Went to start the car today and it was odd. I opened the boot and the alarm went off (I had key).
Got in hit start and the dash lights came on and flickered and the engine didn't even turn over.
I don't do many miles at all now and the subaru has a quite a significant draw on the battery so the thought that the battery had drained didn't seem that odd (been through learning process that the power tailgate goes funky with a slight voltage drop).

I have a booster so I hooked that up and the car started first time. I'm still thinking flat battery.
Drive the car into the garage and hook up the charger. It has a test mode and I ran that first....battery 77%.
Let it charge for 10 min the got in a tried again, car started no problem.
Drove to the tip (with booster) and car started again no problem leaving that.
Got home, washed cars and went to put Subaru back in the garage. Again started no problem.
Hooked up the charger again and this time the test said 66%.

I've left the car on the charger now (I usually do bth due to lack of use).

I'm wondering if the battery might be on the way out. It's 5yr old. I know the tests can be misleading with chargers showing a dud battery as good.

I strongly suspect your battery is failing, have you tried a load or cranking test?

4 hours ago, Warrior193 said:

I strongly suspect your battery is failing, have you tried a load or cranking test?

I’ve not got kit to test tbh.

If you don't have a multimeter, can your charger read voltage while it's not charging? If so, leave it clamped to battery terminals while not charging and get someone to crank the engine over on the starter - voltage falling below around 10 volts indicates failing battery.

A decent multimeter need not be expensive and is a very useful item for the toolbox.

16 hours ago, Warrior193 said:

If you don't have a multimeter, can your charger read voltage while it's not charging? If so, leave it clamped to battery terminals while not charging and get someone to crank the engine over on the starter - voltage falling below around 10 volts indicates failing battery.

A decent multimeter need not be expensive and is a very useful item for the toolbox.

I did have multimeter but operator error casued it to blow up 🫣
Good call on the V drop on cranking. Charger does have a V display.

Tried this.

Charger showing full battery. At 12.1v with the charger idle.

Voltage dropped to 10.1V when cranking. Up to 14.3V with engine running.

Any advice on the numbers welcome.

Got a lidl multi meter at the weekend so possibly can do more now.

Provided there is no load on the battery, 12.10 volts on a EFB/AGM is less than 60% charged.

10.5 volts on cranking is reasonable given the relatively low SOC.

14.3 volts on charging is a little low - but again may be due to low SOC of battery.

What charger are you using?

Battery should have been 100% charged. Had a long (80mile) run the other day and it’s been on the charger since.

Might just change the battery anyway. They are not too silly prices for the Subaru. The car is very low mileage for age (21k in 5yr) so may have had the battery discharged in the past

2 minutes ago, Aspman said:

Battery should have been 100% charged. Had a long (80mile) run the other day and it’s been on the charger since.

Might just change the battery anyway. They are not too silly prices for the Subaru. The car is very low mileage for age (21k in 5yr) so may have had the battery discharged in the past

Is your Scoob battery a standard lead-acid - or EFB/AGM?

3 hours ago, Warrior193 said:

Is your Scoob battery a standard lead-acid - or EFB/AGM?

Can’t see easily without removing the bracket. The car does have start/stop built in so i goes it will be an EFB or I’ll get an EFB to replace