Powerpack II - The Sequel

By Elliott Reitz

In pursuit of the perfect electric-drive power system, preferably in a neat little package.


Originally Published on www.evworld.com: September 04, 2008


As stated in my previous article, the deployment of electric cars can be hastened by the standardization of their energy sources into easily swappable powerpacks.  This article examines a few powerpack types that could be build today and their resulting characteristics.


PHOTO CAPTION: Renault Kangoo Compact Concept car is a follow-on to the popular Kangoo, versions of which included an all-electric model and one of the first modern 'range-extended' electric vehicles developed around a small, integrated motor-engine generator package.


The powerpacks can contain batteries, motors, fuel cells, or according to the driver’s needs at that specific time. Some of the responses point out the challenges of standardizing the battery form, voltage, and other parameters. These are far from the ideally packaged perfect solutions but do highlight the benefits of having a mixture of the various types available to the same vehicle. And as any technology advances, the swappable powerpacks allow car owners to choose the technology that meets their needs, including affordability and range.

Each Powerpack technology has strengths and weaknesses. To explore this consider a common footprint in feet of 2 High x 2 Wide x 1 Deep that was proposed in the first article. With that set of dimensions and an associated design for mounting and connecting (clamping in), we can simply consider the resulting powerpack characteristics based upon internal makeup from available components. Thus let’s consider:

  1. Lead Acid (LA) – 2 examples considered

a. 4 marine deep-cycle 12V battery (similar to a car battery)

b. 12 Wheel chair batteries (UB12350s)

  1. NiMH D cells – available from ebay
  2. Lithium-Ion D cells – available from ebay
  3. Alkaline D cells
  4. A portable home generator

How do they compare?

A comparison is clearly subjective and depends on the market force assumptions used to form the selection criteria. Obviously a break-through could change the selection significantly but we shouldn’t hold our breath because the same solution (powerpacks) can also enable these break-through technologies to become available and will not hamper them.

First assume the EVs have mounting provisions for auxiliary powerpacks that are 1x2x2 feet. With the powerpack assumption, a shorter range becomes acceptable for most EVs because of the option to plug in a powerpack for range extension only when it’s needed. Also important is the option to increase acceleration power via a different type of powerpack when desired.

For many consumers, cost is an important interest. The nearly $4/gallon gas price is much more expensive than $0.20/KWH electricity. Investment cost also matters since people won’t buy what they can’t afford. Technology is assumed to be progressing linearly.

For output voltage standardization, 150VDC nominal with limits of 120V to 180V accommodates the 170V sinusoidal peak of AC single phase power in the US and Europe. This 20V difference from the nominal voltage will be excellent for most chargers to avoid having to do any more than rectify the house power.

The D-Cells (Lithium-Ion, and NiMH) and Gas Generator easily provide this voltage via serial/parallel bank wiring. The LA powerpacks can meet this via composition from the smaller wheel-chair sized batteries wired in serial (car batteries would only get to 48V in the assumed powerpack size).

Table 1 represents a decision criteria sequenced according to the assumptions in an engineering trade study and displayed in a scorecard format.

Table 1. Powerpack characteristics by technology

powerpack trade matrix.jpg

So which one wins? Lead-Acid (for now)
Lead-Acid is clearly the first choice to have on-board an EV based on the assumptions. LA outperforms the other technologies in:

• Power/Weight ratio

• Cost

• Weight

• Charge/discharge time


The rapidness of charge/discharge possible is especially noteworthy because it compares favorably to present-technology super-capacitors that weren’t included here. Reference: http://peswiki.com/index.php/PowerPedia:Capacitors. However, this number is estimated on CCA ratings and does not mean the LA batteries can be fully charged or discharged in that amount of time.

The low investment cost is important for keeping the total EV cost reasonable for mass production (the everybody market). And at $400 per powerpack the business model is workable for swapping powerpacks. And note also that the electric motors can produce allot more torque off the line than traditional gas motors with engaging clutches.

The primary drawback of LA batteries is the low scores on Range based on energy storage. That’s where the other choices come in very handy to some EV owners but not all of them. Without the powerpack assumption, the range via the Energy/Weight (WattHours/lbs) ratio would be much more important and would lead to different results.

The TM4 motor generator powered what could be considered one of the first range-extended electric cars, a Renault Kangoo developed in collaboration with Cleanova. Here the generator, motor and controller have been integrated into a single unit.

Two examples were used to represent LA. The wheel chair batteries were slightly denser and heavier providing more total power and KWH capacity. However, the marine batteries had a better power to weight ratio.

Which one looses? NiMH.

