Battery puzzle.

[recman]

https://i.postimg.cc/d14Yb2WG/20260828-141312.jpg

The black one is only a few years ago as opposed to the yellow one which I've had for over a decade.
The black one has been connected to a solar maintainer for extended periods whereas the other has only been on it now and again.
Both have recently gone through a mains powered charge cycle but the black battery won't crank the bike at all whereas the yellow one does, although it will only reliably do it a few times.
The solar panels output measures 21v so now I'm thinking I've buggered the newer battery by exposing it to too much power for too long.
I'm not entirely sure if there's an output controller built into the solar panel, I just assumed it would keep my battery from completely discharging while not in use.

Edit- I don't appear to be able to upload the yellow battery pic for some reason but it's reading 13.11v.

[twowheelwonde...]

Stick a meter on the battery terminals with the panel in full sun before you bin anything. Anything over 15v means there's no regulator and the panel's been boiling it for years. 21v open circuit is normal, it's what reaches the battery that matters. Bare panel with a diode will cook a battery dead flat.

[Kawasaki Jimbo]

Those voltages only look healthy because there’s no load. It’s likely you’d see them drop to 9 volts or less when trying to crank because the batteries have lost amperage. I think you’re right, the solar panel probably has no regulation so the batteries have been overcharged.