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What about using modular battery packs in the design.

A 10 mi. range battery pack could be included in the base price of the car. Extended range battery packs could be optional add-ons.

This could enable the car to be brought to market at a cheaper cost since the battery capacity is so expensive.

This would also allow the user to upgrade at a later time with possibly newer technology battery packs that give more range (and are cheaper).

I believe the battery system is already smart, so it seems it could be designed in enough of a modular fashion so that the same control system can work with varying battery capacities and automatically adapt as they are added or changed.
 

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battery packs not only have to be designed for total kWh storage but also peak instantaneous amps at design voltage. This isn't a milliamp laptop. It would be difficult to get both the voltage and the amps to run the 160 HP motor at full power with half-a-pack of cells.
 

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Half a pack

battery packs not only have to be designed for total kWh storage but also peak instantaneous amps at design voltage. This isn't a milliamp laptop. It would be difficult to get both the voltage and the amps to run the 160 HP motor at full power with half-a-pack of cells.
I was thinking that they should be able to get away with at least half of a pack, but that it would increase overall cost if you wanted two of those half-packs because each of the two packs would need to supply the same total voltage, so each would need to have the same total number of cells (each cell having lower energy density, but good or better power density) as the one large pack.

I would also like to have the option of 20-mile battery-only range if it would lower the price by $5k, because I only have a 12-mile (round-trip) commute to work. It doesn't make much sense to me to have so much expensive battery capacity that will go unused 90% of the time. In fact, the lack of this option might tip the scales for me between an economic no-brainer and something I have to justify on environmental/moral grounds. Also, less battery capacity would mean less weight, so higher efficiency. All-in-all a perfect option for me. I'll be surprised if these options don't appear in the future. It just makes sense.
 

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This is an idea that I and many others have been advocating. I was posting on Tesla's blog about them doing this for their proposed sedan before the Volt was conceived. Their are technical and practical limitations with how short the range can be, but I believe 20 miles is definitely doable for A123 batteries. Below this has it's challenges especially since there are inherent pack costs regardless of the total capacity.

To me it appears to be a no-brainer to design the battery system with modularity in mind. This starts with the physical layout of the pack and the modules (or blades - this term seems to make sense) and pervades the cooling system and power electronic controls. I believe 20 mile increments (4KWh) make sense. As hvacman points out, there are engineering issues that need to be addressed. It's not just a matter of removing a bunch of cells from the pack. these issues don't seem insurmountable though.

The blades could have some onboard control and BIOS information. The battery pact (or system) would read the blades BIOS information and control it accordingly. The blades would need to able to output the same voltages bus the chemistries could he different from one blade to the next. Electronics onboard to each blade could control voltage variations from blade to blade to keep them outputting at the same voltage, perhaps synchronized off of the lead (first) blade.

Biggest issues to overcome might be cooling and resultant vehicle weight variation.

Benefits Include:
-Affordability with configurability. One size doesn't fit all and buyers could size the battery capacity to their needs.
-Expandability. Add more blades if driving patterns change
-Upgradability. e.g. buy 20mile capacity initially and then add 20 more later that has newer technology (lower weight, lower price,netc.). There is expected to be tremendous improvements in battery technology over the next 10 years just as there were in the last 10. It would be a shame not to be able to take advantage of them or to have to throw out your entire battery and control system to do so.
-Efficiency. No reason to lug around the city 150lbs or so of battery capacity that rarely is needed.
-Maintainability. The pack as shown by GM looks somewhat modular but each section looks different and if a module fails then all may need to be replaced if they aren't "self-balancing".
 
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