According to teaktown is a 3.7v one. They cite Ninty japan site as their source. Here is their very in deph analisis: http://www.tweaktown.com/news/55938/switchs-4310mah-battery-takes-3-hours-fully-recharge/index.html
Further Analisis:
http://www.tweaktown.com/news/55965/nintendo-switch-battery-life-analysis/index.html
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