Fast rising voltage on lead acid battery while charging
Sealed Lead-Acid batteries are usually arranged so that they only have just enough electrolyte (dilute sulfuric acid) to operate (often referred to as starved electrolyte).  This is to allow the battery to operate in any position, even up-side down.  Normal car batteries have much more liquid (flooded cells) and can spill if inverted.

If sealed electrolyte batteries are left for a long time to completely discharge all of the sulphate ions in the electrolyte will combine with the lead plates to form lead-sulphate so that the electrolyte changes into just water.

Pure water does not conduct electricity very well so that when you attempt to charge the battery the voltage will rise very rapidly and the battery will not charge.

This state is often called "sulfation" where the plates contain lead-sulphate that cannot be easily converted back to lead or lead dioxide as normally happens during charging.

Sometimes a small current will flow and the battery will eventually charge (days or weeks) but almost certainly the battery will never regain its original capacity. I have never succeeded in recovering a battery in this state.