Could one reason be that the seller had stored the batteries for too long so they had already lost capacity when being sold?
The two battery packs without the soldering issue run for hours and hours before a recharge.
I haven't tried the "fixed" battery out yet. I would only trust that when going for a walk at night, not for rides.
The demand for these batteries and lights is pretty high, esp. at the going rate of $20 to $25. I doubt they stay in the warehouse for very long.