Lots of info here. Most of it "enough right" to be troubling.
The state of your electrolyte depends on the charge state of the battery in question.
If the battery is dead, the sulphate from the acid is stuck to the lead plates and the electrolyte is mostly distilled water (about 2% acid and 98% distilled water). Just top off the battery with distilled water.
If the battery is FULLY charged, there is NO sulphate on the plates and the electrolyte is mostly Sulphuric acid (SG 1.277) of a 36% acid 64% distilled water concentration.
At 50% charge (1.172 SG), the ratio of acid to water is 24% acid and 76% distilled water.
ANY loss of acid will effect the fully charged capacity to some unknown degree. Only by doing a fully charged specific gravity test will determine the capacity reduction you have experienced.
Adding ONLY distilled water is the safest method
unless you have titration facilities to calculate the exact amount of acid to replace for full functionality.
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