Sealed lead acid batteries

Your best bet would be to look up the specs on this. The figures vary a fair bit.

It's *an* indication, just not a *good* indication.

With a known tested battery, once you have a discharge curve outline, it can be used to give a reasonably good indication of where the charge level is.

However, and especially if you're going to abuse the battery, as it ages, the curve shape changes and will no longer give you a good indication (till you retest again anyway).

If good charge indication is really important in this application, doing something like what's done nowadays with a dynamically determined level (not sure if there's any official terminology for this) would be best. It continually estimates the level going on how much has been taken from the battery, based on the capacity that was determined last time it was fully discharged. Or something along those lines.

How long is a piece of string? You need to check the specs of the actual battery. However, pretty small ones are used in car jump start packs so they can be made to deliver a lot of amps.

It can be if the battery is good. But not if it's faulty. To check one properly you need to do a heavy discharge test. Most decent battery suppliers should have an tester for this purpose.

On a related note...

Conventional wisdom is that such batteries can be destroyed by a single "full" discharge -- regardless of the rate. This happened to me with the battery pack for a Sony DT-10. I accidentally ran it down all the way, and it would no longer take a charge.