Rejuvenating SLA batteries

Has anyone successfully rejuvenated any SLA that has become sulphated? There’s lots of info on how to do this on the web ranging from videos to reviews of desulphating chargers. That’s fine and dandy but I’d be interested in hearing from anyone who has recovered an SLA.

I have a number of old SLAs that are somewhat knackered to very knackered. I’ve no intention of using them on an activation partly because I’ve got my feet firmly in the Lithium category now and also because these are my old contesting batteries and they are 4V or 6V 100Ahr cells and weigh around 23kg each!

I’d be happy to see if they can be recovered if anyone has achieved this personally. Failing that it’s time to weigh them in… I wonder how much 112kg of surplus lead is worth? :wink:

A trick - I believe works is to take the plastic top off the cell covers, remove each rubber stopper and poor in some heated distilled water, apparently this works well. Although I have never tried it. I could experiment this weekend.

A quick call to the local scrap merchant: 100kg of old lead-acid batteries is worth £30. As these batteries haven’t been used for 15 years I’m going to take the £30!


Good call, I think, Andy.

My attempts at de-sulphating old car batteries in the days when a new one would have cost more than the car was worth, were almost completely ineffective. There were lots of articles on the subject at the time, and I think my attempts were reasonably well informed. The batteries were just knackered!

I know that SLABs have a different structure and plate composition, but if they have been lying discharged for 15 years, there will be armour plated sulphate deposits by now :o)

I would expect them to be showing some signs of swelling, too, which is never good…


Yes, I rejuvenated 2 SLABs, following a procdure described by DJ3TZ in an article in the German “Funkamateur” magazine FA 1/13. The process essentially consists of a controlled overcharge at low current, followed by a discharge. Such a cycle takes several days, and you have to run several cycles in sequence.

I did the experiment just out of interest. Prior to the test, my batteries (2.9Ah and 22Ah) were not used for several years. After 3-4 cycles, capacity was up by a factor of about 4 and pretty much back to its nominal value. But the internal resistance did not go down and still was outside the manufacturer’s spec.

My personal conclusion was that for high-current applications the process was close to useless, and for low-current applications it’s not worth the big effort.

Markus HB9BRJ


I have done this with some old UPS batteries, they as you say are useless as batteries. But as a large capacitor they are not bad ! I use them to kick start the 19 Set when its running on fixed supplies so the inrush current is limited and doesn’t trip the supply off. Not quite a paper weight.

Hi Jonathan,
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I didn’t know you’ve got this new callsign!
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Best 73 de Guru - EA2IF

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£30 hmmm

Buy your self another battery :wink:


Yes, they would probably be recoverable to that extent. Thanks for mentioning UPS Jonathon. That reminded me I’ve got 2 perfect APC 1400 SmartUPS in storage (hidden under a pile of ethernet cables in the server room at work). The UPS themselves are fine but the cells are wasted.

The question is whether the effort in time (I’m time poor) is worth it and I don’t think it is. The big cells came from a BT exchange and were new-ish but a battery had failed and the whole batch were scrapped by BT, an enterprising BT engineer organised the disposal! I can remember paying £15 for 3x 6V 100Ahr SLA batteries. I acquired 2x 4V 100Ahr for free and they were used to power the 2m/70cm contest gear for 4 years. But they’ve not been used since I moved to GM in 2000 and are dead now.

If I can dispose of them, the UPS cells plus a few dead 12V 7Ahr SLAs for £30/100kg then I get a load of space back, the environment benefits from them be recycled, my wallet benefits and best of all I get brownie points from Mrs. FMF. :smile:

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I think you will find an inconsistency in your statement when the power fails.

That’s why they were scrapped! The cells don’t have the capacity to maintain the UPS hold up time anymore and they weren’t big enough for the job. So the both old UPS units work FB but are a bit QRP. If you want to restore it to QRO operation you need 4 big SLAs.

They were replaced with these 2 bad boys which can power all the routers, the DHCP/printer server/domain controllers, both 16 core VMware machines and the 3 local compute servers along with the 48TB NetApp for 2hours!

That’s just for our office, the heavy lifting is done in the datacentre in Munich where there are countless servers and rooms of SLAs and big diesel generators etc.

Well I took the old dead cells to my local scrap yard/ scrap metal company, 112kg of used car batteries and big SLAs netted me £40 cash. :slight_smile:

Hi Andy,
No doubt the best decision. Rejuvenation of sulphated cells left for a while doesn’t work. The internal resistance will never be low again even if the terminal voltage can be raised.

I’ve been told by someone who worked with batteries and standby power sets that lead acid batteries can lose 50% of their capacity by being completely flattened even if recharged within a couple of hours. The lead acid battery rejuvenation additives have been tested in labs and found to not be worth the money.

I’ve found the GelCel more resilient to renovation than the wet ones if flattened rather than just discharged to say 11 V. I once tried to put 7 AH into a flat car battery by paralleling it with a 7 AH GelCel. Killed the GelCel and still could not start the car. A lose lose situation.

I have recovered NiCd batteries when they have failed due to internal shorts from metal whiskers but their new life was a short one. I seem to have recovered a 2S LiFePo battery that went down to a couple of volts but I’ve not done a proper capacity test so maybe it too needs to be subject to recycling.

The warnings about minimum discharge voltages and storage rules aren’t given out for nothing. Some of us are however born optimists and will try to breathe life back into a pile of useless chemicals.

I believe the scrap metal people offer about 40 cents per kg around here for car batteries. That’s about 20 p.


That’s the same ball park Ron, I got 36p per kilo. The scrap yard I went to was much bigger than I expected as it was actually crushing/shredding cars for metal recovery along with buying in scrap copper/aluminium/lead etc. I’ve never seen so many copper pipe offcuts, about 10 skips worth! It’s on a farm and the entrance doesn’t suggest the scale of the operation.

The good thing is that these will be recycled and reused reducing the amount of landfill and pollution. Scrap metal is highly regulated in the UK after years of telephone/railway signal cable thefts. They’re not licenced to handle NiCd cells (the Cadmium being a worse poison that Lead) and they checked every SLA to make sure it was Lead. All the cells had manufacturer’s labels saying Sealed Lead Acid on them, the guy told me that for non-car batteries, no label = no purchase. Something to consider if people in the UK want to scrap SLAs.

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The batteries in these 2 UPS units both failed with a few weeks of each other. New batteries were shipped and I’ve just finished unloading 72kgs of batteries to take to scrap dealer.

I have never seen so many 12V 5A SLA cells!