bikebikebike

Is there a problem with device operation that makes adding external power problematic?
Some devices, eg: phones, Varia lights, allow for this , yet many will not operate when a charging port is powered up.
Is there a problem with the internal power management that presents a challenge here?
I could postulate several, or is it just more convenient not to have that feature?

Steve B.

The 2 Garmin's I own (1000 & 1030) as well as the Hammerhead K2 I own can all operate while connected to an external battery.

Iride01 

Devices use to only want to one thing at a time. Perform their intended function, charge or upload data. They didn't have the smarts to be able to handle all that at the same time. So various conventions came about to let them try to figure out what they were connected to. If they were connected to something that supplied a lot of amperage, then they must be connected to a charger, therefore they only charged. If it was connected to a low amperage thing, the maybe it was a computer, so they went into data transfer mode. But they would still slowly charge too. It's getting somewhat better now that many of these devices have more sophistication to them.

Not all USB cables are created equal. Some have a resistor on one of the pins to let the device figure out the things above. Trying different usb cables might give you one that will fool the device to thinking it's something else sometimes.

unterhausen

It's a little more expensive and takes more work to design a pass through charging system, so in order to save money, a lot of devices don't allow it. A lot of devices that seem to be expensive to us have very cheap internals. I imagine they don't think it will hurt sales to delete this feature.

xroadcharlie

When I connected my Garmin GPS to the USB port in my car with the wrong cable it tried to interface with the cars computer and of course failed because it's a charging port only. It charged but wouldn't work. With the correct cable, One that only has charging contacts it both charges and works correctly.

canklecat

Ditto, the above. Some devices, such as the Light & Motion Rando 500, can be charged and run simultaneously, but only at low power, and only with a special USB cable. With ordinary USB cables it can only be charged.

I've checked all of my devices -- lights, computer, video cameras -- and only some can operate normally while being charged from a USB battery pack. Price of the item doesn't appear to be a factor. I have an inexpensive helmet light that can be charged and run simultaneously.

unterhausen

I think if something is really cheap, they don't even bother to check. It might not be a great idea to charge while using though.

bikebikebike

Thanks to all.
You helped me clarify the question I need to ask of cable and device makers/vendors.
The Wikipedia page on USB Hardware, in the section on charging has the details alluded to above.
https://en.wikipedia.org/wiki/USB_hardware#cite_ref-57
So many details don't get presented on consumer goods, or are even made hard to find.
It's easier when you get to a good/the right question and know where to look.
Thanks again

canklecat

FWIW, "smart" vs "dumb/plain" USB cables can often be spotted by comparing the sizes of the plug housing. Plain/dumb USB cables have smaller plastic plug housings. The "smart" USB cable included with my Light & Motion Rando 500 has oversized plug housings colored orange.

Some of my other devices have special USB cables, but these aren't color coded, or the plug housings are only slightly larger than plain cables, so it's not easy to spot the special cables after I've dumped them all into a basket or drawer. I try to tag the special cables but forgot to do so with my bike computer USB cable. Without the special USB cable I can only recharge the bike computer. With the special cable I can transfer data via USB to my computer, rather than just wirelessly.

Unfortunately all of this complicates the USB standard, which was originally intended to minimize confusion over varying cable and plug standards. The only remaining standard is that most USB cables will recharge most USB rechargeable devices.

bikebikebike

The wonderful thing about Standards, is that there are so many to choose from.

adamrice 

FWIW, I have a multi-head USB cable (handy for travelling or touring) that has a convertible USB-A/USB-C tip at one end, and four tips at the other end: USB-C, USB Micro B, and two Lightning tips. Only the USB-C tip carries data, and is marked with the USB logo; the others only carry charge, and are marked with a lightning bolt.

On the original subject, I have a relatively nice Lezyne headlight that does not operate while charging. Lezyne does sell a special external battery to extend its runtime, but that plugs in through a barrel connector rather than the USB port used to charge the light.