I have three:
- Luxos U,
- Sinewave Revolution,
- AXA Luxx 70 Plus.

The Luxos U has a built in pass through cache battery, the others on my list do not. The Luxos U pass through cache battery is quite small and only has minimal capacity.

With the Luxos U I usually ride the bike for at least 10 minutes before I start charging anything to give it a head start. If you measure the amount of current going into your device (in this case your Garmin) it might be taking power out of the built in pass through cache battery faster than your dynohub can replenish the built in battery. In that case I have found on my Luxos U that everything is working fine until the built in battery is depleted. Then the Luxos U stops sending power for a few minutes to start to replenish the battery in the Luxos U and after a short while it then starts charing your device again. This on and off charging that you experienced is not ideal. And it is not limited to slow hill climbing, it can even occur on the flat when you are riding faster if your device can suck power out of it faster than the dynohub can produce it. When my Luxos U battery is depleted and the red light on teh handlebar button goes out, I unplug whatever I am charging for a while (say 15 to 30 minuts or more if I am on flat ground) to let the Luxos U internal battery catch up. If I am on uphills and going pretty slow, I just stop trying to charge devices from my Luxos U.

On my other chargers that lack the internal pass through cache battery, I use an external one. The ones that I use are no longer sold, so don't bother trying to find them. They are:
- a cheap solar power bank I bought from Ebay, shipped from China without a brand name.
- a Brunton Ember 2800 solar powerbank.
- and a Steripen solar power bank.

I think that the solar power powerbanks are designed so that the battery can accept power input at the same time that it is charging another device. I also had some small cheap powerbanks that did work as pass through cache batteries, but they all eventually died, the list above are the only ones I have that have not yet died.

Someone out there has a website where they gave high marks to a Goal Zero series of powerbanks that purportedly could be used as a pass through cache battery. So, I bought several and they DO NOT WORK with my Garmin model 64. In all fairness the 64 is very picky about the power supply, but because they do not work for me I would not recommend them to someone else.

And of course there are the cycling specific pass through cache batteries that cost a fortune.

So, I think you can either buy an external pass through cache battery that is big enough to supply your device while you are out hill climbing and use that with your Luxos U. Or, you can do what I do, just stop trying to charge your device for a whlle when the Luxos U battery become depleted (the red light goes out).

A lot of people that do bike touring will carry a large power bank and just charge that all day long, then at night they use that to charge their other devices. I instead opt to charge my devices while I am rolling instead.

As a third option, if you were trying to charge up your devices during daytime and then use the headlight at night for some long multi-day rando events, you might want to carry a larger powerbank and then recharge that powerbank during daytime and use that keep your Garmin powered up at night.

In the photo I am using the Sinewave Revolution (small white thing strapped onto top tube near head tube) to keep my devices charged up. There were not a lot of other places to plug in for power. And the road quality was not so great, so I was not going really fast. I bought the Sinewave for that trip because I think it might have the best waterproofing of all the USB chargers for dynohubs out there. I had not yet bought my Garmin 64 when I was on that trip, I was using an older GPS and using an Eneloop charger to charge up my AA and AAA batteries.