I have a couple of 12V sealed lead-acid batteries, which I have to charge. But when I hook one of them on the charger at 12V/1A (the battery is 10Ah) the voltage from 12.6 drastically increases up to 14V in 5-10 seconds. I tried hooking it up at 0.4A charge current. But it's no use, the battery just raises its voltage to the max. The battery is surely not ok, but what is the case here?
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.
The battery is most likely sulfated. You can try leaving it sit at the equalize voltage (15.0V for a 12V gel-cell battery) but it probably won't bring it back to life.
You can try searching out DIY de-sulfater circuits on the web - maybe they will help, maybe they won't.