Auto batteries: reserve capacity vs Ah (Amp*hrs)

The tutor defines two terms relating to auto batteries.

The reserve capacity (RC) of a car battery is given in minutes. It’s the duration the battery can discharge 25A at ≥ 10.5V.

Ampere hours (Ah), on the other hand, is the product of steady discharge in Amps that is possible for a duration of hours before the battery voltage slips below 10.5V. Ah is often given in a format like 50Ah@20h, meaning that, over a 20 hour duration, the battery delivered a steady 2.5A at or above 10.5V.

Mathematically, reserve capacity can be converted to Ah as follows:

Ah ≈ RC*(5/12)

However, this equation might be more a “rule-of-thumb”, since the total amount of energy available from a battery decreases with speed of discharge. In other words, a battery that delivers 2.5A for 20h might likely not manage 50A for 1h.


Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Physical chemistry: vapor pressure: why a car may be harder to start in the winter

The tutor implicates the Clausius-Clapeyron equation to explain why combustion is more difficult at lower temperatures.

To burn, a fuel must evaporate.1

In colder temperatures, fuel has less tendency to evaporate. For two specific temperatures (T1 and T2), the Clausius-Clapeyron equation can give a comparison of the vapor pressures (p2 vs p1) of a liquid with molar enthalpy of evaporation ΔHvap:

ln(p2/p1) = ΔHvap(T2 – T1)/(R*T2*T1), where

R = 8.314 J/(K*mol), the idea gas constant.

T1, T2 are absolute temperatures.

Note, from my post from Feb 20, that ΔHvap for gasoline is approximately 38.1kJ/mol.

Example: Compare the vapor pressure of gasoline at 12°C (285K) vs -5°C (268K).

Solution: Let T2 be 268K (-5°C). Then

ln(p2/p1) = 38100(268 – 285)/(268*285) = -1.02

Taking the antilog gives

p2/p1 = 0.36 = 36%

Apparently, gasoline is about 36% as likely to evaporate at -5°C as at 12°C. Not much surprise, really, that a car can be more difficult to start in colder temperatures.


1Only Vapors Burn 1947 Chemistry of Fire US War Department

Mortimer, Charles E. Chemistry, sixth ed. Belmont: Wadsworth, 1986.

White, J. Edmund. Physical Chemistry, Harcourt Brace Jovanovich College Outline Series. San Diego: Harcourt Brace Jovanovich, 1987.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Electricity: getting a shock from the car

The tutor shares some research about why he gets shocked leaving the car.

When the weather turns dry and summery, I love it. However, I face a summertime hazard: getting an electric shock when I leave the car. Sometimes the shock makes me wince as I hear the “crack”.

Last night I researched the reason for these shocks. Possibly, it’s mundane: as my leather belt and even my dry skin rub against the polyester seat, they develop positive charge (the seat, negative). When I touch the metal door, my positive charge attracts electrons through my fingers – zap! In dry weather, the process is emphasized, since there is less water to bleed away static charge.

I’ll be talking more about static electricity:)



Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Autos: routine maintenance: signal bulb replacement

The tutor shares his experience changing a signal bulb on the minivan.

While I drive much more now than I used to, I’m not a casual driver. I try to minimize the risk of problems by keeping the car maintained.

Last night, on the way to pick up my eleven-year-old from an activity, I flicked the turn signal. It clicked at double-speed, which typically means one of the blinker bulbs is out, or possibly a blinker relay is out, depending on the model of car. Inspection revealed the problem to be at the left rear.

I’m no automotive expert, but I do have some mechanics training from long ago. We used to own a 1988 Olds, on which I changed a few bulbs, the rad hose, etc. I’ve heard that new cars can be different, though – tougher for the owner to service. Our minivan is a 2008, so I wondered if this repair would be worth trying myself.

From the glove box I fetched the manual. With both text and a picture it shows how to approach changing the bulb. Parked halfway out from the lighted garage, I found the two screws to remove. The’re Torx, which surprised me. At first I couldn’t find a screwdriver for them, but eventually found a socket in a roadside repair kit we keep in the car.

I had the light fixture off after a few minutes. Unlocking the harness to the bulb, I plucked it out. The time was after 8pm by then – as far as I knew, too late to get the replacement bulb. I re-attached the light assembly.

This morning, just after 7:30, I walked into an auto parts store and showed them the bulb. I bought two replacements – total charge, $3.30. Having rehearsed the previous night, I replaced the bulb in the parking lot. I was back home at 8:01am, ready to leave again at 8:10 to bring everyone to school.