NiMH is too heavy and normalized scores like Power/weight do poorly. In household AAA, C, and D cells, NiMH is a good selection because of the reasonably high per-cell capacity (where it would be compared with alkaline cells) – but only in owner-selected applications where the recharging advantage would be worth the cost.

And the others? Niches.

Gas Generator Niches

The gas generator is especially good for longer trips. At over 100lbs it’s clearly a benefit to be able to disconnect it from the vehicle and leave it at home (or locked like a bicycle somewhere). Without mechanical coupling to the EV wheels, the gas motor is working at an optimal RPM for it’s maximum efficiency. 2-stroke generators are also becoming popular in the portable market. These have very light weight and with constant RPM and slowly variable load, 2 stroke motors are much easier to tune for clean burning and efficiency than otherwise, which is why they’re not currently used in autos but are so popular in tools like chain-saws and weed-whackers. In theory these efficiencies would cause the gas mileage will be substantially improved from the traditional auto and even from the early “parallel” type hybrids. However, without the direct mechanical connection there is substantial loss in the energy storage and delivery process. Therefore, the niche for these devices in a powerpack will primarily be to extend range and even enable very long trips with gas-only refills. These generator powerpacks will also be excellent to have for home backup generators, especially once the home charging stations become similarly modular with several plug-in stations (for charging, and home backup).

Lithium-Ion Niches

Lithium-Ion is an expensive investment but can extend range and power substantially (keeping the Lead-Acid charged). The ones I looked at don’t discharge/charge very fast and therefore aren’t a good option for excessively long journeys unless they can be swapped on-route. They also don’t provide as much instantaneous power as LA powerpacks. Thus their niche seems to be the medium range commuters and day-trippers who have allot of money to spend on luxury items. It is noteworthy that there are allot of investments going on with this and other battery technologies. Hopefully these efforts will continue to drive the price of this option down and make it a competitive selection outside of the current niche.

Alkaline Niches

These have been common for a long time and have a fairly decent capacity for their cost when using just a few in a common item such as a flash-light. NiMH have been challenging their niche-role for some time but haven’t taken over due to their higher price.

Others Niches

Other technologies not explored here include capacitors, and fuel cells. Again specialized powerpacks will have different characteristics including costs. Also not directly considered are any fundamental break-through with any of the technologies. In general, technology advancements tend to follow a Moore’s law type of pattern with an occasional break-through. Thus we could estimate the year when a given technology should become cost effective. Many car companies are doing just that and we’re hearing which technology they’re banking on. One has selected Lead-Acid and will get a jump into the market first. Another has selected Lithium and is banking on power-delivery improvements, fire-safety, and cost improvements to make their cars deliverable to customers.

The Take-Away?

Well first is a realization that Lead-Acid is a Top-Gun good solution to power an EV. So much so that an EV manufacturer may choose to build in a primary set of LA batteries that aren’t powerpack swappable. LA batteries are already in wide-spread use for wheel-chairs, scooters, and golf carts. In car applications range becomes a much bigger issue and thus there is a serious need to supplement the charge LA batteries can hold. And that’s one of the foundational premises of this article series – which leads directly to the conclusion that powerpack standardization can hasten the delivery of EVs to the mainstream market. Similarly any EVs may be designed with a primary power source technology. But regardless of the selection, until 5 minute recharges and low cost packages are a reality the need for powerpack options remains.

The secondary take-way is the generator does belong in a Powerpack or other easily attach/detach configuration. It is heavy for little surge power but useful for a continuous recharge, especially when continuously running at maximum efficiency. Thus it’s primarily a range extender (assuming no mechanical direct drive) and quick-mounting puts it into proper usage.

A third take-away is that the batteries are so heavy that we should be figuring out how to adjust the number of batteries we have on board according to how far we would need to go in a normal day. For example, to/from work 20 miles plus 10 for the grocery mart and another 10 for reserve. Thus 40 miles worth of batteries. A short commuter could use 10 miles worth of batteries and attach the generator to go further only when necessary.

The most impressive aspect of this study is the wide variation in the technology characteristics and how they match up differently to various market segments. Short range low cost commuting is readily served by Lead-Acid technology. Expensive EVs can take advantage of the energy storage of Lithium-Ion for good range on a single charge. Generators can provide low investment long range. Powerpacks also have secondary benefit creating a home energy centers for distributed power generation and storage across the grid. Powerpack standardization can give the technology choice to the EV drivers in both business models of investment and rental/swap-out. And best of all, powerpack standardization enables EVs to use technology that’s available today without any waiting for a promised technology solution that is still very expensive. And when great battery solutions do come, the powerpack standardization will again enable EV owners to adopt it without changing the whole car.


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