I find batteries to be a bit of a 'dark art' at times, I've had ones that I've Molly-coddled but they've died premature deaths, and others that''ve done it hard and lasted long past their u.b.d (mainly standard lead- acid), an example of that is a cheap 6N6-3B that's been in a Honda CL90 that I bought for a project with my youngsta mid-90's, used as a total-loss ignition for a while and then sat unused for at least 18 years, probably 20, until I dug it out a couple of months ago and the battery showed 0.7 volts, but I put it on charge overnight, and it's been holding ~5.9 volts ever since....(as per pic). probably ruin much load for looking, but maybe make a spark, and recharge if I can get a charge....
I know enough to be dangerous, but've been led to believe that AGM as above, but in electrolytic gel rather than SLA is the general best option...I have a Duracell labeled gel-AGM in the Gutzzly, that is supposedly a Motobatt in the US...
, but still goes flat if I don't treat it right....
Having said that^^, finding gel- AGM in 6v may be harder to find