So I've got a 110Ah AGM leisure battery (Varta LFD90, been in use about 3 years) connected to a 200W solar panel via a Victron SmartSolar 75/15 MPPT controller. Over summer it was brilliant — sitting at 12.7–12.8V fully charged no problem. But since October it's been struggling to get above 12.4V even on decent sunny days, and overnight it's dropping to around 12.1V with only a small 12V fan and phone charger running.
I know solar input is lower in winter obviously, but I'm in Yorkshire and we've had a few clear days recently where the panel was putting out a solid 8–9A for a couple of hours and I'd still expect it to recover better than this. The SmartSolar app is showing absorption completing fairly quickly — like within 20 minutes — which I think means either the battery's already full (doubt it) or it's not holding charge properly anymore.
Has anyone seen this pattern before a battery completely gives up? I did a rough capacity test — ran a known load overnight and it looks like I'm only getting maybe 55–60Ah out of it before it hits 11.9V, so roughly half rated capacity. Three years of fairly heavy seasonal use, mostly partial state of charge. Is that about the lifespan I should expect for an AGM in this kind of setup, or is there anything worth trying before I just replace it?