An observation on Self Discharge of NiMh cells

[SilverFox]

I think I may have a record here...

Aircraft800 sent me some Sanyo 2700 mAh cells. He cited that they had developed a high rate of self discharge. I told him that I would like to take a look at them and ended up with them.

I ran a couple of Break-In cycles on them, and they came in at around 2200 mAh. Since Sanyo lists the 2700 cells as 2500 mAh minimum, this works out to around 88% of the original capacity, so that seems pretty good.

I put them aside for a few days and then checked them again. They were down to around 0.9 volts and were dead. OK, that seems to be accelerated self discharge.

I decided to charge them back up and do some more testing on them. There really isn't anything that can be done to reduce the self discharge rate, but I just wanted to play around. I charged them up and left them on the charger to trickle charge. The next day I decided to do another discharge.

Wow, was I surprised...

Sitting on the charger trickle charging overnight (well a low trickle charge as I was using a C9000), I lost over 500 mAh of capacity!!!

These cells are beyond crap... they are junk.

Aircraft800 I am sorry you wasted the postage on these cells...

Tom

 

[Wicho]

I take it you are just referring to the ones you have right now and not Sanyo 2700's in general, right?

 

[SilverFox]

Hello Wicho,

Yes. I don't know what Aircraft800 did to these cells, but they are beyond any hope of use.

I have some other high capacity Sanyo cells that are doing quite fine.

Tom

 

[Wicho]

Phew! I'm safe then.
Maybe they were re-branded Energizers?

Thank you for all the info you post on batteries - it has been incredibly educational to me.

 

[Turak]

Tom,

You should try to carefully disassemble them.

I'll bet you that 'if' you can managed to get them apart, fairly intact and manage to unroll the layers......

You will find one of the following has occurred;

1. The separator material has thinned out in some places so much that the reactive layers are essentially causing the separator to act much like a lower value resistor would, draining the energy off.
chief cause> charging/discharging at extremely high rates (i.e. 15 minute chargers) or missed terminations during charging resulting in cooking the batteries (i.e. hot , hot , hot batteries)

2. The separator material has actually developed small pin sized holes in it or just been damaged in general.
chief cause> trickle charging for extremely long periods of time (i.e. just leaving batteries on a trickle charger till you need them) or shorting the batteries causing excessive current draw (i.e. flash current testing) or dropping the batteries on hard surfaces.

3. Small dendrites have formed and poked through the separator layer.
chief cause> trickle charging for extremely long periods of time or just plain old extremely poor quality batteries.

I would be interested in your findings.

 

[SafetyBob]

Aircraft800, please tell us how you used these cells. I think most of us would be interested. I thought you had said in some post a long time ago you were using these in a hotwire of some sort. Is this a good lesson for us to stop right, exactly when we see the light start to dim? Recharge right as quickly as possible (as in 30 minutes of dim, come inside and charge)?

This reminds me, I need to go and abuse some Eneloops to make sure they are still good.

Bob E.

 

[Aircraft800]

Tom,

Besides charging them with a "Universal Smart Charger" (junk IMOBO) 2 or 3 times, and about 30 total cycles. I would never leave them discharged.

Can the heat caused by the overcharge from the USC 2 or 3 times kill them that quick? I felt them getting HOT while charging, too hot to hold for more than 5 sec.. I don't think the ROP killed them since I was only pulling roughly 1.7A (12 AA stacked 6s2p).

I'll bet the 12 Titanium 1800 died the same fate, but the charge terminated on those. Any luck testing those?

Yea, that charger is gone now..

 

[paulr]

I've had a bunch of energizer and sanyo 2500's go crappo on me despite careful treatment. Haven't had any problems with eneloops yet.

 

[MadZebra]

My Sanyo 2500 behave exactly the same. 0.9V after very very short time - useless crap. I had to replace them with old (7 years?) Philips and Panasonics - because they still hold a lot. I won't buy Sanyo anymore...

 

[Ig]

I have 8 useless 2500 Sanyo and 4 a little better "HR" marked 2500 Energizer. No more Sanyo for me.

 

[Nebula]

I have 8 useless 2500 Sanyo and 4 a little better "HR" marked 2500 Energizer. No more Sanyo for me.

Agreed....2500s are regarded as crap cells, especially the Energizers. However, these were 2700s ... generally highly regarded. My 2700 Sanyo cells are doing very well (so far). Others can weigh in if they wish, but I don't think that the Sanyo 2700 cells have a bad reputation. YMMV.

