iPhone resetting at 40% after installing a new battery

Hey, I recently installed a new battery in my 6s, but it now resets at anywhere between 30-50%, when it turns back on it reports either 1%, 10% or whatever % it was at last. I believe I installed it correctly (fullcharge 8hrs, full drain, full recharge, reset phone using power and home button). A low battery log is below. Any help is much appreciated. Thanks

Doug

{"bug_type":"120","os_version":"iPhone OS 13.6 (17G68)"}

Incident Identifier: 1FF01042-711D-43CE-9373-DBD4C8BB1404

CrashReporter Key: 3715515bb36c1781301c1f0507a7d67523f1a2d7

Date: 2020-08-01 06:41:45 +0100

OS Version: iPhone OS 13.6 (17G68)

runningboardd: daemon<com.apple.SpringBoard>28-55-3:FBSystemApp-PreventIdleSleep SystemIsActive == 255, held for 00:00:24

mediaserverd: com.apple.APEndpointManagerCarPlay.timed PreventUserIdleSystemSleep == 255, held for 00:00:10

locationd: com.apple.locationd.CLLocationAwarenessProvider.159052520(Active location session has begun) NoIdleSleepAssertion == 255, held for 00:00:06

apsd: com.apple.persistentconnection[apsd,121,APSDaemon-courierconnecttimer-PendingSimpleTimer(0x102763b70)] NoIdleSleepAssertion == 255, held for 00:00:02

apsd: com.apple.apsd-connectcouriers NoIdleSleepAssertion == 255, held for 00:00:02

nfcd: nfcd[pid:139,_NFSecureElementManagerSession,sessionID:0] PreventUserIdleSystemSleep == 255, held for 00:00:02

Screen Brightness: 0.247989

Hardware Model: N71AP

Awake Time: 00:00:00 (0)

Standby Time: 00:00:00 (0)

Partial Charge: 0

Capacity: 51

Voltage: 3486 mV

Voltage Droop Time: 40233984

Voltage Droop Transitions: 24