Paul Remde mentioned that he replaces his batteries after two years of use.

The sealed lead acid batteries we use, start life with a given capacity,
rated in amp hours. After manufacture, and during the first several
charge/discharge cycles, these batteries will actually gain a small amount
of capacity. After that, it's all downhill, with the battery losing capacity
over time and discharge cycles.

In the alarm industry, we replace the batteries at 5 year intervals. At this
point, most of the batteries will be down to about 70% of capacity. However,
these batteries have a pretty easy life, rarely being called upon to provide
power and generally not bieng discharged deeply.

In a glider, life for a battery is tough. Deep discharge cycles and wild
temperature swings being the norm. Generally, discharging a lead acid below
80% charge will have an impact on its remaining life, below 50% is not
recommended. So life will be short, so to speak.

I agree with Paul, replacing batteries often is cheap insurance. I change
mine at 3 years and always store them fully charged.

all the best,

bumper (12 volts and proud of it!)