Anyway to resurrect some NiMH Cells

[keantoken]

I heard that the discharge method worked by 'burning out' bad spots in the battery itself. I'll have to keep the freezer method in mind, though.

Hmmm. I have a 12V lead-acid (or whatever) rechargeable battery, and I was told that if I put 24VDC across it and leave it like that for a while it would charge fine. I have the AC-DC converter and everything ready, but I haven't tried it yet because I was worried about there being an excessive current jump at the beginning of the charging process. I'm more worried about the battery than I am the converter. Any precautions? The battery itself says the charging current and voltage is to be about 14.5-14.9V at 2.5A.

I know this isn't my thread, but it's a convenient place to ask and I don't think it's worth making an entire new thread about.

- keantoken

[soongsc]

Originally posted by keantoken
I heard that the discharge method worked by 'burning out' bad spots in the battery itself. I'll have to keep the freezer method in mind, though.

Hmmm. I have a 12V lead-acid (or whatever) rechargeable battery, and I was told that if I put 24VDC across it and leave it like that for a while it would charge fine. I have the AC-DC converter and everything ready, but I haven't tried it yet because I was worried about there being an excessive current jump at the beginning of the charging process. I'm more worried about the battery than I am the converter. Any precautions? The battery itself says the charging current and voltage is to be about 14.5-14.9V at 2.5A.

I know this isn't my thread, but it's a convenient place to ask and I don't think it's worth making an entire new thread about.

- keantoken

There is actually another thread that talks about desulfators.

[unclejed613]

i have read texts going back to the 1920's, and from what i gather, the goal is to remove sulfation by the generation of internal heat. the 24 volt method does that by passing large currents through the battery. you can do this to liquid electrolyte batteries (make sure you remove the vent caps first), but not gel cell batteries (the buildup of hydrogen in the cells will burst them). the charge-discharge method also seeks to break down the lead sulfate (PbSO4) back to being metallic lead and sulfuriic acid solution. sulfation is an oxidation reaction, de-sulfation is a reduction reaction. the only problem is, after this has been done enough times, enough of the metallic lead precipitates out as a powder to sink to the bottom of the cell (happens in liquid cells, but not often in gel cells) that it forms a leakage resistance across the cell, eventually with a low resistance, and it becomes a shorted cell. with gel cells, the lead powder is held in place where it precipitates, and the only measurable effect of this is that the cell capacity decreases with decreasing plate surface area..

i remember seeing a battery rejuvenator at an auto shop when i was young. it had a large DC power supply in the base and a huge set of load resistors on top. it also had a timer which would be set for however many charge-discharge cycles were required.

[dangus]

There was a schematic for a desulfator at Home Power magazine which can be found on the web. Like some commercial versions, this gizmo attempts to apply brief pulses to the battery that magically remove sulfation.
Lead Acid Battery Desulfator

My technique for reviving nicads involves a 12V car battery or equivalent, a resistor to apply a normal charging current, a voltmeter to monitor the cell voltage, and a length of maybe 18 gauge wire arranged to apply a very brief dose of 12V to the cell. By brief, I mean just swipe the wire. Watch the voltmeter and see if the cell begins to charge; if not, apply 12V again. For safety, hide the victim nicad around a corner or under a sturdy box in case something goes entertainingly wrong.

With the cost of dollar store nicads, and NiMH cells on sale, though, it's questionable if it's worth the effort. Maybe if it's a weird size cell and you just want to revive a vintage laptop or something.

[TroelsM]

This might be a bit TJ, but I think it would be clever to have all the battery-stuff in the same thread and the problems might be related. (Mods: move if you feel like it)

I have a large number of sub-c cells that charge just fine and otherwise seems to be OK, - BUT they all seem to have a "very" high internal resistance.

When loaded with 2-3A the voltage drops to approx 1-1.1V. When heavy loaded (stalled cordless drill ~ 10-20A ) the voltage plummets to 0.3-0.4V !

When loaded with 1-3A the mAh is OK (1000-1500mAh).

The cells are cheap and used, but I have to many to just throw them away.

Regards TroelsM

( Yes I know, it´s a total waste of time and I should go by something new and better, but I would like to understand this problem...)

[unclejed613]

i've got a pocketPC with a LiIon battery that recently has begun to get "memory". i thought they weren't supposed to, but the battery will go from 50% to zero in a second with no low battery indication at all..... i wonder if the freezer method might work...... it's worth a try

[keantoken]

I just remembered something: frostless freezers I think operate on powerful infrasound or ultrasound frequencies in order to break apart ice crystals forming on objects. If what I remember is true, then this may have some effect on a freezer's ability to resurrect batteries as described in one of the earlier posts.

Not sure though, just an idea.

- keantoken

[Coulomb]

Originally posted by unclejed613
i've got a pocketPC with a LiIon battery that recently has begun to get "memory". i thought they weren't supposed to, but the battery will go from 50% to zero in a second with no low battery indication at all..... i wonder if the freezer method might work...... it's worth a try

The method seems to work the best on High Current Drain batteries like those used in Power tools, Laptops and RC toys.

Anthony

[Duo]

Lithiom ion cells themselves don't really get memory, but their voltage to charge level relationship goes off as they age and this fools the battery control circuitry in terms of remaining charge. My laptop battery drops to 3% almost immediately after disconnecting the power supply (as indicated by software), but continues to operate for about four hours. It's that the battery management circuitry isn't always designed to deal with this.

As for frost-free freezers; in my time as a professional appliance technician I never saw one that used ultrasonic technology to break down ice crystals. Virtually all of them use a defrost heater which melts away the ice at intervals set by the defrost timer.

And for NiCD batteries; the best method of restoration I know is to apply very high current pulses to the cells individually, and then balance them. I had a power drill with NiCD battery pack which wouldn't charge properly. A number of the cells had more or less shorted at 0V and so the others were being overcharged. This happens with many old NiCD cells.

My solution for shorted NiCD cells is a capacitor bank of about 1800µF at 160V. I have a large SCR connected to copper probes which are pressed against the ends of the NiCD cell and then the SCR is triggered. This is not the safest of methods as you're working with a rather high voltage power supply and the probes can have the tendency to weld into the ends of the battery if you don't have a good connection. For smaller cells like AAA and AA I wouldn't use quite as much energy storage; maybe half depending on the application.

In maintaining these old NiCD cells, moderately slow charges seem to work best in getting the most energy storage. I usually charge at 1/10C (one tenth of rated capacity in amp-hours) until there is a detectable quick rise in cell temperature or until the voltage curve bends in (you can research this charge technique on the web in many places).