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Old Sep 13, 2014, 06:14 PM
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Charging/Discharging NiMH Batteries?

I am not familiar with NiMH batteries. Is it best to cycle them (charge/discharge) or just always keep them topped off?

What is the "best practice" to use with these types of batteries?
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Old Sep 13, 2014, 06:45 PM
PGR
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I have some information about NiMH packs in my website:
http://pgrpages.com/nimh-battery-pack-maintenance/

The short version is cycling doesn't really help low self discharge (LSD) cells like Eneloops, but it's a good idea to cycle any NiMH pack at least once to test capacity and for dead cells. Regular NiMH cells can benefit from a few cycles but you can also overdo it. Also, NiMH cells store fine at full charge but they'll lose their charge pretty quickly if they're not LSD cells. Consequently, the non-LSD cells are best used hot off the charger.

Pete
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Old Sep 13, 2014, 07:15 PM
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The short version is cycling doesn't really help low self discharge (LSD) cells like Eneloops, but it's a good idea to cycle any NiMH pack at least once to test capacity and for dead cells.
Thank you for your input!

But how would a charger know if a cell was bad in a pack. It's not like LiPOs where it can "see" each cell.

Anyway, my pack looks good. Charges to 5.4v and it's rated at 4.8v.

BTW, is it possible (or better or worse?) to charge batteries like Eneloops on a HiTec X1 Touch vs. the Eneloop charger?
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Old Sep 13, 2014, 10:58 PM
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But how would a charger know if a cell was bad in a pack. It's not like LiPOs where it can "see" each cell.
Do a charge-discharge-charge cycle at about 0.3C and pay attention to the discharge and recharge mAh. If it's close to the rated mAh of the pack then the cells are probably OK. I'll assume that you're talking about the 800mAh AAA pack in the other thread you started so 0.3C would be 240mA

Quote:
BTW, is it possible (or better or worse?) to charge batteries like Eneloops on a HiTec X1 Touch vs. the Eneloop charger?
I charge AA and AAA Eneloop packs with programmable RC chargers all the time but I haven't been a big fan of Hitec chargers for charging NiMH packs because the "Delta Peak Sense" couldn't be set below 5mV. I just looked at the X1 Touch manual and see that it can be set as low as 3mV which puts it on par with my favorite NiMH charger which is the Hobbico AccuCycle Elite.

With that in mind I'd say that the X1 Touch is probably an excellent delta peak charger for NiMH packs, but I haven't actually played with one. And I assume that an "Eneloop" charger is for loose cells which definitely makes the X1 Touch a better choice for packs.

Pete
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Old Sep 15, 2014, 01:57 PM
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As long as the cells are well enough matched it should be fine. The most common problem I see with larger NiMH packs is that one cell gets a bit weak and that cell discharges more than the others. Eventually it gets so bad that the better cells charge the weak one reversed and that really kills it.
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Old Sep 19, 2014, 03:51 AM
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800mAh AAA pack in the other thread you started so 3C would be 240mA
PGR I think you meant 0.3C right?
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Old Sep 19, 2014, 08:16 AM
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PGR I think you meant 0.3C right?
Yup.

Mornings haven't been kind to me lately.

Pete
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Old Sep 19, 2014, 09:53 AM
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Yup.

Mornings haven't been kind to me lately.

Pete
No worries Pete

BTW, when you cycle a Nimh battery at say 1/10c, how do you keep from over charging?

Do you just keep the charger on for strictly 14 hours?
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Old Sep 19, 2014, 10:16 AM
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When charging at 1/10C or less, heat generated due to overcharge is minimal and does negligible harm to NiMH cells. Performing low current (1/10C or less) charges on a periodic basis ensures that all cells are balanced to the same state-of-charge and prevents damage due to potential overcharge when performing higher current peak charges.

I personally charge my Eneloop AAA and AA cells at 1/20C or less for a longer period of time (a day or so) to further prevent heat generation. Have been doing this will all series-connected Eneloop packs since they were introduced ~10 years ago and have never lost a single cell or suffered any measurable degradation. All of my transmitters are outfitted with Eneloop cells and are almost exclusively charged with the low current dumb chargers that were supplied with my transmitters.

Mark
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Old Sep 19, 2014, 03:26 PM
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+1, excellent advice


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Originally Posted by mrforsyth View Post
When charging at 1/10C or less, heat generated due to overcharge is minimal and does negligible harm to NiMH cells. Performing low current (1/10C or less) charges on a periodic basis ensures that all cells are balanced to the same state-of-charge and prevents damage due to potential overcharge when performing higher current peak charges.

