Honda autos have something called the Dual-Mode charging system, which sets the alternator at 14.4V when heavy electrical loads are present, but drops it back to 12.4V otherwise. The 12.4 V level leaves the battery only partially charged - about 60-70% - and I assume the thinking is that so long as it will still start the car, there's no point wasting gasoline keeping it charged any higher than that. Unfortunately, a partially charged lead-acid battery will soon sulfate up and fail. I had to replace the battery on my 2012 Civic twice over about five years. So any benefit Honda gets from quoting high MPG numbers is paid for by the customer having to buy a new battery much more frequently than necessary, now at $150 a pop.
So I've taken to driving with the headlights on, even during the day. That keeps the battery charged. But I was hoping to find a way to defeat the Honda system, and keep the alternator at 14.4V as God intended, so I won't have to waste gasoline on the headlights.
Earlier models (Civic 8th gen) could be hacked with the "ELD bypass", which consisted of strategically inserting an 820-ohm resistor into the Electronic Load Detector circuit in the fuse box, but apparently this no longer works on 9th gen and later cars. I was thinking of simply hacking the alternator, but it is now controlled over CAN bus, so that doesn't seem promising.
Well, I was hoping to find someone who had solved this problem, or maybe a Honda engineer who could offer a solution.