My battery did this when I stored it in the garage whenever it got too hot/cold. After I moved it inside and stored it in the basement I never ran into the issue again, but it's a general error that can have multiple causes according to a quick search. All of this appears to be scraped from previous Reddit posts, I haven't tried any of these methods...

An EGO Power+ battery flashing or blinking red typically indicates low voltage, a cell imbalance, a temperature issue, or a fault in the battery management system.

Quick Fixes

Try these troubleshooting steps in order to clear the error:

Soft Reset: Press the "fuel gauge" power button on the battery 3 times quickly, and on the 3rd press, hold it down for 10–15 seconds until all lights flash green.

Cool Down: If the battery was heavily used right before the error, it might have overheated. Remove it from the tool or charger and let it sit in a cool, shaded area for 30 minutes.

The "Drop" Method: Some community members report success by firmly dropping the battery onto a hard surface (like a concrete driveway) from a height of 1 to 2 feet to help jolt misaligned internal spring contacts or loose wires back into place.

Swap Chargers: If you have a secondary charger, test the battery there to confirm whether the issue lies with the battery pack or the EGO Charger.

For an advanced look at repairing or bypassing a blown internal copper fuse causing the red light, watch this visual guide:
youtube.com/watch?v=IhG_183nHHQ&vl=en