The Torx screws surprised me – luckily, I had a socket to remove them. However, the bulb replacement was as easy as I remember any being on an older car.

By the way: I re-treated the minivan windows with rain-x over the weekend, as well. (See my post here for more info.)

I’ll be sharing more car care anecdotes in future posts:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Driving: rain-x and Fog Relief

The tutor brings up two products he finds indispensable.

I’m not a casual driver, but I have to drive sometimes. More often, my wife does the driving.

I try to make the car as easy and safe as possible to drive. Some examples are replacing the tires when they’re wearing out, keeping the tires at proper pressure, and keeping the windows clean.

Around five months of the year, the weather here is very rainy; then, just having the windows clean doesn’t seem to be enough. To maintain visibility during those winter months, I treat the car windows with two products: rain-x and Fog Relief.

rain-x (its name is in lower case) is for external application. I find it makes a dramatic improvement to driving visibility. On a window treated with it, the rain won’t film; it just beads and rolls off. Heading into the winter months, I religiously treat the car windows with rain-x.

Fog Relief is applied to the inside of a window to prevent its fogging up. I find it does help. Sometimes, fog will still form on a treated window; however, the advantage I’ve found is that, once you wipe the fog off with a clean cloth, it doesn’t return.

I highly recommend both rain-x and Fog Relief.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Energy efficiency: electric cars vs gasoline-powered cars

The tutor continues to investigate energy alternatives. The comparison between electric and gasoline cars might depend on your point of view.

Someone told me a while ago that electric cars are 50% efficient.  I was surprised to hear that number, and didn’t necessarily believe it.  I remember reading, maybe in grade four, that the human body is about 40% efficient.  I have a hard time believing that a mechanical entity is more efficient than a biological one.

Well, first I had to check my facts. Eventually I did find confirmation at the human body is 40% efficient at converting glucose to usable energy.

Next, I found, from the US Dept of Energy, that electric vehicles are, in fact, about 60% efficient, while gas cars are about 19% efficient.

Here’s where the road forks: are you an academic or a consumer?

If you’re a consumer, the comparison is done: electric cars, at 60%, are about three times as efficient as gasoline cars. The next step is to compare prices, to find out which might be the bargain. Energy prices can be volatile; however, my July 29, 2014 post found that gasoline, diesel, and electricity, per unit of energy, may have surprisingly similar costs.

If you’re not interested in the cost, but rather the true energy efficiency from an academic point of view, the comparison between electric and gas vehicles must go further. Where did the electricity come from? If it came from burning fuel, then you need to factor in the efficiency of that process as well.

The US Energy Information Administration suggests that more than 86% of electricity in the US comes from fuel-consuming plants (this includes nuclear power). Around a third is from natural gas. I’ve done a weighted average of the efficiencies of the various sources (based on information found here) and come up with an efficiency figure of roughly 35% for the generation of electricity in the US.

So, if your electric car is 60% efficient, but the electricity you use to power it is produced with about 35% efficiency, the realistic energy efficiency of the car is 60% of 35%, from an academic point of view. It finally works out to 21%. Remember: gas cars are around 19% efficient. To an academic, the electric-vs-gasoline efficiency might be compared as 21/19, or 111%: the electric car has 111% the efficiency of the gasoline car.

To a consumer, the electric car is roughly three times as energy efficient as the gasoline one. To an academic, the two are much more similar, the electric car having 111% the gasoline car’s efficiency.

Which side speaks to you?)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Auto batteries, part two: cold cranking amps and internal resistance

The tutor continues to explore auto batteries (aka car batteries).  While this topic is rare during tutoring, it’s probably relevant to virtually everyone at some point….

I recall, decades ago, a friend of mine bought a new battery for his car. “It’s got 600 cold cranking amps,” he smiled. “No more doubts about starting.”

I soon learned the meaning of 600 cold cranking amps: his car started perfectly from then on. At the same time, he was impressed by the number because of its technical meaning; being a mechanic, it spoke to him in a more precise way.

Years went by: I finished my degree, then went back for (of all things) some mechanics training. Today, when events from the past pop in my head, I try to answer questions that I let go at the time.

So it is with the issue of “cold cranking amps”. The other day I suddenly realized: if a 12-volt battery is pushing 600 amps, its internal resistance must be less than 12/600=0.02Ω. The obvious question: do car batteries really have internal resistance that low?

