Totally drained NiMh pack - can it be resuscitated, and how?

[Former Member]

[This posting has been removed]

[ChrisB]

Forget, Eneloops, just get normal Nimhs, from somewhere like component shop. I use standard Sanyo Nimhs and charge on a normal wall charger (150ma). Very rare I have any issues.

I've gone through several Eneloops and LSD Nimhs and I won't use them again.

[Jonathan M]

Well, I don't have a wart-charger, so just used my smart-charger at 1/10C, which stopped after just four hours with the battery at just under 4.8v - clearly below par. I've a spare Sanyo NiMh which I'm now using in the model, and will just bin (i.e. recycle) the Eneloop on the basis that its worth neither the hassle to try other methods of resuscitation nor - more importantly - the risk of then flying with it.

Edited By Jonathan M on 10/10/2017 16:58:02

[onetenor]

Flash the pack with a high voltage from say an arc welder. Go not HOLD in contact merely brush the contacts briefly .Then charge as usual with either a wart or proper charger. Works for power tool packs too.It breaks down the dentricles which form inside the cells shorting them internally.

[Mike Blandford]

Why did the charger stop? Many "smart chargers" include "backstop" timer and stop after that time even if the pack has not been peak detected.

Also, it is not unusual for a "false peak" to be detected on a fully discharged pack.

Since the pack was fully discharged, you do need to charge it at 1/10C for 12 to 14 hours (perfectly safe to do so at that charge rate), and then run a discharge test. For a 2000mAh pack, I would suggest a discharge at 0.5A, and you would expect at least 3.5 hours. You would very likely only get the full 2000mAh if you discharged at 200mA, but this woud take 10 hours.

So I suggest putting the pack back on charge at 1/10C for another 4 hours, and then repeat again.

Mike.

[Jonathan M]

Two interesting options...

Onetenor, I don't have access to an arc welder - would a 12v lead-acid work for your technique?

Mike, re the charger stopping, it doesn't have a time limit set so might simply have registered a false peak (currently set to 5mV, but I can increase this if need be).

Also your last post is slightly ambivalent: are you suggesting I continue charging (at 1/10C etc) and repeat if need be until I've got as full as possible a charge, or discharge first, then charge up again?

[Peter Christy]

Posted by Jonathan M on 10/10/2017 19:44:53:Onetenor, I don't have access to an arc welder - would a 12v lead-acid work for your technique?

Yes, but as per my earlier post, and OnTenor's more explicit one, don't apply the voltage for any length of time. Simply brushing the wire briefly over the terminal a few times should be enough to "unstick" any frozen cells. Monitor the voltage after each attempt. As soon as it comes up to its nominal voltage, stop!

You will only need to do this if one or more cells are actually stuck at zero volts. Try a trickle charge first. If after, say half-an-hour, the pack is still under voltage, then try flashing it. But if it comes up to voltage with a short trickle charge, don't bother.

--

Pete

[Peter Miller]

, Consider the batteries and chargers and nothing else that we had back in the 70s and they worked.

Now we have so many different batteries and chargers and analysers and checkers and transmitters that tell you what is happening to the receiver battery etc we don't half get our knickers in a twist.easily.

[Nigel R]

"I don't have a wart-charger, so just used my smart-charger at 1/10C, which stopped after just four hours with the battery at just under 4.8v"

You really need a 'dumb' charger for this job.

Sounds as if the smart charger assumed it was a 3 cell pack and stopped at "max volts for 3 cells".

[Mike Blandford]

Don't do the discharge test until you have completed at least 12 hours charging at c/10.

Mike.

[Engine Doctor]

Re flashing the batteries with a high Amper . Just my two pennyworth from my own experience. It's only a temporary fix that sometimes worked with nicads . Despite all the wonderful claims on YouTube etc it doesn't work well with NiMhs or not in my experience . I tried it on a drill pack and got the duff cell to accept a charge but it was way down on the other cells and went flat very quickly. It might be ok for boats or cars but I certainly wouldn't try or advise trying it on a flight pack. Replace with good quality pack. Don't be tempted by the high capacity AA packs of 2500mah and above. They are fragile internally and can only give you something near there stated capacity at very low discharge rates. After losing a good model due to  a cell going dead a few years ago I now use twin 5 cell packs on any decent models with a 2100 mah cap maximum ,giving me 4200 mah.These are connected to Rx via a "y" lead with diodes fitted. Or sub "C" cells if high current is needed. Any model with a single battery fitted is  tested using a load of 2.5 amps checking voltage is stable. If voltage drops quickly then it's bin time. I have tried using Life batteries but charging and balancing is a pain compared to easy to use NiMh packs.

[Martin Harris - Moderator]

Published advice is that balancing LiFe cells is far less critical than for LiPos and only advised as being needed occasionally. I do usually balance charge mine but then I don't find it too much trouble. I'm afraid my experience of NiMHs in general hasn't been great and I tend to avoid them.

[Mike Blandford]

You know these cells have a nominal capacity of 2000mAh, and that you fully discharged them. Before trying a discharge test, you need to charge them with at least 2000mAh, hence needing to charge at C/10 for over 10 hours.

If you do this, then do a discharge test (0.5A as I suggested above) you will have a good idea of the health of the battery.

Mike.

[Jonathan M]

OK, I've established the following:

There isn't a faulty cell, so flashing with a 12v Pb to 'repair' isn't necessary.

Mike, your analysis and recommendations make a lot of sense.

The early cutoff at too low a voltage (4.8v) was most likely due to having the peak-detect set down to 5mV. I changed this up to 8mV, kept the charge rate at 0.1C and the battery quickly (say 1.5 hrs) came up to about 5.6v. I left this going for several more hours, then saw the battery was showing over 6.0v (!?) so switched it off before going to bed. This morning the pack has settled at 5.7v. (My other normal pack has just been topped-up with the peak-detect reset to 6mV and shows 5.8v as usual when both the charger and the 0.5A load-tester show FULL.)

Next step is to do a full slow cycle of discharge at 0.1C (cutoff at 4.4v or 1.1v/cell as normal) and re-charge at 0.1C (peak-detect left at 6mV). This conditioning will take a long time, but I'd ideally like to see the battery showing 5.8v when FULL before proceeding.

Then I'll do Mike's test: discharge at 0.1C down to 4.4V, charge at 0.5C for 2 hours and check this half-charge both in the model and with the load-tester.

Jon

[Don Fry]

As Peter Miller says, seems a bit complicated. Jonathan, this is a battery which has been run flat, and it's function is to power the internals of a model plane. Why not estimate the maximum drain the plane can drain, (Poetry), charge it up, and shove it on a load test to see if it holds a voltage big enough to keep the receiver going at the maximum load, for say 20 minutes. That will prove it is up to the job of powering the internals when you feel like a hooligan and are working the machine hard. Thereafter carry out regular capacity checks to verify that the battery is not deteriorating, and load tests to see it can do its job.

When it fails under test bin it. Then buy a new one and test that before use.

[Mike Blandford]

I did suggest the discharge test is done at 0.5A to save time, although a C/10 discharge may well show the capacity available more accurately.

I also suggested: "Recharge at c/10 for 2 hours, then test in a model that servos drive OK (so testing with a battery that is only charged to 1/5th capacity."

Then I'd do a normal fast charge.

Mike.