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

[Jonathan M]

I must have foolishly knocked the RX switch on a few weeks ago, as I've now got an almost new but completely dead Eneloop 4.8v 2000mAh pack.

Is this now permanently dead, or can it be charged back to (reliable) condition for use in the model? If so, what rate should I use on my smart charger - 0.1C (for better balancing) or my usual 0.5C (for better peak voltage detect)?

[Tom Thomas]

I'm no expert, but I'd bin it and buy another.

[JohnP25]

Have you tried charging with your tx's charger?

[Former Member]

[This posting has been removed]

[Chris Channon]

Hi Jonathan, charge it up and instead of risking a model, if it holds a charge consign it to w/shop duties ie servo tester, setting up linkages etc.
Just an idea, have a great day
Chris C

[Peter Miller]

I have left my transmitters on quite a few times and flattened the batteries over the years.

I just charged them up to full using the normal TX charger and carried on using them.

I have never had a a problem with them..

Same with receiver packs.

I just keep a check on them to see that they hold their charge and only bin them if they don't

On transmitters you have a reading of the battery voltage allthe time so if you are worried you can keep an eye on them and see if they are going down as you fly.  Never happened to me.

 

Edited By Peter Miller on 04/10/2017 08:08:54

[Frank Skilbeck]

Often, transmitters have a low voltage alarm and then a low voltage cut off, so the batteries are not completely drained, but it depends on the Tx and as Peter says you do have a voltage readout on the Tx.

But for a receiver Nimh pack that has been completely drained, I do a slow charge and then cycle it a couple of times to check it's capacity and ability to work under load, if it doesn't match up then it's off to the recycling.

[Jonathan M]

Thanks for the views. I didn't want to try recharging it until I'd checked the consensus, also what charging rate.

I'm usually of the opinion that one shouldn't spoil the ship for a ha’porth of tar (or in this case an Acrowot, 70 FS, RX and five servos, say £350 all in), but its worth trying Frank's method of charging and testing - if only to find out for myself.

[Peter Christy]

You may find that an "intelligent" charger will refuse to charge it, as it won't register any voltage and assume an error. Equally you may find that one or more cells are "stuck" at zero volts.

Try a trickle charge first (1/10 C) and monitor the voltage. If it comes up to 4.8V or more quickly, you are good to carry on charging as normal.

If it only comes up to 2.4 or 3.6V, then you have a stuck cell. They can often be unstuck by a short burst of high current. You can achieve this with a 6V (or thereabouts) source of some kind. Connect the battery negative to the source negative, then quickly dab the positives together a few times until the voltage rises to normal. Then trickle charge as above.

DON'T apply the high current for any length of time - a few seconds at most - but I have recovered many "dead" cells using this technique.

Finally, cycle the battery several times to check its capacity. If in any doubt, use it as a bench test pack, as suggested by Chris above.

Best of luck!

--

Pete

[Don Fry]

Why does running a NiMh battery flat harm it. And having recharged it, and checked it it holds charge, delivers load as specification, why would you trust it any less than its new replacement.

[Martin Harris - Moderator]

The theory that I've read is that the lowest voltage cell in the pack reaches zero first and if there's any significant imbalance, becomes reverse charged i.e. the polarity effectively reverses across that cell. This does that cell no good at all, but a well balance pack may get away with being flattened. In years gone by, it was said to be good practice for NiCds to discharge them completely for long term storage - with the proviso that each cell needed discharging individually, which fits in with the above.

I have used flattened nickel batteries after doing a few test cycles without problems - although I have the benefit of rx battery telemetry to give me warning of any impending problems. I even recovered a LiPo that had been completely discharged (no discernable voltage when tested) in a tree for a couple of weeks and it gave about 90% of its pre-discharge performance for many cycles. It's always possible (although unlikely) that a new battery could have a manufacturing fault so the "replace option" is not 100% safe.

Edited By Martin Harris on 04/10/2017 10:31:12

[Kevin Wilson]

It was always considered good practice to completely discharge nickel based batteries to zero.
I have frequently taken nimh and nicd to zero without harm.
I would recharge this pack at 1/10C but only because Eneloops are not quite as robust to high currents.
I would not expect any charger to grumble about low voltage on a nickel setting as nicd/nimh do not have low voltage issues encountered with lithium batteries.
If in doubt, do a few cycles and check the capacity.
When I was rc car racing I regularly cycled my race packs 5C discharge 3C charge as a conditioning routine. It would restore their capacity.

Edited By Kevin Wilson on 04/10/2017 11:32:29

[Kevin Wilson]

As some previous posts seem to be based on modern lithium cell rules I will share my experience with LiPo as distinct from my use of Nickel technologies.

I have not been so lucky with lithium cells taken to zero. 50% will not recover. and those that do show reduced capacity and I have little confidence in them. Those do go in the bin.

It seems the less time a Lithium cell is at zero volts the better it can be recovered. One I had at zero volts for a few days recovered completely, One at Zero for a few months lost one cell completely and the other cells had <20% capacity. So quite similar to Martins example above.