I started searching the net. The answer was harder to find than I’d expected, but here are some numbers:

car battery resistance link

So I guess it’s true: 600 cold cranking amps – or even more – is possible, based on internal resistance alone.

I have yet to define “cold cranking amps”; I will do so in a coming post. Cheers:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Auto batteries: seasonal reflections

Tutoring high school chemistry includes electrochemistry.  The tutor shares some reflections about auto batteries as winter approaches.

I recall more than one mechanic talking about how a device might work fine for a long time but have a hidden weakness. When that device is, for whatever reason, put under abnormal stress, it likely fails. The failure is surprising: hasn’t it worked for months (or years) with no problems? Why, then, does it suddenly fail, at a time you really need it to work?

From my experience, auto batteries can give that “false sense of security.” It’s not the battery’s fault, of course; it’s just the life cycle in most of North America (ie, summer to winter).

From purchase, an auto battery of good quality is likely strong and reliable for a few years anyway. Depending on driving habits, it may maintain its vigor much longer than that. However, time is working against the battery: potentially, the chemical process of sulphation, among other factors.

So, the battery likely weakens over time, yet continues starting the car just as expected. Let’s imagine the battery becomes considerably weaker in late May. The driver likely won’t even notice: from late spring through early fall, the weather is warm. In the heat, the car’s oil might be more agreeable to letting the engine turn over. The days are bright, warm, and dry: the driver doesn’t use the headlights, heater, or wipers as much. Life is easy for the battery. Yet, during those carefree months, the battery may already be too weak to start the car in the cold.

When the inevitable “first winter storm” comes, the car may not start. The driver is surprised. (I’ve been there.) Really, though, the driver’s been on borrowed time for weeks or months already. The battery just wouldn’t reveal its weakness until put under stress.

I’ve been reading up on auto batteries lately (just for the pictures, of course:). In coming posts I’ll discuss some of my findings about this fascinating topic we all depend on.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Ethanol: viable as a transportation fuel?

Continuing about autos, the tutor turns the ignition on another topic: ethanol as a fuel in North America.

Sometime in the 2000s, if not before, ethanol’s use as an alternative auto fuel became a hot topic. Its use in Brazil is famous; why not in the US?

As I understand it, a couple of motivations spurred the use of ethanol:

  1. Possible ecological benefits.
  2. Relieving the high US expenditure on imported fossil fuels.

The first motivation, I believe, has come under serious question. For instance: some people mistook “renewable” to mean “reduced carbon footprint”; the two concepts aren’t necessarily related. Wood, for example, is a renewable fuel; yet, burning wood to meet America’s energy needs would probably not be ecologically sound.

The second motivation is possibly more rational. However, to make a fair judgment, one must realize that ethanol has only two thirds the energy of gasoline; therefore, the price of ethanol needs to be less than two thirds the price of gasoline for ethanol to be viable.

Today, the price of gasoline is $3.43/gallon, according to The price of ethanol, at $2.17, ( is less than two thirds the price of gasoline. Therefore, today, ethanol makes economic sense as an alternative to gasoline – theoretically, anyway. However, the prices of both fuels are volatile.

In future posts I’ll be exploring the ecological dimension of ethanol as a gasoline alternative as well as its economic aspect.

My children await their return to school; all the best to those trying to reach a settlement:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

Driving: fuel consumption

Progressing through summer, the tutor recalls these issues he’s always loved discussing.  During tutoring, he rarely gets to….

One source of fascination, for some drivers (and MOST people in the transport business), is how fuel consumption increases with speed.

Any cyclist knows that on level ground, their speed tops out very quickly even under optimum conditions. Riding my mountain bike avidly in my early 20s, I noticed that it’s hard to ride 20km/h for more than an hour – yet 15km/h can be sustained indefinitely with good conditions. On a racing bike, both speeds would be higher, but the point is the same.

Driving, your car doesn’t get tired; rather, it just uses more fuel. How much more? Well, the good people at the US Department of Energy are offering this rule of thumb:

For every 5mph over 50mph, it’s like you’re paying $0.25 more per gallon of gas.

Converting to Canadian units, we can translate:

For every 8km/h over 80km/h, it’s like you’re paying $0.066 more per litre.

Since I rarely drive, I don’t know the pump price today; I hear it’s around $1.40 per litre. If you drive 100 km/h (as opposed to 80km/h), you’re effectively paying about $0.16 more per litre, or around $1.56.

Then again: time is money:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.