So - I have installed new batteries.

I was up a few days ago, and the mains battery charger started behaving oddly when I switched it on. After a brief investigation, I discovered that the batteries had lost a lot of electrolyte. The tops of the plates were clearly exposed.

I topped them up and tried charging them again. This didn't work. There may be more extreme resuscitation methods I could try, but the original batteries were already a long way through their useful life and I didn't fancy having to rely on semi-dodgy batteries...

I had also been thinking about switching from conventional lead acid to AGM, so this seemed a good time to do it.

I don't really know why the electrolyte was low. I check it fairly regularly. I normally switch the charger off when I'm not on the boat, but did leave it switched on for a couple of weeks by accident recently. I find it hard to believe this was relevant, as I used to leave the charger switched on permanently - up until two or three years ago, when I learned that this might not be a good idea.

The charger is a 25 amp NOCO Genius charger, which should be safe enough to leave permanently on ...

The drop in the electrolyte level did look like the consequence of a boiling event, though.

Anyhow, I now have AGMs. So I don't have to worry about heel angles anymore. Or about topping up.

A small issue:

The Leoch AGMs are rated as '115 ah', and the Trojan batteries I removed are rated at '225 ah'. However, both are rated at 115 ah (or thereabouts) for a ten-hour discharge period. The higher rate for the Trojans is for a twenty-hour discharge. The arithmetic here doesn't quite make sense, because 10 amps over 10 hours is 100 ah, and over 20 hours is 200 ah... I'll need to go back through the specs to understand this.

I should still be able to discharge 10 amps for 10 hours and keep the bank (two batteries) about 50% charge. This looks fine for overnight sailing, for instance.

I have a Sterling Alternator to Battery charger capable of putting 90 amps (alternator maximum) into the batteries during the boost phase of the charge.

I also have a 200w solar panel to deal with lower demands.