concerning semi-dead lead batterys

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Been searching a bit on the forum, but din't find a post regarding this, so i'l pop the question in a new thread.

I got a hold of 4 lead 12V, 2\1.8Ah batterys for free the other day, yet 3 of them mesure from 6 to 10.5V.
is this caused by dead cells?
is there a way i can restore them?

I'v heard that you can "jumpstart" the batterys by applying 24V, but my common sence say i should perhaps take that with a grain of salt..

hope you guys can help me out, id' really like to make use of theese batterys.
and before someone asks or reccomends: no i cant afford new ones.

I'm afraid but they are probably dead. If you can see somewhere how old they are, like more than 8-10 years, they are goodbye.

If you have charged them with 2.35-2.45 V per cell for a couple of days and the still won't produce 2.1 V per cell when loaded, sorry.

(We sell products at work with lead batteries, so I know some about the subject. One good way to kill a lead battery is deep discharge and let it be in that condition for a while!)

thanks for your reply!
I saw the production date on 2 of them, and they are 7 years and 11 months old, so I'l try giving them a shot, let them charge for a few days.

There should be no problem paralell charging them?
It'l be 8 days if if i can just try one at a time..

The cells of your batteries are sulfated. This means that a non conductive layer of lead sulfate has been built up around the lead plates, preventing them to make electric contact with the electrolyte. This happens when cell voltages are allowed to drop below 2 Volts for prolonged periods of time.

To dissolve that non-conductive layer (and recover most of the capacity of the battery) it must be excited into piezo-electric ringing at its natural resonant frequency (about 3Mhz). There are several switching circuits capable of doing so. Following there is a link, search for 'desulfator' in google to get more.;read=9100

This method is clever, cheap and ecologic since it allows lead-acid batteries to be recycled several times before their capacity has dropped enough to make them unusable.

I've also built my own desulfator, it just dumps a charged inductor into the battery at 15Khz in a flyback fashion, and it works.
If there's no switching desulfator, small current over charge is also OK: Charge the battery with constant current < 0.1C for a long time.

NEVER charge sulfated battery with high/standard voltage and big / standard current.

This would cause serious overcharge to sulfated units, and the positive panel will be permanently destroyed.
I don't agree. Desulfation requires some high frequency AC to be applied to the battery in order to excite the resonance of the lead sulfate layer and dissolve it. That layer appears as a capacitor in series with the battery so DC will never flow across it, only AC can do the job.

DC will only flow through the unsulfated parts of the lead plates, storing charge in the cell but not solving at all the sulfation problem, that strongly increases the equivalent series resistance of the battery (seen as a voltage drop much higher than expected for a given load) rendering it useless.
One trick i've used is to just pop the top covers (and rubber plugs) off and drip a bit of water into the cells,and trickle charge them for a day or two..
Sometimes they'll come back to life,sometimes they won't..
Thier life-span after that point seems to vary also..
I've got a couple batteries that are still going strong after ~3 years,while most of the others have slowly died off..
peranders :

The lead sulfate layer has piezoelectric properties so it rings at 3Mhz by itself, it doesn't require 3Mhz continuous AC excitation. All that is needed to keep it ringing is just excitation pulses at a lower frequency, usually between 10 and 30Khz

About refilling :

Standard lead-acid batteries should be refilled with distilled water until the plates are gently covered but not further. However, sealed lead-acid ones (SLA) are not designed to be refilled and doing so usually renders them useless after a short period of time (that's my experience).

firstly, thanks for helping out! appreciate it.

the link left me uncertain, but i think i got it:
a high current, high frequency(10-15KHz) psu at the battery's native voltage, 12.71?
this will desolve the sulfate layer, in how much time? there is no need for the ac to be a clean sinus chaped one?

I imagine making a RC chain oscillating at 10-15khz, switching a mosfet or BTJ into the battery. would this basic idea be dead wrong?

