The Lenovo 5B10W51870 is an internal rechargeable battery pack for a laptop, intended as a replacement inside a compatible device. Its part number and battery model help identify the pack for service and parts records.
The manufacturer identifies the battery model as L21D4P72 and describes the pack as lithium-polymer. It has a 52.5 Wh energy rating and a battery voltage of 15.48 V; the distributor lists this exact part as a four-cell battery. No compatibility with specific laptop models is stated.