The problem's (somewhat) solved. I removed the battery and it is dated May 2014, so I reckon it's done well. It had an isolator/cutout that wasn't working reliably, so that may have been a contributory factor and I've deleted it. But annoyingly (on the basis of when all else fails, read the instructions), my Ctek is a XS 0.8 with no Mode button. After reading the instructions again (i.e for the first time !), here's what it says:
The XS 0.8 is CTEK’s smallest 12V charger and it’s suitable for charging smaller batteries as well as maintenance charging normal sized car batteriesThe XS 0.8 uses state of the art technology for charging smaller batteries on motorbikes, jet skis, ATVs and lawnmowersDarn, probably the wrong charger, and I don't have many jet skis or ATVs -- buyer beware etc. Anyhow it is now showing fully charged, so I'm happier, and in a couple of days if it still shows being maintained I will re-install it in the car (not a job I want to be doing all the time). Alternatively a like-for-like replacement for the Bosch S5 004 battery is £70 delivered, so I suspect that's the way to go. Far better than being stranded somewhere thanks to tired electrical gremlins.
![[Linked Image]](https://www.tm-img.com/images/2021/12/05/Bosch-S5-on-charge-Dec-2021.jpg)