Lithium batteries often have a safety feature which prevents charging if the battery voltage has fallen below the safe voltage. You may have killed the battery by leaving it fully discharged. Follow the advice in If your Apple Watch won't charge or it won't turn on - Apple Support
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I let my watch battery die completely for three days. Soon as I put it on the charger the green snake of death comes back up after an hour or so on the charger the watch still show a red snake of death. I’ve tried different ports all through the house even my laptop. Getting very frustrated with this watch I just bought it in February 2018. Apple Watch series 3 with cellular
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Lithium batteries often have a safety feature which prevents charging if the battery voltage has fallen below the safe voltage. You may have killed the battery by leaving it fully discharged. Follow the advice in If your Apple Watch won't charge or it won't turn on - Apple Support
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Watch not charging .....