I'm considering upgrading to a pair of Valence 40ah lithiums, these are used medical batteries that are fairly inexpensive.

I've been doing some reading, and saw (from lithium sellers, mostly) that lithiums offer 100% of available capacity, while lead acids offer only 30-50%. That would mean my current ~100ah batteries have a useful output of 30-50ah, so the smaller lithiums are equal.

Is that true though? I always thought amp hours were calculated using the normal discharge limits of the battery, not pulling it into damaging territory.