I was never a big fan of carrying around lead/acid batteries. They are bulky and prone to short circuit in case of accidental contact with metal objects. Over a decade ago I started buying automotive jump starters, usually at Sears. These were still a bit heavy, and the alligator clips were huge. But if you could find one without a built in air compressor it would be easy to handle and easy to keep charged. Yes it was more expensive than, say, a motorcycle battery. But you also did not need to buy a battery charger and figure out where to put it in the garage.
Sometime in the last five years I noticed some Li-on jump starters that were even smaller. I tried one of these out and was (after some minor modifications) really impressed with how well they worked. They seem to last for three years, not unlike a lead/acid battery.
When I bought that first one I discovered that it had a built in safety interlock. It is basically a control module between the plug on the battery case and the alligator clips. Inside this module is a low current relay. If you were to connect the clips backwards, the relay would not engage, so no power delivered. Likewise, if the battery was totally dead there would be no voltage to trip the relay and no power delivered. So in order to use the unit as a field power source, I had to remove the interlock module. This required little more than a soldering iron and some patience.
I have been using multiple of these, several different brands, for years now. They charge with a wall wart or 12vdc car plug. My favorite model charges through a USB C cord. On your way to the launch and forgot to charge the altimeter? Plug it into the USB port on your power source. Headed out on a long recovery and your cell phone is low on juice? Plug it in.
I use these to power the launch controllers and 12v public address systems. Once upon a February morning I was having trouble getting good current. I stuck the battery in my pocket for ten minutes and once it warmed back up it worked just fine. They are not cheap but they sure are a good value. They are generally $50 and up. Most include a flashlight so you got that going for you. Oh, Mike needs a Jump Start? I've done that too.