Unplug the battery and set aside (picture 1) to test its voltage.

Rather than test the voltage, the first time I unplugged the battery I immediately plugged it back in, powered it up, and found it had 100% charge.

Seconds prior to completing the above task, the battery had been stuck near 18% charge for weeks. Exposure to cold weather and potentially moisture at the battery connectors potentially played a role in this light's temporary demise.