Hello, noob here.

I recently had to isolate due to corona. This meant my 2015 Fiesta was left for 2 weeks turned off, a large proportion of the nights below -0C.

Lo and behold when I tried to start it up afterwards, it just wouldn't. Not even an engine crank to work with. All exterior and interior lights and dials worked fine however.

I called the RAC who (After 4 hours) turned up, plugged their battery pack thingy onto my battery and gave it a boost. I'd successfully turn the engine on and keep it running on idle for 10mins (throwing in the odd rev here and there). We then turned the engine off, and he saw how much battery drained after I turned the engine back on again. We did this procedure 3 times. After the 3rd, my car was ticking along on idle, with the battery neither a 'pass' or 'fail', but an in-between 'critical' according to his machine. He suggested I get a new battery (which would be a stop-start EFB type for my Fiesta) to be safe. After all, for a 2015 reg with an OEM Varta battery in it (60Ah, 590A, 12V), 6 years of battery life sounds about right? (current battery photo attached).

Now before I buy a new £110 battery + £25 mobile fitting, I ran a battery test myself (hold trip reset button + turn key, then cycle through car status settings on trip display). With engine off, the 'bt' (battery test) value was around 115 (guessing that is 11.5V?), and after I turned the engine on and idled, it went up to 145 (14.5V?). Now from reading around, those values appear normal.

Question- given these in-car values on the battery test appearing normal, do I need to buy a new battery or not? I'm happy to buy one, but don't want to get one too prematurely if there is plenty of health still left in it. Perhaps any car left for 2 weeks alone, in freezing temps would suffer in a simliar fashion to how mine did? Thanks!

 

(side question, the ford manual says get 'exactly the same specification' of replacement battery. Now I can't find exactly the same... a suggested one on Halfords website (with the correct dimensions don't worry) is 65Ah and 650A. (Up from my current 60A, 590A) Is that rise in values ok for my car? Maybe it's better for my car?!)

I wouldn't trust the in car values for battery health, I'd get a multimeter out and double check. That being said, 11.5v is too low and will struggle to start the car. You need it to be 12.5v (certainly above 12v).

The RAC man can also place a load on the battery and measure that, where as a multimeter and in car measurement cant. I'd trust his machine saying critical (car batteries are either good or bad, no inbetween. If it's not saying good, then it's bad). Given you also have less than 12v before starting the engine, I'd be going for the easy and cheap option of a new battery.

The battery you listed will be fine, provided it's an EFB (for the stop/start). It just means the new one has slightly more capacity than your current one (which is a good thing).

Had exactly same problem as you did

If he has jumped started it and left it running its not putting much charge into the battery, idling the vehicle doesn't  really charge the battery as its only turning at 800 rpm, i would put the vehicle on a mains battery charger with a 5amp charger overnight or for around 12 hours and see how you go.  then test it, testing a flat battery will 100% fail a test, even new batteries that are flat ( under 12.5v ) normally fail a test unless fully charged. 

Ok I've gone ahead and ordered a new EFB stop-start battery. Thanks for the help everyone!