[brucepick]

Good advice to monitor the 12V system.

Expect to see this apparent oddity:
Voltage dropping from the "charge" level of 13.x or 14.x, down to 12.5ish, then dropping gradually to about 12.1V before popping back up to the charge level, usually 13.x V.

That is done deliberately by Honda's ELD circuit, which they've been using for something like 25-30 years. The idea is to avoid the load on the car's system, that comes from constantly keeping the 12V battery at the 14V that normal alternator systems put out. Energy gets wasted as heat that way. ELD = Electrical Load Detection. It's essentially an amperage detection circuit, along with voltage detection. As long as volts is above the threshold (which seems to be 12.0V on my display), and current use is below some preset limit, it doesn't charge the battery. Once the 13.x charge system is cut off, V will slowly float downwards, often taking several minutes to reach the low limit cutoff. Then the ELD system reactivates charging, and it goes back to 13.x V.

The fact of the battery absorbing charge is one of the "loads" detected by the ELD system. As it approaches being fully recharged, it draws current more slowly. So when that load, plus whatever else is running, comes back down to some preset level, the ELD system discontinues charging and system voltage gradually drops again. It's a cycle.

There seem to be a few built in bypasses to the logic. If headlights are switched on, it jumps to full charging voltage as long as they are on. Early examples of ELD let the headlights dim, then they'd brighten again under full voltage. Was very disconcerting to drivers. Also it stays at full charge voltage for a while after startup, just on principle. Heavy use of fans, wipers etc. would likely be enough load to keep it at the full charge level. Most stereos don't use enough current to force it into charge mode, but if you have big subs and a separate amp - that probably would keep it running at the 13.x V level.

If you swap in a significantly larger or smaller 12V battery, the rate of voltage drop will change (a higher capacity battery will have the voltage drop more slowly). But the system still functions.