The Wk03 works to charge flat top 18650. The host comes w a button top that is not intended for the Zebra.
The Red led is my own mod. I use it every morning for Retina therapy. Also at other times to relieve headache, sore muscles, and to speed healing of any scratches and bruises.
I recently discovered the Red Light also relieves the itch from mosquito bites. I just hold the light against the bite for a short time, less than a minute. After which I forget the bite is there.
I generally prefer lights without USB port, and instead prefer cells with integrated port, if I’m anticipating needing to charge, however, if you view the light with integrated charging as a (very, comparatively) rugged charger with light function, then I can clearly see the benefits.
My current “travel” lights (S2+, Sofirn HS10, Sofirn SP10-something, plus a basic Vapcell charger are a bit hap-hazard, it would be good to standardize on a single cell format.
On 18650 batteries, the ones w USB are 5mm longer than flat tops, and will not work in the 18650 Zebras.
otoh, for my 14500 lights, I like USB 14500 batteries. They are no longer than AA Eneloop.
but not all my 14500 have USB… so now Im going shopping for an AA light w built in USB, either with a Red LED or that can be modified 
this Wurkkos HD12 looks tempting:
How to charge an 18350 on the road…
Use the HD15
Aluminum foil will help you standardize.
Of course most rechargeable lights will work for this but why not carry a 90° light that can be used as a headlight or clipped on to various things to be hands-free as the backup.
Of course you do want to make sure you can recharge buttons or flat tops as Jon said. We keep the HD15 in both vehicles as it’s a true multi-tool.
Dats Tuff… !
Thank you
The HD15 does seem like a great option as a travel charger and battery backup for a Zebra.
I also like that the HD15 has Red plus White LEDs , and that it is a headlamp… all great features.
