This is really a separate topic, but from the above I bet you know the answer: What type of charger should I buy (yes, I can use the stock charger, but I also want a car charger, and probably a multiport-charger....)? This is my first device with USB>2 charging, so I haven't had to learn about this yet, and what I read is confusing. I don't see any chargers for sale advertising compatibility with Pump Express. They are all either QC or USB-PD. Are either or both of these compatible with Pump Express? Do I just have to match volts and amps? How do I know if the Cosmo can negotiate the volts and amps it wants with the charger?
Thanks!
Summary: Any 5V ~2A USB charger from a reputable brand will be OK.
Details: For
non-USB chargers, it's really important to match the voltage, polarity and connector, and to make sure the charger (and cable) can deliver enough current (enough Ampere). It's usually not a problem if the charger can supply more current, but there may be exceptions to that. Luckily, USB simplifies those things a bit, but adds other complexities...
Any USB charger for a car or mains power must support the original standard of 5V and 0.5A. That
will work with any USB-charging smartphone, including the Cosmo, but a full charge would probably take a the whole night or more. For keeping a smartphone topped up, a basic 5V, 0.5A charger should be OK, as the phone draws very little current when its nearly fully charged.
The next step up for you, is any USB charger that can give ~2A (at 5V), because that's approximately as much current as a Gemini or Cosmo can draw, when the battery is close to empty. It's no problem if the charger can supply more current (say, 3A or 5A), as the phone will only draw as much current as it wants. It might be a good idea to check that the USB cable used can handle 2A. Otherwise it might cause a voltage drop, making the charging unnecessarily slow. Using a cable rated for more current, is totally fine, thought it might be a bit clumsier and more expensive, for, in this case, no real gain.
USB Fast Chargers that can supply more than 5V will always start at 5V and will only increase the voltage if the phone tells it to. Thus it's harmless to use a USB Fast charger supporting the wrong fast charging standard for you phone. The charger will just not understand what the phone says, and will stay at 5V.
While
newer versions of fast charging standards, such as QC 4.0 (Quick Charge, created by Qualcomm -
https://en.wikipedia.org/wiki/Quick_Charge) and Pump Express 3.0/4.0 (created by MediaTek) are more or less compatible with USB-PD (USB Power Delivery, the "official" standard -
https://en.wikipedia.org/wiki/USB_hardware#PD), the Gemini and Cosmo uses Pump Express + (basically version 1.0), which is both old and uncommon, so unless you can find a car charger with support for Pump Express +, your next best option would be any 5V ~2A USB Charger. That's what I'm using most of the time, as it works decently with almost any phone or tablet charging via USB (laptops charging via USB, usually requires some version of USB-PD to get enough power to even begin charging).
While, at places like aliexpress, there is plenty of chargers being advertised as supporting Pump Express,
BEWARE, as there's usually no certain way to tell safe, sane chargers apart from dangerous junk. Usually they doesn't specify which version of Pump Express they support, anyway, so chances are that, even if they're safe, they will be no better than a plain 5V ~2A USB charger. If, on the other hand, you can find a charger specifically supporting Pump Express + from a reputable brand, please post a link here on OESF. Best of luck.
Edit: Oh, and, if using a Pump Express + charger in the left-hand side port of a Cosmo or Gemini, it will ask for 9V at ~1.7 (when the battery is close to empty) and thus charges at around 9 x 1.7A = 15W. The Planet Computers chargers also supports, per the Pump Express + standard, 7V and 12V, but I've never seen my Gemini ask for it, and I don't think the Cosmo does either. I've heard that the right-hand port of a Cosmo only supports regular (5V) charging, and will, presumably not ask for more, regardless of charger. A Gemini won't changer at all from its right-hand port (but if I plug a charger in there, it still draws a few hundred mA, so it's doing something).