LOL, there was frost on my windshield from ten last night onward–when the cloud cover goes away it gets butt ass cold out at night! SE Portland, about 240 ft elevation.
I think the key word is “adequately”. They may have meant “fully” charge the battery? 20 MPH equates to X engine RPM. And X engine RPM would be the minimum engine RPM required to spin the alternator fast enough to generate the required voltage to fully charge the battery.
What do you mean, remember? Most of my rigs still do.
Sounds like it just had an under-sized alternator.
Back in the day (up until the 60s) DC generators depended on enignine rpm to spin the armature fast enough to generate power. It was quite possible for your battery to run down from extended idling.
Alternators generate polyphase AC, and the alternator’s power output is controlled by adjusting the current through the spinning field coils- this is what the voltage regulator is actually doing. If an alternator can’t service the vehicle’s electrical demand even at idle, it’s too small.
Manitoban here, I drive with a block heater cord hanging out the front grill all the time 
I don’t think that the block heater runs warm enough to prevent freezing, but it does help the engine when starting.
That’s not correct. I was worried about my battery having found it drained. I drove to the garage after a jump start. The tech at the garage found it started fine, turned the car off, pronounced the battery fine, and walked back inside. I got in the car and there was not enough charge left in the battery to start the car.
depth of discharge is important. if you run the battery completely (or near completely) flat, you risk taking it to the point where it can’t be recharged due to excessive sulfation. if you just run it down 20-30% from leaving the lights on, re-charging it properly will let you get on your way.
There’s a world of difference between a battery that’s drained and needs a jump start and one that merely had the headlights on for two hours. Apples and oranges.
ETA: In other words, my advice was appropriate for the OP’s situation. I would not have given the same advice for yours.
Two hours’ lights is not enough to hurt a healthy battery. Just turn everything off for an hour, and the battery’s own chemistry will give you enough power to start the engine. If, after an hour, you’re not ready to go anywhere, let it sit until you need it. If the battery was dying of old age, though, it might not be enough.
Huh? What?
My current and previous motorcycles (not sure about the ones before that) have rectifiers in the loop to charge the battery with excess power from the alternator, then direct it elsewhere (headlight, perhaps?) when the battery is full. I’m admittedly naive about vehicle engineering, but I figured a car would have something equally sophisticated in its electrical system since there’s more room for parts under the hood than under my fuel tank. Not true?
–G!
I had two Volkswagen Beetles (late 60’s model years I think), and at least one of the manuals said they did not generate enough power to charge the battery if you were using the headlights. The manual warned that if you only drive with the lights on, you have to use a battery charger too. These would have had generators instead of alternators, I think, but they’re examples of cars not intended to be able to charge the battery in all use cases.
To me it seems more and more likely as the years go by that cars would be designed to keep the battery charged regardless of the balance of uses. For instance, I think some cars spend hours at a stretch idling, perhaps with their lights on, such as police cars warning of nighttime construction projects. But I don’t think I’d assume it about all cars.
motorcycles typically have “shunted current” voltage regulators/rectifiers and permanent-magnet alternators. the alternator is always producing whatever current it’s capable of all the time, and any power not needed to run the bike is simply dissipated as heat by the voltage regulator. it’s why the regulator has a large heatsink and is placed where the air stream can cool it.
cars have field-excited alternators; the spinning rotor is made up of wire coils around pole pieces. the voltage regulator controls the amount of current being passed through the field coils to control the level of current induced in the stator windings. The voltage regulator has complete control over how much power the alternator generates (up until its limit, of course.) the rectifiers are built into the alternator and cooled by the alternators external (or internal) fan. they run a lot hotter and regularly relying on the alternator to charge a depleted battery can eventually lead to one or more of the rectifier diodes overheating and failing.
Yamaha and Honda tried using field-excited alternators in some of their bikes in the late '70s and '80s, but they weren’t very reliable and eventually they went back to shunted current systems.
I will never forget the Texan in Montreal (1990-ish) who thought our neighbour had an “electric jeep” when he plugged in his Suzuki Samuri in the winter. She was very confused when she saw him filling up with gas a few weeks later.