Sine excitation ar 10-15Khz won't work. The current waveform applied to the battery should contain 3Mhz transients in order to excite the lead sulfate piezoelectric resonance. This is easily done with a 10-15Khz square wave, charging an inductor and then letting it discharge into the battery. The turn-off process of the switch should be fast to ensure that enough 3Mhz component is produced. It's like plucking the strings of a guitar.
yeah, ok.

think i have access to some tone generators with squarewave posibilitys.

another possibility would be a RC chain with a schmitt trigger..
using fast transistors with a high slew rate..

but does the current output capability have to be as big as the desulfators on the linked page?

I can't figure out how a signal generator could be used for desulfating.

Look at these circuits:

A inductor has to be charged to a certain current level (let's say 2 or 3 Amp.) and then left discharge into the battery through a flyback diode. It may be done in a synchronous way with a timer or in an asynchronous way with a flip-flop and current sensing comparators.
Re: This may be of interest!

Eva said:
I can't figure out how a signal generator could be used for desulfating.

Look at these circuits:

A inductor has to be charged to a certain current level (let's say 2 or 3 Amp.) and then left discharge into the battery through a flyback diode. It may be done in a synchronous way with a timer or in an asynchronous way with a flip-flop and current sensing comparators.

Took a while, but i think i got it now.

looking into building myself a desulfator now, if i just have the parts i need available.


mwmkravchenko said:

This is the english edition of elektor. If you want a PDF of the article here is my email address.


thanks, you got mail.
Now my desulfator looks like that:

An externally hosted image should be here but it was not working when we last tested it.

The conduction period of the switch ends when the peak current reaches the desired value (4..5A), then the inductor is allowed to discharge into the battery during a fixed time. The entire circuit oscillates around 80Khz when the batteries are approaching good condition.

This produces 80.000 positive going current transients and 80.000 negative going current transients per second and ensures gentle excitation of the lead sulfate piezoelectric resonance.

The adjustable 12V supply allows operation without discharging the battery, actually I use it to provide moderate charging at 13.2 to 13.8V

Note that it's built in a breadboard so I'm making changes continuously.


I've been desulfating two 6V 10Ah batteries connected in series for a week. Both are 8 years old and I got them in very bad condition, showing less than 3 volts each and not responding to standard charging.

I've just tested them for 37 minutes with 4A load (12V 50W halogen lamp). Results just before disconnecting the load :

Battery 1 : Started at 6.57V and dropped to 6V in the first few minutes, then it kept 6V till the end.

Battery 2 : Started at 6.3V and dropped to 5.8V in the first few minutes, then it maintained 5.8V for most of the time. In the last few minutes it started to drop again and I disconnected at 5V.

The unit no.2 has a very weak cell but I still hope to recover it.
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Seems like a lot of work to "fix" an old and questionable storage battery. I have purchased 12V 11AH batteries on eBay for as little as $8.00, one is in my UPS and has lasted 2yrs - handly outperforming one I purchased from a reputable vender for $60 the yr before that.

I have dissected a couple of old SLA types and the plates tend to buckle with use due to the thinning of one of the plates with regular cycling.

They are recyclable so the incentive to do this I would suspect is more for the challenge than for practicality?

This sounds like a good tool for saving accidentally sulphated batteries that are still well within their normal service life though..

Not so much work. Once you have a working desulfator mounted in a PCB inside a box, you just have to connect it to the battery, apply power and forget about it for a week or two.

Seems quite reasonable for me ;)

That circuit is also useful to prevent loss of charge and sulfation in batteries of vehicles when they are not going to be used for several months.
Joined 2004
Paid Member
Actually the vehicle idea is pretty cool, there might just be a market for it. I remember having to trash more than one car battery over the years, and I bet there is a good market for such a device amongst motor cycle riders in cold climes (like here) who have to store their bikes for 6 months of the year. :D

Call me ignorant, but I don't recognize the flag of your country.. Clue me in.. LOL

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