Testing MacBook Air Battery

I have a MacBook Air, Early 2015 model, that suffered water damage last week and now only works when plugged in. The Apple System Diagnostics test shows that the battery is dead - or more precisely, that there is zero current flowing from it; more on this in a moment - but that everything else in the computer is working. Led to believe that the Genius Bar has better diagnostic equipment than I, I took my MBA in for an appointment, where upon hearing the words, "Water spilled" whisked it off to the back, returning 15-20 minutes later with the proclamation that yes, there was water damage, 4 sensors worth indeed. They also told me that they tested the battery, and the batter was fine; the damage was to the I/O controller, so the only way to fix it was to replace the motherboard.

What I want to know is, can they test this at the Apple Store, and if so, can I test it at home? There was something about the guy I was working with that was a little off, like he was defensive when I tried to ask him more detailed questions, and this detail about the battery being fine only came out at the end of the conversation, as I tried to work with him to understand my options fully.

There is a big difference b/w a hosed motherboard, at which point the combination of cost and complexity of repair means I might as well get a new machine at around $1k, and a new battery at $80 that I can replace myself.

The way I would naively test it is to swap batteries with another macbook air - I have my previous two, both of which are from the same period from 2010-2017 which an iFixit product page claims should have compatible batteries. Does this work? Is this risking any sort of damage from putting non-working batteries into a working power system, or vice versa?

Thanks!!!

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Here's the full story of what happened:

I spilled water through the vents at the back of my MacBook Air. I had fallen asleep at the table at the end of a late night work session, and knocked the glass over in my sleep, so I don't remember the exact state of the computer before the water hit it - ie on or off - but I believe I had been asleep long enough that it was probably off or asleep as well (ha). I immediately turned it upside down so water would drain out, and then pulled out my handy Torx and Pentalobe screwdrivers, took off the back, and drained a scary amount of additional water out. Next I took out the battery, again finding significant amounts of water underneath and around it. At that point I very gently with the edge of a paper towel, after grounding myself, soaked up any obvious, visible water, I considered taking more components out to help things dry even more, but when I started reading the instructions on iFixit I realized that the risk/reward was no longer in my favor, particularly given the working environment available to me (a living room floor, no anti-static or grounding equipment). Unfortunately, it was only at this point that I read that I was supposed to handle the battery only be the plastic around the edges and not touch the exposed lithium ion battery packs. Oops.

I left the MBA open to dry for about 36 hours before putting it back together and booting back up. As stated previously, it works fine as far as I can tell, when plugged in, but dies immediately as soon as the plug is removed.

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