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I've an old lead acid battery with the following specs:

  • 12V, 7.2Ah
  • Valve regulated
  • Sealed lead-acid type

I've no idea about batteries prior. This battery has been in the attic so far and I would like to use it now. But, it is almost dead. I tried to recharge by applying 12V to it for one hour. But that doesn't seem to work. So, how can I recharge the battery or how to find out whether the battery is dead?

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  • \$\begingroup\$ What voltage does the battery currently show? If it's below ~10V it is likely junk and useful only as a door stop. \$\endgroup\$ – David Nov 2 '14 at 10:05
  • \$\begingroup\$ I don't have a voltmeter to measure it. But, I'm pretty sure that it is completely below 10V, as I tried to glow a 5V fan that doesn't even showed the pact. \$\endgroup\$ – Gowtham Gopalakrishnan Nov 2 '14 at 10:07
  • \$\begingroup\$ @David Is there any way to get that working? \$\endgroup\$ – Gowtham Gopalakrishnan Nov 2 '14 at 10:08
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    \$\begingroup\$ @Gowtham, I'd guess that the lead plates are by now coated with an insulating sulphate. There's some "pulse desulfators" in the market that claim to do something helpful to the battery, but I think that physical brushing of the plates would be the only real way to get rid of the sulfate and reviwe the battery. And that's not feasible with a normal kind of battery. \$\endgroup\$ – PkP Nov 2 '14 at 10:19
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    \$\begingroup\$ Dispose of it, it's obviously knackered. Float charging needs to be at about 13.65 V, anyway. \$\endgroup\$ – Leon Heller Nov 2 '14 at 11:39
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Ouch. My best guess is that your battery is toast.

Lead-Acid batteries suffer from several damaging chemical processes if they are allowed to remain discharged for long periods of time. One of the real problems is something called "Sulfation". You can use Google to search out that term and also look at the many attempts to build de-sulfation devices. Wikipedia has a good description of the problem: Wikipedia

In general, I have good results with Lead-Acid batteries so long as they are kept charged. We build our own chargers: Equalize voltage is 15.0Vdc and float voltage is 13.5Vdc. The battery is charged with constant current until the current drops below the current limit value, then the charger drops into float mode. There are better 3-stage chargers available now but we have had great success with those chargers - some of that gear is more than 20 years old and the users routinely get 5 to 7 years of battery life.

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If there is any crack or any physical damage to air inlet in VRLA battery, it will not work.Please ensure. There is still hope for its revival. You have 2 charging options- 1. constant voltage at 14.4V for 8-10 hrs and then 0.3 A constant current for 4 hours. 2. constant current charging 0.3A * 4Hrs+ (-0.5A discharge * 3hrs)+ 0.3A * 2hrs. The second option will improve the conversion of sulphate crystals to usable Lead dioxide.Even after this if you don't get the voltage above 13V, scrap the battery. Hope you will try.

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Although sealed Lead-Acid batteries do suffer from sulfating another common problem is that since they are operated under starved electrolyte conditions the electrolyte can end up as pure water that does not conduct. (all the sulphate ions are in chemical combination with the lead in the electrodes).

To rectify this situation you may need to charge it with a high-voltage (20-30V) current limited supply (put 1k in series) and leave it for a few weeks.

If you are lucky there will be enough current to reconvert the water to sulphuric acid and restore the battery to operating condition. After that you can charge normally.

Although everyone talks about "sulfation" as a problem, the normal operation of the battery converts the lead/lead dioxide in the plates to lead sulphate as it reacts with the electrolyte to create electricity, it is not in itself a problem condition. What does happen if the lead sulphate is allowed to sit for a long period it can convert into a form that is not easy to transform into lead/lead dioxide during the charging process.

kevin

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