Agreed, while running alternator is supplying current while the car is running. My understand is that when the battery is deeply discharged, the alternator favourably tries to recharge the battery first and supply accessories later. Since it's already having a hard time charging the AGMs (which need higher voltage) when deeply discharged, alternator struggles to provide enough voltage and you see the lights dim etc.
I will try to clean the battery terminals as suggested.
I have 90k miles. The battery is about 2 years old. It has been very well cared for. Never been driven offroad, submerged etc. The author should not have more than normal wear tear. It has been my daily for 10 years.
Actually after riding the voltage booster with the plain fuse, the voltage has been closer to 14.1, which it had been prior to the deep discharge.
I have an aftermarket voltmeter that I installed. While it's not a high grade by any stretch, the readings have been reliably consistent ie after full discharges ( has happened on three occasions in the last year) the voltage is read low (13.4-13.6 running even at higher revs) and when fully recharged back at 14.1 running, unrelated if idling or high revs
I'm certain I have a high parasitic draw - likely culprits are dash cam (has an auto cut off below battery voltage of 11.5) and remote starter. I need to isolate that
The battery is used only for starting the engine, and for supplying electrical loads when the engine is not running.
When the engine is running, the alternator supplies all the electrical power to the vehicle. If the lights dim or the wiper motor slows down at low engine speeds, there is a defect in the alternator, some defect in the wiring (typically high resistance at the battery clamps or ground connections), or the total electrical load exceeds the alternators output at low speed.
How many miles on the vehicle, and how long has the battery been in service? If there are more than 85K miles on the vehicle, and especially if it has been frequently driven on dusty dirt roads or submerged in muddy water, the alternator brushes may be worn out, or the brushes may be sticking in their holder.
A voltage booster will not compensate for a dying battery or a defective alternator. Also, most plug-in voltage boosters are "polarized"and must be correctly oriented in the fuse holder, or the alternator will not charge the battery.
You mentioned several voltage measurements; how were these made (what kind of voltmeter), and under what condition?
If you have a good-quality (accurate) digital multimeter, here are the basics voltages you should see with a standard Toyota charging system:
1. At least 1/2 hour after engine shut-off, at a temperature of 66-72F, battery voltage should be ~12.6-12.9 volts (no voltage booster).
2. With all accessory loads turned off and the engine running at 2,000 RPM, the voltage at the battery terminals should be 13.2 - 14.8 volts.
Where did you measure the 13.7-13.9 volts, and under what conditions?
Finally, with engine off and all accessories off, maximum parasitic current draw should not be more than a few tens of milliamps. More than this, and you will have problems starting the vehicle after remaining parked for a week, even with a good alternator and a good battery.
Clean both battery terminals and clamps down to bright bare metal, and unbolt the battery ground connection at the fender liner, clean to bright bare metal, and re-tighten.
Verify voltage levels as specified above, and have the battery load-tested. It is possible to simultaneously have a dying alternator and a dying battery.