Would charge to about 12.3v but drop rapidly to about 11.8v after 5 minutes. 20 minutes and the low battery warning came on at about 11.5v.
Those are figures with the charging off, aren't they (ie, charged, but not charging)?
Anyway, a drop of 0.5 volts in the first 30 mins after the engine running is reasonable, depending on how much the battery charged, that ambient temps and a whole load of other factors (some people see this drop in voltage and jump to the conclusion that there is something awfully wrong, but that rough value of voltage drop is quite usual).
Was replaced with a Bosch S5 85Ah.
Bosch make quality batteries (well, I believe that Varta make the batteries for Bosch), so that should work well.
Do you know what physical size the battery is (ie, 110, 75, etc)? Which engine? And does that fill the battery box, or does it look as if a longer battery would fit, if you wanted to.
I'm concerned that mine is fitted with the wrong size of battery and it might suddenly fail, some time. So, I suspect that I might have to replace mine at short notice. Or, maybe, I'm worrying about nothing.
For anyone wondering, you have to remove the air box to remove the battery.
I think with my smaller battery, you could, just about, fit it without removing the air box (top section). But access is so much improved if you take it off, I don't know why anyone would want to. And, unlike the idiots who fitted mine, you actually use the clamp-down device, rather than just leaving it rolling around and the battery secured largely by the cables...