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in my case i have some 4v lead acid battery laying around.... and i have not used for a while so obviously they are dead or dried out.... i tried to search some way or result on how to refill it... but i got up with two answers.... which is really confusing.... also many videos states that they are using distil water to refill the battery and others are just using the liquid/acid of a new bought battery to refill the 4v battery......... so basically i couldn't understood what should i use to refill my battery, distil water or the liquid/acid of the new battery..../??

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    \$\begingroup\$ If it is "dead" as you say then lead sulphate will have formed on the plates. No amount of refilling with acid or water will recover the battery. It's dead and you should recycle it and buy a new one. \$\endgroup\$ – Steve G Jul 18 '18 at 11:53
  • \$\begingroup\$ but it is still giving me some volts... 1 is about 1.7v and another 1 is about 70mv \$\endgroup\$ – D vlogs Jul 18 '18 at 12:18
  • \$\begingroup\$ Sounds dead to me. Generally you top up a battery using distilled water. You would only add battery acid if it has leaked somehow. \$\endgroup\$ – Steve G Jul 18 '18 at 13:01
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If these are "flooded" cells - originally had liquid electrolyte sloshing around - then you should add distilled water. If they are gell-cells or AGM, then water won't help, as there is no liquid electrolyte.

If flooded cells have been unused and left to discharge for several months, the plates are probably sulfated, and the cells will not accept a significant charge, so should be discarded.

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