Hi started using a battery tender for the winter months, optima came with my bike cocoon. Any way it’s been on for about a week and keeps it at around 13.4volts,
I took it off today to go shopping and within 20mins the batt had drop to 11volts to low to start.
Now never had this problem before the car would sit for a week with no problems.
It’s been on all afternoon and up to 12.9volts but I can watch it dropping on the multi meter.

Battery is not connected to the car V8 facelift Range Rover Autobiography 22" wheels.
1976 Harley Davidson Iron Head Sportster

That's pants!

How old is the battery? There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
KIA E-Niro 4+
2021 Discovery 5 D300 MHEV commercial

4 years this month, bosch silver V8 facelift Range Rover Autobiography 22" wheels.
1976 Harley Davidson Iron Head Sportster

Take it back.

They have a 5 year warranty  There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
KIA E-Niro 4+
2021 Discovery 5 D300 MHEV commercial