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I have two 12V 4.5Ah sealed Lead Acid batteries from an old electric scooter. The scooter is a decade old, and packed up a few months after purchase. The batteries were literally left as is for almost ten years, and I am unsure if they were connected to the circuit or not during that time.

Okay, so the one measures 1.1V, and the other -0.6V! The negative voltage is very odd as the batteries were in series and I can't think how that could be physically possible through (dis)charge..

I do not have any desulphation equipment or similar, all I have is an alarm "UPS box", which slowly charges SLA batteries, although it never gets them above 13.4V as that is what it outputs when no battery is connected, also it's all linear - nothing fancy.

Is there anything I can do to resurrect these batteries with the equipment I have, or should I just have them recycled? I have a fair amount of knowledge in electronics if there's anything I could do. Also, are there any dangers hooking it up to my charger or hooking up one with a voltage too low in general?

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Recycle them. Lead acid batteries that have been sitting discharged for 10 years are pretty much spent beyond reasonable repair. Even when properly cared for, 10 years is a long life for a lead acid battery. Yours were abused at deep discharge level in addition. Dump them and move on.

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  • \$\begingroup\$ To expand on this -- batteries this dead could likely be revived by a sufficiently magical and expensive charger. This charger, if you break it trying, will cost you far more than new batteries. I learned that lesson with a 12v 7Ah gel-cell and an iCharger 308. \$\endgroup\$
    – Bryan B
    Commented Sep 14, 2017 at 13:19
  • \$\begingroup\$ and if you are feeling guilty about the environment: lead-acid batteries are remarkably recyclable, something like 99%. They extract the chemicals, melt the lead down, purify them, and turn them into brand new batteries. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 21, 2022 at 19:40

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