KK

 

[SilverFox]

Hello Aircraft800,

The Titanium cells are doing much better. I tossed the vented ones and just checked the remaining 10. After a month they are all showing 80% on the ZTS.

I noticed one of them as a little piece of silver tape on it. What's the story with that one?

Too hot to hold is too hot. I would guess that was the contributing factor to the demise of the 2700 cells. My final attempt at testing them is to charge them up on the Energizer 15 minute charger. If there are soft shorts, high current charging my blow them off...

Tom

 

[Turak]

Tom,

Your just killing them even quicker with the 15 minute charger......

It does not put out anywhere near enough current and does so for WAYYYYYY to long to get rid of the 'soft-shorts'.

The process you are trying to use to was well known way back in the Ni-Cad days....but you CAN NOT use a charger or a power supply to do it. We actually used a machine that was used to charge magnets. It could supply an energy pulse (roughly a 2-3 uS pulse) on the order of almost 2 Joules.

What you need to do is get a GIANT capacitor...I am talking one close to 1F. You can then charge the cap up and then release the energy from the cap in to the battery. The capaciter can release FAR more current than the charger, combined with the fact that you need to release the energy VERY quickly.

By trying to do it with a 15 minute charger....all you are doing is heating the battery up, causing the separator material to thin out or and holes to open up even more.

You need something that can release energy on the order of 100-200A for 2-3 micro seconds.

Using this method with Ni-Cads in the 'old' days....would cause the Cadmium salt shorts within the battery to litteraly fry and open up, thus reviviing the battery by eliminating the short. Problem is that there was always a remnet of the salt pathway and it would quickly reform and cause the parasitic drain again in a very short period of time. So while it helped, it was only a very short term solution that ultimately failed again after the batery was used a few times.

By the way......trickle charging for extreme periods of time was the MAIN CAUSE for Ni-Cads developing parasitic energy loss symptoms.

FYI.....NiMh cells do not develops the internal shorts the same way as the Ni-Cads did, so the above process has not been shown to be of any real benefit to NiMh batteries.

NiMh batteries and their parasitic energy loss symptoms are almost always because of the thinning of the separator material, or just shoddy construction.

Keeping the temperatures down during charging/discharging is the BEST way to minimize the 'self-discharge' symptoms for forming. Also we are not just talking about the external temperatures....but also the internal temperatures that you can not 'feel by holding them in your hand'.

Basically, when you charge them at high current rates (anything over 1C)....just because they do not get 'too hot to hold', does not mean that you are not doing internal damage to them

To answer AirCraft800's question regarding missing a termination a couple times during charging....all it takes is 1 missed termination and cooking the batteries 1 time....to cause problems.

 

[Turak]

Sorry about the double post.

 

[Burgess]

to Aircraft800 --


Happy Birthday !


_

 

[Aircraft800]

Hello Aircraft800,

The Titanium cells are doing much better. I tossed the vented ones and just checked the remaining 10. After a month they are all showing 80% on the ZTS.

I noticed one of them as a little piece of silver tape on it. What's the story with that one?

Too hot to hold is too hot. I would guess that was the contributing factor to the demise of the 2700 cells. My final attempt at testing them is to charge them up on the Energizer 15 minute charger. If there are soft shorts, high current charging my blow them off...

Tom

When I slide them into my flashlight, the covers sometimes snag on the sharp edge where the tailcap seets. I used a chip chaser to knock off the bur, now I don't have that problem.

 

[Aircraft800]

to Aircraft800 --


Happy Birthday !


_

WooHoo!!! Thanks!!

(Off Topic)

 

[SilverFox]

Hello Turak,

Yes, I am very familiar with zapping cells...

I am just playing with these cells to see what happens.

I was amazed when I put these cells on the 15 minute charger. I had run them down to 0 volts under a very low current (10 mA), and they had bounced back to just over 0.9 volts. I popped them into the 15 minute charger and they charged normally. The cells warmed up, but did not get hot.

Now I will set them aside to see if their self discharge rate has improved...

Tom

 

[EssLight]

Aircraft800, which "Universal Smart Charger" were you using?

I recently got one, a CHUN-0212DC, it seems to work well for fast charging of a Versapak battery, but when I charged 6xAA NiMH in series, the batteries got much hotter than I would have preferred.

 

[TONY M]

Phew! I'm safe then.
Maybe they were re-branded Energizers?

Thank you for all the info you post on batteries - it has been incredibly educational to me.

Maybe they were. I had energizer 2700's (I think) and they were crap. The self discharge rate was terrible. I had no real use for them at all, so I just gave them to a friend. :thumbsdow