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Simplest way - fill kitchen sink with water; turn light on and put in sink. Check every 15 minutes until light goes out. If light was on at 2:30 but out at 2:45; light is good for at least 2:30 of burn time. A battery in good condition will have 70-80% of its original life remaining.Hmm, how does one arrange a burn test before a sale? According to the seller she used to get 3.5+ hrs on a charge, and that it's been lightly used. Any way to test this, short of hanging out at her house for a few hours while it's on?
Use a bucket, not a sink.
Sinks have a tendency to drain, and if you walk away and forget about it, you might just have a ruined light when you get back.
Simplest way - fill kitchen sink with water; turn light on and put in sink. Check every 15 minutes until light goes out. If light was on at 2:30 but out at 2:45; light is good for at least 2:30 of burn time. A battery in good condition will have 70-80% of its original life remaining.
For the light you're talking about; if it works and the original 3.5 hour battery has at least 2 hours remaining, I'd probably buy it for $300 and be happy to get a year's worth of use out of it.