In April 2019 I replaced the original Banner battery, it was 5 years old, so a reasonable life.

The new battery was a YUSA HSB075, from Halfords, with a 5 year guarantee.
In October it was going flat regularly, alternator charging properly, so Halfords replaced it, under warranty.
Now, a year later, it is holding charge for no more than a week, today after 8 days in the garage it is showing 6.2v, so on charge again.

Back to Halfords, but I'm getting a mite fed up with this. I'll get Williams to check the charging rate and voltage, I can't believe that the car is pulling any power with the ignition off: the only device that is "always on" is the radio, it needs a non switched power line to retain station memory.

Any thoughts, collective?

If your MOG has the factoryfitted immobilizer that‘s the problem.

The factory itself writes in the manual „if the car is out of use for more than three! days connect it to a charger.

My BANNER battery does her work after two years with a time out of use for a week.

My expierence with cheap batteries is very worse. They would work in a daily, but not under „toy conditions“.

René

Although Yusa appear to be a good quality battery, I would go back to Halfords and ask them to replace it with another make a offer to pay the difference if necessary.

Wouldn't Halfords be able to check your alternator was charging properly?

Ages ago my wife's Golf did this. We found an earth leakage on the headlight switch.
Good luck as this stuff is very hard to find. We only discovered it when the switch failed & the problem stopped.
Do you have a trickle charger? That might help

The car has no immobiliser...
I think the next step is to charge the battery and turn off the "master switch", all Aero 4s have this on the front of the electronics box.
If the battery still discharges it is the battery and yes, I'll go back to Halfords. But if it stays charged then it is a "car issue".....
It is booked into Williams next month for Service, MOT, etc, so they can see what they can find...

Joy.

Disconnect the negative terminal on the battery and bridge it with an ammeter to find if there's a drain.

Or you could find the keep alive fuse(s) for the ECU or BCU. Remove and put your ammeter set to 10A range across the socket(s). You might be surprised

ETA: if the battery has been allowed to drain to an incredibly low six volts it will have taken a fair chunk of its life due to sulphation. It might not even take a charge now. Whatever, I would keep quiet about that if looking for a replacement from Halfords.

Note also it's the battery that determines the charge current. The alternator is just regulated to a max voltage - you should be seeing at least 14.4 volts on a near fully charged battery at a fast idle, measured at the battery terminals ideally.

I have a clip on current meter that plugs into my Fluke which is worth purchasing for your workshop they are very handy bits of kit, far safer than disconnecting batteries and taking fuses out and it is a perfect interface/buffer for your meter.

Or this sort of stand alone device.

Peter, I have been told that if you store your ignition key to close to the vehicle when not in use, then some part of the electrical system doesn't shut down fully. I'm an electrical numpty so can't comment directly, but find it helps with mine to keep the two apart. I had no success with a Ctek and have reverted to a Halfords smart charger, which so far works a treat and keeps it starting happily.

Oh don't say that .... I just ordered one last night..........