Measuring battery volts without a load does not give you a good indication of battery condition.

You need to give the battery a significant load, (like headlights) and look at voltage drop, which will then indicate the internal resistance, which limits the current that the battery can deliver (CCA ?)

Some of us may remember the battery testers that looked like a spring between two prods.......

I have five car batteries, one on a solar charger, a lead/acid and three Ca batteries in cars. The Pb battery sits in a car with light use.  I use a fog lamp unit directly connected to the battery.  None of my cars have "Smart" charging as such. (not that I would call anything smart that fits a big battery, and then does not fully charge it in normal use...)

 

Ca 1 has recently been replaced.....  Not yet measured the replacement, time I did !

H11 Load 55W Bulb	Battery Capacity	No Load V1	Load Current A	On Load V2	Voltage Drop V3 (V1-V2)	Battery Internal Resistance m Ohm (=V3x1000/A)
Solar/Ca	390/43Ah	12.44	4.27	11.58	0.86	201.41
Pb	590/60Ah	12.28	4.25	11.93	0.35	82.35
Ca 1	590/60Ah	12.43	4.29	11.91	0.52	121.21
Ca 2	500/52Ah	12.47	4.31	12.29	0.18	41.76
Ca 3	500/52Ah	12.40	4.30	12.20	0.20	46.51

1 hour ago, Paulkp said:

Solar/Ca

typo ?

My poor abbr......Calcium battery in a box with a small solar charger permanently connected, with the other 4 in cars.

Partial success. From the Haynes manual I found the lack of 12v on a fuse feeding the panel and the GEM that should have had volts with the ignition switch in position 2. I changed the switch today (long delay due to delivery by Evri!), and the panel now turns on and the engine turns as it should. I believe the engine might have started if it was not such an icy day, because it did seem to fire a couple of times when I first turned it, but would not pick up. I warmed up the new battery and tried that, but now it just turns without firing. May wait till the weather warms a bit and try again!

35 minutes ago, SteveJS said:

I believe the engine might have started if it was not such an icy day, because it did seem to fire a couple of times when I first turned it, but would not pick up. I warmed up the new battery and tried that, but now it just turns without firing. May wait till the weather warms a bit and try again!

In these near-zero temps, the engine will benefit from glowplugs.  Switch the ignition on for 5-10 seconds before turning the key to start position.