Sump pump backup keeps screeching. Have replaced battery, charger, and still when plugged in keeps screeching. When unplugged, new battery shows to be fully charged.

SOURCE: Little Giant Sump Pump Backup System Alarm Keeps Screeching

is there water? sensor maybe bad, or pump failing to cycle

SOURCE: my battery on my samsung g600 keeps going flat,

send the phone away for a warranty repair thats if its still in the 12 months warranty. sounds like the phone. since your phone is working with the charger in it, that points to the battery but if you bought a new one then it has to be the phone unless you are very unlucky in getting 2 dysfunctional batteries. But the best way around this is to send it away under warranty

SOURCE: I replaced OEM battery with one from non-OEM

Install Latest bios update from support.dell.com.

Ensure battery has At least 10 % charge and Ac adapter is connected during bios upgrade.

SOURCE: We have an Aquanot II

https://app.qleapahead.com/rtp/libraryget.aspx?asset=88182,63

Above is the owners manual for your pump system. Included are phone numbers that you can call to find information.

Gary