4pcs? of blade every edge can used if?one damaged by hard stone or other objectsRear wheel it heavier than S510,can keep from slippery when running on slopes and hills.Charging connection S520 has a better connectionfront wheel? it is softer than the old one ,against the vibration when working , with stainess steel holder ,against rusting

Brand Name:DEVVISCutting Width:21cmCertification:EMC,Other,cePower Type:ElectricityFeature:Self Propelled,Anti-Slip,Antistall,4-StrokeForward Speed:35m/minModel Number:E1800Type:Robotic MowersSmartphone App Control :YesWater-Proofed charger:YesAuto Recharged:YesRange Function :YesSubarea setting:YesMowing schedule:YesIntelligent Spiral /Go Straight :YesLCD display :YesPassword-Anti theft :YesBump sensor/ Rain sensor:YesOverload protect:YesMower Size:52*39*27cm(LWH)Charging Sation :48.2*15cm(LWH)Cutting height:2.5-5.5cmWorking capacity:1500m2+_20%Coverage (for one charging):500m2+_20%Battery :4.4ah lithium battery