My car is old enough that it came with an iPod 30-pin connector to feed the audio system. I currently have a ridiculous series of adapters to allow me to listen to iPhone 6s and simultaneously charge the iPhone.

The iPhone is in a Car mount , with Lightning port provided by an official Apple Lightning to 30-pin Adapter (0.2 m).

Since the Apple adapter has a very short cable, next there is a 30-pin extension cable (male to female).

That is long enough to reach the 30-pin socket in the glovebox, but the car only provides FireWire iPod charging voltages. So yet another adapter injects 5 volts into the 30-pin system feeding the iPhone via a mini-USB side port.

All this actually works (although I am embarrassed to have implemented such a thing), with the limitation that the iPhone only charges at the rather slow rate of 2.5 watts (actually less than that, probably due to all the cable resistances). This is the same rate it would charge from a straight up original USB 1.0 or 2.0 port, before all the fancy 'faster' USB charge rates became a common thing.

Is there a way to trick the apple Lightning to 30-pin cable adapter into allowing higher charge rates? What might be required?

I certainly can provide a high current source (say 2 amps) into the mini-USB side port on the DockStubs+ adapter, but it is unclear to me where the gating factors would reside that signal to the iPhone how much current is available.

I suspect the charge rate limitation is baked into the Apple 30-pin to Lightning cable adapter, but the Internet has been unable to reveal any information in this regard (at least, not from my searching).

Is there a spec for the 30-pin connector that allowed higher charge currents from the 5 volt source?
Update: This pinout shows USB data pins 27 and 29. Whether those have the same function in 30-pin world I cannot be sure.

I dug out an old 30 pin to USB cable (pre iPhone era one), a 30 pin to lightning adapter, a 12W iPad charger and a Kill-a-watt. Plugging this all into an iPad Air 2 revealed a 12W charging rate, so exactly the expected 2.4A 5V charging rate. Tried the same with an iPhone 6s and did see it go up to 6.2W on the same setup.

I'm guessing it's not going to the full 12W rate due to the battery not being below 80% like the iPad was. It's at least validation that you should be able to get it to work at more then the 2.5W level you are currently seeing. Letting the iPhone drain a bit and I'll check again later to see if it ramps up to the full fast charge rate.

Wow, thanks for doing that.

I can add that I did probe the mini-USB adapter and the data pins D- and D+ do pass through to the 30-pin connector, pins 27 and 29. So I do see the appropriate signaling voltages for charger current capacity, at least to where the 30-pin mini-USB adapter feeds into the 30-pin extension cable.

Since all of this gear is trapped inside my car dashboard it is no fun exposing each connection point. Even the 0.2m adapter cable has the 30-pin end hidden inside the dash.

The USB car charger I have in there is a ZAGG dual outlet, and both sockets have the 2 amp (10 watt) data signal voltages. When I plug my iPad or iPhone into the other outlet I do get close to ten watts charge power. Both ports are the same and swapping ports makes no difference to the results. I am only testing with one charger port at a time, no dual loads.

I am using a DROK OLED USB 3.0 Meter to monitor voltage, current and power flow into the load device.

It is unclear to me whether the monolithic Lightning to 30-pin adapter is electronically the same as the version with the short flexible cable.

I just realized I did not (because it is buried in the dash) verify that the USB signaling pins (27 and 29) within the 30-pin extension cable actually extend to the far end of said cable.

The iPhone was below 80% charge during my testing, and the Drok USB tester is quite useful for this sort of thing.

Side note: I do wish Apple would expose the charge rate on the actual iOS device, would make charger validation and checking so much easier. That little lightning bolt battery icon doesn't reveal enough.