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Reconditioning old/dead lead acid batteries.

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I recently became interested in reconditioning of old lead acid batteries. Reconditioning in context of lead acid batteries usually means desulphation of negative electrode.

There are lots of very similar articles online about how to desulphate those batteries. Half of them talk about using magnesium/sodium/cadmium/lithium(choose one) sulphate salts. The other half describe various circuits that send high frequency current pulses through the battery.

There are few scientific articles on the subject, but they tend to have very narrow focus.

For example one article looking into magnesium sulphate use found that 2% of it in the electrolyte lowers the negative electrode internal resistance and it has some other minor benefits. Unfortunately, they didn't make any comparison of capacity before and after. Then another article tested the pulsed desulphation and they too found benefits, but they didn't test battery capacity before and after, just the way it charged.

Finally, there is another article written at batteryvitamin.net that claims both methods manage to at the very best restore 10% of original capacity.

So the conclusion is? I have no idea.

So, having two old tractor batteries I decided to test the magnesium sulphate method. The tractor batteries are 6 volt, 165amp-hour units that used to work in series. When I bought the tractor 3 years ago it came with the batteries. By the newish looking stickers I estimated the batteries to be no more than 5 years old at time of purchase. Two years ago in winter, the batteries failed to start the tractor (in - 15C) unless charged for few hours. Then they failed to start it even in summer (+25C) so I bought a new set. The old set sat in an unheated shed for two years. Few days ago when I measured the voltage there was none - 0 volts. On both batteries. I dumped the old electrolyte to a bucket (there was 9L of it). And I made a solution of 2kg of magnesium sulphate (7*hydrate) in 9l of warm deionised water. I filled the batteries with that solution and I started to charge them using two lab PSUs.

Surprisingly they behave very differently despite the fact they were used very similarily.

 Battery no 1 upon charging with constant current of 0.5A went to 7.4V (this is equivalent to 14.9V if this was a 12v battery) and sits at 120mA and that voltage.

Battery no 2 behaves much better. I initially set the current limit to 1A. It reached the limit at 6.3V (12.6V equivalent). I kept it like this for 12h and the current dropped to 200mA. So I set the current limit to 2A and voltage to 7.3V. The battery has been taking 2A of current at 6.85V(13.7 equivalent) for last 12H.

24h have passed since I started charging them. Initially the plates looked light gray wth white sprinkled over. Now they look dark grey with reddish-black patches.

I have two more batteries to try to fix. They are in much better state. One is a diesel car battery 12V, 75AH.This allegedly has lost the majority of its capacity. Its voltage in storage is pretty good at 12.5V. The second is a backup power battery 12V 100Ah. This one allegedly is completely dead and fails to charge. Its storage voltage is 11.5V.

Is there anyone here with experience of attempting to restore old batteries like this? I would be very interested in hearing your positive and negative experiences. Am I wasting time trying to fix them? I was very skeptical regarding most of positive restoration claims I read before, but seeing how well one of those old tractor batteries charges I'm not so sure.

My plan for the older of two remaining batteries is to add enough magnesium sulphate to achieve 2% concentration and to try to charge.

I may try cobbling together a pulse desulphator for the remaining one.

Please don't reply just saying to buy a new one. I know I can do that, but I like fixing and restoring old things such as those batteries. If I'm successful restoring at least 50% capacity I'll use them for backup power. If my attempts fail I'll sell them for recycling.

You can make an easy pulse desulfator out of a triac light dimmer (set to about halfway), a 5-10uF motor run cap, and a bridge rectifier. Triac light dimmers work by cutting the AC waveform, pass that through a cap and you get pulses, when you then rectify with the bridge rectifier.


--- Quote from: NiHaoMike on December 02, 2021, 01:38:50 pm ---You can make an easy pulse desulfator out of a triac light dimmer (set to about halfway), a 5-10uF motor run cap, and a bridge rectifier. Triac light dimmers work by cutting the AC waveform, pass that through a cap and you get pulses, when you then rectify with the bridge rectifier.

--- End quote ---

Thanks. There are hundreds of circuits online too. I would probably use my signal generator with a power mosfet transistor as I don't have a dimmer. However, I'm more interested in if there is anyone with positive or negative experience of actually using those circuits, or chemical methods.

How much capacity can one reasonably expect to restore? Is it only 10%? If yes, there is no point. If it is ~50% it is very interesting.

My experience is that it's far more effective at restoring a weak battery than trying to revive a totally dead one, and is more effective on conventional lead acid than sealed lead acid.

I used to have a Belkin UPS that charged the battery with brief high current pulses using some circuit in conjunction with the inverter transformer. The battery lasted about 5 years compared to 3 years for APC which used the same kind of battery but DC charging. Main downside is that the high current pulses cause interference with nearby analog audio equipment.

There are a few discussions around EEVBlog about the subject. One of them with some interesting results is at:


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