Some of asked me how I do this, so here are pictures. It's not pretty, but it's reliable and convenient.

I have a velcro strap, a thick rubber band, and a ball bungee. The velcro strap is closed at just the right girth to let the battery in while it's pressed against the rubber band, which I'm using as a bumper.



To mount the battery, I open the ball bungee and slip the forward end of the battery into the velcro strap. Then I tie the ball bungee around the battery.



To remove the battery, I pull it backwards. It snaps right out. The battery doesn't rattle because of the rubber bumper. I think it has fallen out once or twice on very bumpy roads, but it dangled from my phone, so I didn't lose it.

I know this is off topic, but nice lugwork.

I keep things like batteries in my handlebar bag. When it starts raining, I want to minimize the things I have to do.

It's a 1974 Raleigh International that I repainted with a brush, and not well. I masked the lugs with liquid latex, and that turned out to be a great idea. It just peeled right off.

There's a risk the battery will be damaged in the rain, but I'm not very careful about it, because the water has to get right in the right places. It's also not terribly valuable.

very nice
Rick

On my bike and driving style it wouldn't last many minutes on the bike.
So i do prefer Top Tube Bags:
sks-germany.com/en/products/traveller-up/
vaude.com/en-INT/Products/Bike-Bags/Frame-bags/Carbo-Bag
topeak.com/global/en/products/bikepacking-series/966-toploader
deuter.com/DE/en/bike-accessories/energy-bag-3290017-black.html
abus.com/int/on-road/Locks/Holders-and-brackets/Holders-and-Bags/Oryde-ST-2200?type=adp

is this for your dyno hub? phone? sorry I didn't get that part

In other threads, we've discussed how using a phone as a bike computer doesn't work because of battery life. This solves that problem. I once took a six-hour ride and set my phone to keep the screen on forever. I used two GPS apps at once, though I suspect the extra load from the second app is small. At the end of the ride, my phone's battery was still at 100% and the external battery was at the bottom. I had carried a second external battery in case the first didn't last, but I didn't put it to use.

Some things will operate while being charged, and some won't. My various USB-charged bike lights do, so I can also use the external battery to run a light. I notice headphones won't work under charging.

I don't have a charging system connecting with my dynamo hub. That's expensive and complicated, and I can't justify it with my kind of riding. I'm never away from a power outlet for more than an hour or two, with the occasional exception of a weekend bike ride, and this battery fits the need.

interesting method. I have some lipstick batteries that I keep in a twofish pump block under my stem. It works pretty well. I put a rubber band tightly wound on either end of the lipstick battery to keep it from slipping. But I have mostly moved to just having the battery in a rear pocket of my rando bag. I have a charging cable more or less permanently mounted to my stem with double sided velcro.

[MENTION=136384]unterhausen[/MENTION], can you clarify the way you mount the cable? I might like to do that, too. The Twofish block is a good idea!

Snap!!
I was just about to comment on the lugwork, but see I've been pipped at the post.

The International and your Super Course have me drooling.

I built my own battery pack for my headlights, with 5 rechargeable D flashlight batteries (in series), *
over 20 years ago ,
then LED lights became more efficient , so that became un needed..

And my commute of an hour home , changed when I moved in closer..

* Used the Dowel across the inside my British Saddle bag to attach it within..

I usually wrap the two sided velcro around the bike and stick it to itself so that it stays on the bike if I remove whatever I have used it to hold on the bike. Figured this out when I started to use it on pumps. Wrap the velcro around the bike, stick it to itself, then wrap it around the pump and stick the velcro to itself. I do the same with the charging cable, one wrap around the stem to hold the velcro to the bike, then run the cable over it and put another wrap of velcro. I have way too much double sided velcro on my bikes in case of emergency.

Thank you, but I'd like you to know I've built them with a ton of frugality in mind, waiting until things come together at the right prices. I picked the Super Course frame out of the garbage. That was a stroke of luck. I got the International frame in a trade for a pair of old wheels I didn't need.