Making elecret within nimh AA cell

[SkyWatcher]

Hi folks, thought a thread was worthy here. I'm noticing something so far which others have noticed when batteries have gone dead and become like low voltage elecrets. I had these generic 2400mah nimh AA's laying around 3 of them and were almost 0 volts, then i decided to try and rejuvenate them with bedini style solid state charger using flyback diode method. It was working fine, i was getting maybe 60-70% capacity then i accidentally left them to discharge to long in discharge cycle and they were almost at 0 volts again, however when disconnecting load they shot up quickly past 3 volts and settled around 3.68 volts, well they wont go below that voltage, they keep climbing quickly back up to about 3.7 volts, i've been pulse lighting multiple led's, then i thought i'd try charging another duracell 2650mah nimh AA and its charging this single cell and now the voltage has climbed even higher on the generic nimh AA's to 3.73 volts. Which also sounds similar to batteries continuing to charge via Bedini's charge process. Anyone else see this behavior with batteries as i've read others have.
peace love light
Tyson

[SkyWatcher]

just noticed i spelled electret wrong in title, any way to edit that.

[ewizard]

Have you tried putting a small filament bulb across them or maybe checking the amp output somehow? I'm wondering if this is a real voltage with any power behind it. My first thought was actually that you had some mislabeled 14500 Li-Ion batteries as they are exactly AA size and are 3.7 volts. But then I don't think you have that mixup if they are labeled Eveready. I have charged primary Lithium batteries that are 3 volt to close to 4 volt before but that's a whole different deal and fairly dangerous to try. Lithiums are prone to explode and start on fire.

[SkyWatcher]

Hi ewizard, yes its real. i'm charging 3 lenmark 2500mah nimh in parallel as we speak and when i disconnect them from charging, the 3 - generic nimh in series fairly quickly return to 3.7 volts.
peace love light
Tyson

[Aaron]

new thread

just noticed i spelled electret wrong in title, any way to edit that.

Hi, can't easily edit thread but can start a new one and transfer posts.

[Aaron]

charging cap

You can try to charge a cap with the electret battery
and then dump it to another battery then bring cap back to electret, repeat.

A cap won't affect the electrets like other loads.

At least that is what I tried before.

[SkyWatcher]

If you want, i hope people can understand what i meant.

[sucahyo]

I think we can only create electret if the nimh internal is completely dry, otherwise it will self discharge. I guess we can try to do the same thing with dry SLA. I 'll try to find a dead but not shorted SLA and see if I can replicate.

[ewizard]

Am I correct in understanding that each NiMH battery has around 3.7 volts on it rather than the usual 1.2 to 1.4 volts? I guess what you are saying and I think I missed it initially is that the battery has become more like a capacitor? Can you share the circuit you are using for the charger or point to a link for this particular Bedini charger?

[wrench76]

Hello Skywatcer, and everyone else. This is my first post on this forum, and I would like to thank everyone here for the wealth of knowledge and inspiration received. I have played with nimh batteries quite a bit racing RC cars. We used some methods (not recommended by manufacturers) to get the most performance from our packs. one of which was to store the packs completely dead shorted! This gave very good results, the batteries we used with that method were some of the first generation of nimh to hit the RC market. Probably close to the generic batteries you have.If your batteries peak at a higher voltage then before that usually means their internal resistance has gone up. I only have some experience charging lead acid and gel cells with the Bedini, so Ill have to try some of my old nimh packs. Have you tried this with any newer name brand cells?We also would store newer generation cells about half charged,then discharge them to 0 volts and charge them immediately.FYI this method is not recommended by manufacturers (due to obvious hazards) that does not mean they dont practice them

[downunder]

Hi ewizard, yes its real. i'm charging 3 lenmark 2500mah nimh in parallel as we speak and when i disconnect them from charging, the 3 - generic nimh in series fairly quickly return to 3.7 volts.
peace love light
Tyson

I noticed some people were confused about the voltage issue, so I have highlighted the relevant information in the above quote. The 3.7 volts is about right for three rechargeable AA cells in series (3 x 1.2). SkyWatcher is not saying that each battery has 3.7 volts by itself.

[ewizard]

Thanks downunder - I was confused about that. I've found that most NiCD and NiMh batteries can be revived fairly fast with some extreme charging. I've thrown otherwise dead cells that would not take any charge onto a 10 Amp 12 volt car charger for a second or two which you really want to be careful doing (I usually use welders gloves and goggles) but it almost always gets them back and I can then charge them normally. That is something I found worked in the past but now that I know about pulse charging like Bedini style that's probably a safer plan.

[sucahyo]

I have played with nimh batteries quite a bit racing RC cars. We used some methods (not recommended by manufacturers) to get the most performance from our packs. one of which was to store the packs completely dead shorted! This gave very good results

That is extreme! I found that my nicad capacity reduce significantly when my kid play them to their death.

I've thrown otherwise dead cells that would not take any charge onto a 10 Amp 12 volt car charger for a second or two which you really want to be careful doing (I usually use welders gloves and goggles) but it almost always gets them back and I can then charge them normally.

I found radiant charger revive the battery much better. Also with much better capacity. With auldey 30000rpm mini4wd car, 1000mAh nicad, 45 minutes radiant charging store about 12 fast lap of 20 meter circuit, while a 2 minutes of 5A 12V only store about 3 or 4 fast lap. Only 2 minutes because the battery would be too hot to handle if charged longer. Fast lap means faster/competitive lap than other similar car using brand new alkaline or full nicad.

[SkyWatcher]

Hi folks, thanks for replies and info. I just looked at the generic nimh AA cells and they are marked X1 BBNM-2400. They seem to be acting yes like a capacitor, though it seems if you load them too much it takes a little longer to recover. This may be completely normal, however based on others research i've read that shows very similar effects, its good to look into these things more closely. These cells were completely dead when bought from store and would not charge in the typical charger recommended, so i assume they were highly crystallized, which is precisely the one ingredient that others have found in experiments cause an electret type effect and since i have been using bedinis solid state circuit charging off the flyback diode it may have benefited the formation of this condition. My batteries may be doing the same thing, it does appear so, more testing will be done. This is the circuit i have been using.
peace love light
Tyson

[SkyWatcher]

Hi folks, i let the 3 - nimh baterries charge the paralleled lenmark nimh AA cells for a few hours this time only 2 in parallel and voltage went from 1.262 volts to 1.283 volts and the 3 nimh AA cells in series is now resting at 3.67 volts, though slower to climb back to that voltage. it could be that i'm exceeding the load for the effect to maintain itself longer, or this is just normal. Though from what i have seen with batteries this seems a little different. If anything this could be giving clues as to what happens when batteries are charge in this fashion with bedini type of flyback pulse charging, where some kind of change occurs that enables it to charge in that way, making it more like a capacitor with electret effects though only more noticeable when discharged very low like this. As far as what i normally see with lead acid gel that has been pulsed this way, i have yet to see such large rebounding back to such a high voltage, yet these nimh AA cells just do not want to give up as if something similar to electret type effects are at play. This needs more experiment.
peace love light
Tyson