Alternator does not charge your car battery the way you think it does

> Have you ever seen or driven a car where, as you sped up, the lights got brighter? That's because the battery was dead and all the electrical power was coming from the alternator with no battery to act as a load-balancer.

Not in a long-long time. Voltage-regulated alternators have been a thing for decades now. By the 90s, most companies had solid state internal voltage regulators. Also, most (probably every) modern cars feed the alternator signal through the ECU/PCM before it gets to the battery specifically because it's crucial that computer-controlled component receive a clean, reliable voltage signal. I learned this the hard way.

Some good stuff there, but I think this article suffers from not distinguishing between a "dead battery" that's dead because, eg you left your headlights on, and a battery that's "dead" because it's aged to the point of suffering internal defects.

A "dead battery" in the former case (maybe we should just call it a "very discharged battery") can, in my experience, often be recharged by your alternator and it doesn't necessarily take a super long drive to do it. That's assuming you're able to get the car started in the first place, possibly by jump-starting from another car, or "roll it down a hill and pop the clutch" which will work IF there's "just enough" juice in the battery for the ignition coil to fire (and enough to provide the "seed" excitation current to the alternator).

OTOH, a "dead battery" in the latter sense (maybe we should call it a "dying battery") simply needs to be replaced, ASAP. It won't ever hold a charge reliably, could (in an extreme worst case scenario) develop an internal short and catch fire, or damage your alternator.

Fortunately most auto parts stores (here in the US anyway) offer a battery testing service that can tell you if your battery is truly shot or not. They'll also often have an alternator tester, but that usually means removing the alternator and bringing it in, which can be a lot of work on some vehicles. But a simple test is to start your engine and then disconnect the positive battery terminal. If the engine dies immediately, you know the alternator isn't emitting any current (the engine dies because there's no current for the ignition coil(s), ECM, fuel-injectors, etc).

A common cause of a flat/dead battery is a structural defect --- either a short or a bridge inside the cells. The alternator can't fix this.

This checks out with me anecdotally, at least. I bought a new car just as the first COVID lockdown let up and right on the forecourt it was complaining about a low battery, apparently due to sitting around for a month or two. "It'll be fine by the time you get it home" they said. Wrong. Even a few hours on a charger and a 300 mile drive only bought me a few weeks of time. I ended up having to trickle charge it for 24 hours to permanently resolve the problem.

Is there a solar panel solution for this? Like a solar panel you can leave on the back window or something, that will slowly trickle-charge the battery?

Alternators don't have permanent magnets, they need current for the excitation winding. This is probably why they won't recharge a dead battery.

How do you know how I think the alternator charges my car battery?

This is one of those science gotcha posts that threads an incredibly narrow line of reasoning. If your battery is (A) dead but (B) you can somehow start the vehicle (how?) and (C) you don't have time to call a tow company to come recharge it or replace it and (D) the battery isn't completely defective and needs to be replaced and (E) you do have time to drive around in hopes that your alternator can charge the battery and (F) I got bored with this list

Battery tenders are great things to have if you don't drive often enough to make up for natural discharging. Especially so if you're in a very cold climate.

>Have you ever seen or driven a car where, as you sped up, the lights got brighter?

Yes, VW Bettle ;)