Wednesday, October 7, 2009

Battery information from an electrical engineer

I received this email from Bill Fabrey. Bill was a founder of NAAFA. He's also the owner of Amplestuff, a store that carries things for plus and supersize folks. Most importantly for us, he's an electical engineer.


You are doing a wonderful job on your Travel Scoot mailings! The photos are excellent, and the information is vital. I want a Travel Scoot, and I don't even have trouble walking!

Just so you will know, the larger lithium-ion battery is less likely to overheat simply because its capacity is so much greater. It has to do with "energy density". If you are traveling with a heavier load, or uphill, or on a carpet or wooden bridge, as in your example, the current demanded from the motor is greater--which leads to more heat in the motor (causing it to trip out now and then--even in so-called heavy-duty scooters with 450 lb weight limits.) The same increased current that heats up the motor also heats up the battery. The smaller the battery, the greater the temperature rise for the same current flow. With lead-acid batteries, the effect is negligible, but lithium-ion batteries, even though they store so much more energy per pound of battery weight, are more temperature sensitive. Heat is the Achillies heel of lithium batteries.

It is analogous to a horse pulling a plow. A mule pulling the same plow through difficult soil at the same rate of speed will heat up faster, and refuse to continue. A draft horse or oxen (or team) pulling the same plow, doesn't get phased.

OK, another example: Driving a model-A Ford up Mt. Washington vs. driving a Lincoln up the same incline. The model-A will boil off all its anti-freeze and refuse to continue.


Bill Fabrey (electrical engineer--retired)

PS - Confession: Despite my advancing years, I have not PERSONALLY EXPERIENCED driving a horse pulling a plow, or driving a Model A. I have, however, driven a Lincoln up Mt. Washington. Yes, it overheated, but within tolerable limits. Even though the luggage plus the passengers exceeded its rated capacity. (Knowing me, you will not be surprised to learn that the passengers were of the generously-proportioned variety. LOL).

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