I'm heading out on a ride tomorrow, and seem to always have cell phone battery issues with GPS trackers, especially if I keep the screen on for any period of time.

My solar charger is a bit awkward, and seems to work better as an external battery pack than an actual solar device.

So, one of my headlamp batteries also has a USB port. I thought I'd check, and it seems to be putting out about 8.38v (no load) at the USB port. The pack would be extremely easy to tie to the handlebars.

However, that is quite a bit more than the typical 5V that USB is rated for.

Safe or not? Currently I have a couple of older Samsung phones, but I'd also rather not fry them, or their batteries.

huh, never checked any of my battery packs, but I would probably skip an 8v pack for usb

there are external batteries same size as the Phone and smaller..

Well, I decided not to use the 8.4V pack... yet.
I got one of those worthless replies from my query to Samsung.

Perhaps I'll try the phone that flew out of my shirt pocket at 30 MPH (and still works).

According to Wikipedia, USB power should be 5 volts +/- 5% or so. I'd say your headlight pack is perhaps purposely nonconforming. I sure wouldn't use it on a USB device.

On many devices, one is fine if the volts are close, and the amps really just need to be higher.

Some devices are designed to do their own voltage regulation, and can take up to 20V or so... But one never knows, which is why I sent the note to Samsung technical support, and was just blown off.

I presume most of the 12V ==> USB adapters actually have voltage reduction, and hopefully voltage regulation circuitry.

I think the 8.4 V light battery packs were just being cheap. Trying to give an extra option, but without adequate circuit protection.

What might be nice would be a super-sized battery pack, capable of say 5x the power of a standard battery pack, but would otherwise seamlessly fit into the back of the phone (custom rear cover).

"No load" is the operative term. Unless your headlamp battery has some sort of regulation that keeps it at 8.4v, it is probably unregulated, meaning the voltage drops when hooked up to a load, like a phone. If your headlamp battery has a usb output, that tells me it was intended to charge usb devices.

It's up to you, but I would carefully test my phone, constantly feeling for overheating (and looking for smoke). Since you are charging your phone battery with another battery, I don't think overvoltage for a brief time (again, while constantly feeling for overheating) will do any quick or irreversible damage.

The real question is whether your headlamp battery can handle enough current to meet whatever your phone draws. My guess is a headlamp draws more current than a phone, so you're probably OK. If it can't, you will notice your headlamp battery getting hot.

Ohm's Law.

Since the headlamp batteries are about 10x the size of the phone batteries, I would hope they can handle the load. But, yes, there is a possibility of some voltage drop. Also, voltage does naturally decrease as batteries are discharged. Maybe I'll check the voltage with the headlamp on high beam.

Seeing "magic smoke" may be the point when it is too late.

The thing likes to charge with the LCD on, and does sometimes heat up. The LCD is nice for ride tracking, but it sucks down juice.