…electrical load is applied. I’ve been noticing for a while that my headlights and interior lights will dim, almost impeceptibly, for a split-second when I apply the brakes. Last night, the effect was much more pronounced–The headlights dimmed to about half-power when I hit the brakes. It’s more noticable at low rpm (when the alternator is turning more slowly.) When I flip the heater fan from “off” to “high”, electrical power is sapped until the fan gets up to speed. At first, I thought maybe the battery was getting weak (4 yrs. old) and couldn’t handle the transitional load, but I tested it with a hydrometer and all cells tested good (I don’t think cold temps would affect the accuracy of this test.) Now I’m guessing that for some reason, the integral voltage regulator is not reacting quickly enough to compensate for applied loads. Anybody got a non-guess about this problem? 
I guess not. Why not take it for more extensive tests of the battery and alternator (free) at your friendly auto parts store? They can test the battery and alternator in the car, as you wait.
A hydrometer test is not valid for what you are experiencing. It simply checks the condition of the acid bath. Get the battery load-tested. This tests the overall power capacity of the battery, and is done free at a lot of shops. If the battery passes, then get the charging system load-tested. This will determine if there is a problem with the entire system, including battery and alternator. Some places will do this for free as well, and others will want to charge a small fee.
I’d start with the easy things - clean off the battery connections and check your grounds.
On the other hand, my 97 Taurus has had that almost imperceptible dimming of lights when I turned on the heater fan since it was new. I’m well past 100k and its 11 years old now, and I’m only on my second battery (6 years old now) and haven’t had any issues… I WISH batteries on my Toyotas would average that long between replacement. 
The problem with the Toyotas may be an undersized battery. In my Toyota cars and trucks over the years, I’ve always upgraded to a higher capacity (CCA) battery than the book calls for. I had one in my late truck last 8 years, and the one in my Supra is 6 years and still going.
I agree a higher CCA battery is a good idea for extra life…
But what I’ve run into over the course of 3 different Toyotas which ate batteries for breakfast, lunch, and dinner was a common defect - the danged sensor that was supposed to turn off the trunk light when the trunk was closed wasn’t working properly. The battery would drain at absurd rates because the trunk light was on 24/7, but since I’m not in the mob and don’t ride in the trunk, how was I to know?
I should have worded it more carefully, but I’ve managed to get better life out of them now, just not until I discovered that was the freakin’ problem. My simple solution - take out the bulb. I’ve gone from 3-5 months of battery life to 5 years.
Thanks, BK! I have access to a load tester at work. Whether or not I can find the instructions that came with it is another matter. I think there are some markings on the face to indicate “good” and “weak” readings for both the battery and the charging system. I assume that to test the battery, I should disconnect the cables. Then, to test the charging system, reconnect the battery and clip the tester onto the battery cable clamps. Sound right?
Edited to add: I will check the connections first…
You can leave the battery hooked up for the battery load test. Not a problem. But, this is not a proper tester for the charging system. You’ll need to get that done.