I personally charge my Eneloop AAA and AA cells at 1/20C or less for a longer period of time (a day or so) to further prevent heat generation. Have been doing this will all series-connected Eneloop packs since they were introduced ~10 years ago and have never lost a single cell or suffered any measurable degradation. All of my transmitters are outfitted with Eneloop cells and are almost exclusively charged with the low current dumb chargers that were supplied with my transmitters.
+1 very pertinent and good advice. I do the same and have had excellent service from my Eneloop's.

Mark
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Old Yesterday, 12:36 AM
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Got it guys. Thanks for the advice.
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Old Yesterday, 07:04 AM
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Quote:
Originally Posted by mrforsyth View Post
When charging at 1/10C or less, heat generated due to overcharge is minimal and does negligible harm to NiMH cells. Performing low current (1/10C or less) charges on a periodic basis ensures that all cells are balanced to the same state-of-charge and prevents damage due to potential overcharge when performing higher current peak charges.

I personally charge my Eneloop AAA and AA cells at 1/20C or less for a longer period of time (a day or so) to further prevent heat generation. Have been doing this will all series-connected Eneloop packs since they were introduced ~10 years ago and have never lost a single cell or suffered any measurable degradation. All of my transmitters are outfitted with Eneloop cells and are almost exclusively charged with the low current dumb chargers that were supplied with my transmitters.

Mark
You are absolutely correct it's the best for Nimh,but what happens if time is not on your side and they are many who want to charge quick which is not so good for that chemistry.Luckily my Tx can take a 2s lipo which I charge in parallel with 2s flt packs.But not everyone is in that boat then delta peak charging is your only option.
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Old Yesterday, 10:09 AM
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what happens if time is not on your side and they are many who want to charge quick
No problem at all. If I'm at the field and my Tx dies I simply charge my Tx pack with my iCharger that I keep in my field box. I will typically charge them at 2A for 10-15 minutes and this will provide me with an additional 2-3 hours of flight time.

I will then plug my Tx into my dumb charger when I get home to fully charge my pack. Best of both worlds.

Mark
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Old Yesterday, 10:15 AM
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Originally Posted by mrforsyth View Post
I personally charge my Eneloop AAA and AA cells at 1/20C or less for a longer period of time (a day or so) to further prevent heat generation. Have been doing this will all series-connected Eneloop packs since they were introduced ~10 years ago and have never lost a single cell or suffered any measurable degradation. All of my transmitters are outfitted with Eneloop cells and are almost exclusively charged with the low current dumb chargers that were supplied with my transmitters.
And here's the other side of the coin:

I've also been using Eneloop cells since they were introduced in 2005. In fact, I had to order my first Eneloop cells from Japan because there were no American distributors yet. I made my own packs for my Futaba 9CAPS and a couple of slope planes with that first batch of cells and those packs still pass my annual capacity tests with flying colors.

Eneloops have been the only rechargeable AA and AAA cells I've purchased since then and I now have them in at least 15 gliders and 4 transmitters. I also have dozens of loose Eneloop cells that I use in my photo strobes and other consumer electronics.

And like Mark's Eneloops, almost every one of those packs and cells are still in use and show no significant signs of degradation despite the fact that I usually charge them with automatic "peak detect" chargers. I used the word "almost" in that last sentence because I have had a couple cells in RX packs succumb to physical damage from crashes, but those are the only exception. All the rest of my packs and cells are still in use and doing their job perfectly.

So I don't agree that slow charging them with a CC charger is better for them than peak detect charging at reasonable rates and proper peak sensitivity settings. Evidence suggests that it's probably as good, but better?. I guess we'll see in the next ~10 years, huh?

For the record, my go-to NiMH pack chargers have always been a pair of Hobbico AccuCycle Elites and my standard settings are 0.3C - 0.4C charge rate, 20mA trickle rate, 4mV peak sensitivity. For loose cells I'm currently using a pair of La Crosse BC-1000 chargers which I set to 200mA for AAA and 500mA for AA cells but I don't hesitate to go to 700 or even 1000mA for AA cells if I have a legitimate reason to hurry. Prior to buying the BC-1000s I used a Powerex MH-C800S in "Soft" mode, but I always felt that it got AAA cells a little warmer than I was comfortable with.

I do a 0.1C constant-current "balance" charge on my packs each spring and then do a discharge test at 0.25C with a CBA-III to verify capacity. If the packs test good (which they always do) I don't worry about them until next spring. I don't even test my loose cells anymore because it seems to be a waste of time. They aren't used in "mission critical" applications and the chargers charge each cell individually so balance isn't an issue. And they always test good, so I'm pretty much done testing loose cells until they give me reason to suspect they're losing capacity.

Pete
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