Depth of discharge depends on spare capacity, use case and autonomy of the system.

It's entirely plausible for remote autonomous systems to be discharged deeply during a rainy period of several days.

> It's entirely plausible for remote autonomous systems to be discharged deeply during a rainy period of several days.

And how often is that going to be, pray tell?

Lead-Acid has hundreds of deep-discharges available. And these deep-discharges in the use case suggested (ie: 12V 20Ah Lead-Acid backing up a 3W Rasp. Pi) take 80-hours to happen with zero-sunlight at all.

So... how many multi-day periods per year do you expect to happen where you deeply-discharge such a battery over 80-hours? Even at 100-times per year (aka: every 3 days you have a full deep-discharge), with a 500-deep discharge lifespan battery, we're looking at 5+ years of durability. Worst-case scenario.

I never said it's a scenario worth worrying about, but also 5+ years isn't anything to be proud of for when you're designing systems.