Want to read Slashdot from your mobile device? Point it at m.slashdot.org and keep reading!


Forgot your password?
User Journal

Journal DaChesserCat's Journal: Multiple stages

I've been reading extensively on the subject of Fuel-Cell Vehicles (FCV's) and Battery Electric Vehicles (BEV's). I'm coming across an idea which I'm surprised no one else has stumbled onto.

In your traditional BEV, you want to accelerate (which means you need a very high power output), but batteries have a very low specific power (kW/lb). Additionally, when you kick in the regenerative braking, the motor is providing a very high power INPUT; the batteries don't particularly like that, either. To be fair, you can draw/feed power at a high rate, but the batteries tend to heat up. If the batteries are producing heat, they're using electrical energy (which you want to get out as electrical current) to do it. This is throwing away energy, robbing you of a significant share of your potential range. Consequently, your traditional BEV's tend to be kinda gutless, and leaning on the accelerator hurts your range. It also means that regenerative braking isn't as efficient as you might think.

Back when CPU's started to seriously outstrip RAM performance (I'm talking 33 MHz 386), the solution was to put a cache between the two. The cache was a small chunk of very-high-speed RAM which could keep up with the CPU. The idea was that a very small part of the actual data in the machine was used most of the time; if that very small part could be held in a small chunk of very-high-speed RAM, you'd get most of the benefits of RAM which could keep up with the CPU, without having to cough up too much money. Aside: the reality is that Static RAM could keep up with the CPU, but it is an order of magnitude more expensive than slower, Dynamic RAM. Consequently, you want a small chunk of the fast, expensive stuff, and a large chunk of cheaper, slower stuff. Anyhow, the cache needed special interface hardware and software, but it provided significant bang for the buck. While modern systems have Level 1, Level 2 and, in some cases Level 3 caches, we still use the system because it still provides significant bang for the buck.

I realize this jumps around a bit, but I'm coming to the point. To draw a parallel from the computer analogy, what we have is an electric motor (CPU) which has a very high input/output (fast data transfer), and very low-power batteries (Dynamic RAM). We need the electrical equivalent of a cache. We need a small chunk of some kind of energy storage, which can handle very high power in and out. This way, we can get the performance and braking we want, and the batteries can work the way they work best (low-power, steady output).

Such things do exist; they're called supercapacitors. They haven't been commercially available for very long (new technology), and there are relatively few companies which can supply them (two, I believe). However, like the cache on a computer, we don't need a great deal of them. Honda uses them for it's Fuel Cell Vehicle; the fuel-cell runs at a pretty steady output, while the supercaps buffer out the surges. Most reviews of its performance say it hustles nicely.

To push the analogy further, we might need multiple levels of "power cache" to make truly practical BEV's. We need short-term (L1) cache which can push the vehicle from a standing start to highway speeds AND climb the entrance ramp on the interstate. That's something which would be purged in, say, 15 seconds. Then, we need something (L2) which can provide a steady push for about five minutes (climbing a long hill). Then, we need the main power store (the main batteries). So far, supercaps look like the L1, and some of the NiCD batteries out there (which, depending on construction, can handle about 10-12C charge/discharge rates) look like good choices for L2.

A quick note on charge/discharge rates for those who don't already know. If you charge/discharge a battery so it goes from dead to full in 1 hour (or fully-charged to dead), that's a 1C rate. If you fill/kill in 30 minutes, that would be a 2C rate. Five minutes would be a 12C rate (60 / 5 minutes = 12C). Completely charging or discharging in 15 seconds would be a 240C rate. Traditional lead-acid batteries are reasonably happy with a 1C rate, but they're happier still still with a 0.1C (ten hours for charge or discharge) rate. Anything faster than 1C, however, tends to cause them to overheat.

I'm looking for BEV makers to catch onto this "multistage" approach. At the moment, most EV owners claim they're getting about 80% of the electrical energy they put in, back out. Put this into place, so the batteries don't have to deal with spikes in output when someone wants to get off the light or climb the hill, or input spikes when they hit the brakes, and the batteries can operate more efficiently. Oh, and they won't have any excuse to be gutless.

I've played more than one computer game where your vehicle has a certain top speed, but it also has a "boost" gauge, which indicates a limited timeframe when you increase your power output. Normally, you have to hit certain items to increase this; in some games, it just accumulates as time goes by. I anticipate a BEV with a "fuel gauge," indicating how much energy remains in the batteries, and a "boost gauge," indicating how much is available in the supercaps/L1 cache. It wouldn't take long for drivers to learn to read this, and use it effectively.

This discussion has been archived. No new comments can be posted.

Multiple stages

Comments Filter:

Experience varies directly with equipment ruined.