Power DesignLine Blog
Can all-electric vehicles take charge?
Bill Schweber
11/7/2009 12:00 PM EST
But even if we do somehow get batteries which are nearly ideal (low cost, high energy and power density, lightweight, long life), and even if we don't worry about where all that electricity to charge them is going to come from, there's still one real challenge: getting the charge (current) into these batteries.
I'm a big proponent of what used to be called "back of the envelope" calculations, where you use rough but reasonable numbers to come up with a realistic estimate of a situation. It's a great way to avoid blindly believing your Spice model when says that you need a 10-zillion ohm resistor in your circuit, when the real issue is your model has errors or you entered some wrong numbers.
So try to do some basic analysis of the car situation, starting either at the sourcing end or the load end. At the sourcing end, figure you have 15- or 20-amp line recharging the batteries for "x" hours, and work over to the load: how long can this amount of stored energy run an electric motor of the size you think you'll need in the car? Or go the other way: if you have an electric motor size and run time in mind, how much energy does it require for the run, and how much charging will that require? (Remember: one horsepower is about 750 watts.) Of course, you should also use a derating factor of two (or 50%) to account, roughly, for the system losses between charging connector and actual delivered power. (For this first-level rough assessment, ignore any other loads the all-electric vehicle may have, such as a heater or air conditioner or multimedia system, but you know that they can only make things worse.)
I'm not going to go through the analysis for you, I assume you can do it yourself, I think you'll find it illuminating: it takes a lot of charging time at 15 or 20 A to deliver enough energy for most all-electric car scenarios. Petrochemical fuels have very high energy and power densities (with respect to both volume and weight) and so can move lots of energy fairly quickly.
And if you can't do the rough, first-order analysis yourself, you probably should do some basic reading-up on the relationship between current, voltage, power, energy, and work. Even if you are not involved with electric vehicles, those are fundamental relationships you need to be comfortable with using for quick estimates of your product and its power source. ♦
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ump
11/12/2009 4:19 PM EST
It is just wrong headed to use the motor's size to gage the needed battery's capacity or recharge energy.
It's the energy required to overcome resistance to moving the vehicle (wind, tires etc.) vs. time that determines the bulk of battery recharge energy (in a perfect world). It's the acceleration/deceleration rates and vehicle weight that will relate to the motor's size. Further, much of the energy used to accelerate can be recoved by deceleration.
So, even for a back of the envelope reality check, the battery recharge time does not relate to the HP size of the drive motor.
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BicycleBill
11/13/2009 10:44 AM EST
Have to disagree, you also have to look at it in terms of work being done, and the prime mover. Othewise, why use a larger hp motor when a smaller one will do?
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AlexKovnat
11/16/2009 8:39 AM EST
Whether tomorrow's road going citizenry goes about their daily business in a hybrid-electric or a pure electric car, I would say the most urgent need right now is for us to work out the technical problems with lithium ion batteries. Where I work, there's a guy who's administering a development program for lithium-air batteries, and there has also been talk of lithium-sulfur batteries. Let us hope at least one of these concepts succeeds.
There is another research need I'd like to point out: Fuel cells. Fuel cells are all right if you don't mind their fussy fuel appetites, which at this moment can be met only by hydrogen. I'd rather not have to deal with H2. There is the possibility of using methanol in fuel cells directly, without a technically difficult reforming step. But even if you want to use a reformer, methanol is easier to reform into hydrogen than gasoline.
Researchers should look into the matter of whether you need chemical-grade methanol for a direct methanol fuel cell to work, as opposed to CH3OH with higher alcohols or other impurities that might be present with non-chemical grade methanol.
Will non-chemical grade CH3OH be substantially more difficult to reform, or use directly in a fuel cell?
If this issue can be worked out, and if researchers can work out the problem of producing methanol via direct oxidation of methane (i.e. 2CH4 + O2 --> 2CH3OH, as opposed to CO + 2H2 --> CH3OH synthesis), I think fuel cell-electric vehicles would be a viable option. And all the more so, if you use first-rate lithium ion batteries for load leveling.
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nexogen
11/19/2009 2:45 AM EST
Look, the problem isn't battery technology, the problem is, at the essence, the money based economy. We can have free energy and free food with the current technology, we can have them in abundance. With this, money would be obsolete. Automation can replace human labor. The only thing stopping this from happening is the money based system. This system is also promoting planned obsolecence and deliberate product inefficiency due to the consumption cycle needed to maintain it. It is also fooling us into beliving that we need oil and that batteries are inefficient. The system needs inefficiency and scarcity, it feeds on it.
You need to give this a good solid thought and realize the implications.
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graduatesoftware
11/23/2009 9:49 PM EST
Absolutely. Charging rate is most definitely a concern for all-electric vehicles, but it is not an insurmountable problem. Using a very rough (and conservative) figure of 1 kWh / mi, the amount of charge that must be restored after a 50 mile round-trip commute is 50 kWh. Dumping 5 kW (200V @ 25A) over 10 hours would recharge the batteries (for a cost of around $5). But let's look at the problem another way. We can certainly discharge the batteries at 50 mi/hr (a number of electric cars already demonstrate this). So we are consuming the 5 kWh in one hour's time without blowing anything up or melting anything down. Therefore (assuming the charge/discharge thermodynamics are the same), we ought to be able to put the charge back into the batteries over the same period of time without risking overheating the batteries, wires, controllers, etc. To do so, however, will likely require some changes to the electric service in your home. Instead of 200V x 25A x 10h, you would need 200V x 250A x 1h.
Another thing to consider in this discussion are the battery technologies that have replaceable charge carriers (such as Zinc slurry in the Zinc-Air battery being developed by Revolt).
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