My experience runs counter to the common view that a Lithium cell taken below 3 volts is a gonner.

As context I only use max 3S 2300mAh and in less demanding applications. All my cells are budget (turnigy, zippy etc) sourced from various internet sources. I always balance charge and always at 1C charge rate.

Edited By Kevin Wilson on 04/10/2017 11:50:44

[Martin Harris - Moderator]

It might not be a bad idea to trickle charge it on a "dumb" charger at C/10 for 14 - 16 hours from discharged (or a little lower charge for proportionately longer if that's not possible) to re-form the pack. I do wonder how many of us take the trouble to do a proper forming charge on new packs and how much that affects their lives? Please note that using a delta peak detect charger does not do this correctly.

[Nigel R]

"It seems the less time a Lithium cell is at zero volts the better it can be recovered. "

As I understand it, charge levels below 30% (ish), the cell chemistry begins to degrade. The lower the charge, the faster the degradation. So if you flatten it then immediately charge it, you're probably OK. Leave it a week or two and it'll lose capacity and gain resistance. Etc.

I always thought nimh cells were a bit touchy when it came to deep discharge, too. I've certainly had a lot of loose AA cells fail to fast charge after being removed from various toddler toys and left going flat in a drawer. Even trickle charging doesn't seem to get much capacity back after this level and time of discharge.

Nicads used to be tough as old boots on this front though.

[Jonathan M]

Posted by Martin Harris on 04/10/2017 11:51:45:

It might not be a bad idea to trickle charge it on a "dumb" charger at C/10 for 14 - 16 hours from discharged (or a little lower charge for proportionately longer if that's not possible) to re-form the pack. [...] Please note that using a delta peak detect charger does not do this correctly.

Why would this be the case? I understood that it is precisely the low charge rate of C/10 that might cause the delta peak detect to not happen, equating to a dumb charger just going on until one manually turns it off. Also my smart charger allows me to change the delta peak value - to a level which the pack can't reach.

[Mike Blandford]

NiMh cells handle a c/10 charge rate even when fully charged. You may charge at that rate for 24 hours or more quite safely. This is the method to use to ensure a NiMh pack is "balanced" i.e. all cells are fully charged.

Over time a pack of NiMh cells will become unbalanced as some cells self discharge at a (slightly) higher rate than others. Every so often a NimH pack should be charged at c/10 to get all cells fully charged.

Mike.

[onetenor]

Flash the pack with a welder I mean JUST FLASH touch the contacs . DO NOT hold in contact, |Then charge as normal. This treatment also works for failing cells/packs

[Martin Harris - Moderator]

Posted by Jonathan M on 04/10/2017 13:04:05:

Posted by Martin Harris on 04/10/2017 11:51:45:

It might not be a bad idea to trickle charge it on a "dumb" charger at C/10 for 14 - 16 hours from discharged (or a little lower charge for proportionately longer if that's not possible) to re-form the pack. [...] Please note that using a delta peak detect charger does not do this correctly.

Why would this be the case? I understood that it is precisely the low charge rate of C/10 that might cause the delta peak detect to not happen, equating to a dumb charger just going on until one manually turns it off. Also my smart charger allows me to change the delta peak value - to a level which the pack can't reach.

"the low charge rate of C/10 that might cause the delta peak detect to not happen"

[TartanMac]

I done this with a few 6v NiMH batteries. One went faulty because one cell packed in and the battery would charge and discharge but once you put it in a model (car) it would drain fast. Be carefull also had a vapex 2900mah battery just pack in when in an aeroplane lucky it was on the ground. When NiMH go bad it happens fast. I don't trust batteries that the voltage has dropped too low. Peace of mind is worth £20 in my book lol.

[Former Member]

[This posting has been removed]

[Mike Blandford]

I suggest charge at c/10 for 12 to 14 hours, then put the pack on a charger with a discharge function and check the pack provides something close to its rated capacity.

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.

If this is all OK, then charge normally (peak detect) and the battery should be fully ready.

I've use flight power packs (NiCd and NiMh) that have been fully discharged then recharged (pre-lipo days) without any problems.

Mike.

[stewart BAKER]

please bin it . it is never worth the risk .its just a battery

[Mike Blandford]

Stewart: I'd like to see a reason for your statement.

In my experience, NiMh (and NiCd) cells are quite resilient, unlike lipos that really don't like being over discharged.

If my suggested method is followed to check the pack, I can't see any problems with then using the pack. In fact, I think I would prefer it to a brand new pack that hasn't been tested for capacity and discharge capability.

Mike.

[stewart BAKER]

sorry I just wont risk possible damage to 3rd parties or property for the price of a battery . I tend to err on the side of caution I replace my on board batteries and transmitter batteries every 2 yrs all my models have telemetry to monitor battery voltage during flight also good for highliting tight or over pushing servo setups. just me being a bit insecure I guess