable difference to the range. I'm not a battery engineer, but I imagine simply wiring this in parallel with the main traction batteries would cause all sorts of issues so you would probably want to use this more like a range extender to recharge the main battery while driving. Trying to quick charge this bank of removable batteries also seems problematic. But since we're musing here, I imaging if you standardized these, you could simply swap the depleted ones for recharged ones on the road. I can swap my empty soda stream CO2 cartridges for full one at a bunch of different stores so I can't see why this wouldn't work for 25 pound rechargeable batteries.

 

[i3atl]

The BEVs that have a heat pump have equipment in that REx 'void' space people think is all open...it is not, at least not all of it.

If you do open it up, post a picture. I've not physically seen it, and just relaying what I'd heard.

Then why state it as fact? http://bmwi3owner.com/2014/02/heat-pump-in-front/

On a BEV, there is a significant amount of empty space where the REx would be, as well as in front of the rear bumper (where the exhaust system for the REx is tucked in). Can't find any good pics of the space in front of the bumper, but this is a good illustration to give you an idea: http://mybrandgroup.co.za/blog/wp-content/uploads/2013/08/ku-xlarge.png