HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Aug 17, 2012, 07:18 PM
Aloft Hobbies
Rotozuk's Avatar
United States, CA, Novato
Joined Sep 2003
5,554 Posts
Alert
How to charge your NiMH batteries.

After posting what I thought was the correct method to charge our glider batteries, I found out I had made a number of mistakes. Please read through the thread a bit more and you also will learn a lot too!

Below is my original post for reference.

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

It has come to my attention that a number of slope pilots are cooking their NiHM battery packs and are not aware of it.

I started to realize this when folk's were complaining that some of the smaller batteries on on the market for small gliders were not holding the rated mAH. For example they were reporting less then 100 mAH for a 300 mAH pack. When I asked them how they were charging, the normal response was around 1C or higher, in this example that would be 300 mAH. In short they had charged at such a high rate that they have caused severe and permanent damage to the battery pack.

For NiMH, normal charging rate should be C/10, or 1/10 of the rated mAH of the pack. NiMH are sensitive to DAMAGE on overcharge when the charge rate is over C/10.


Normal Charging
The best way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.

Faster Charging
Using a timer it is possible to charge at C/3.33 for 5 hours. This is a little risky, since the battery should be fully discharged before charging. If the battery still has 90% of its capacity when the timer starts you would have a good chance of venting the battery. One way to ensure this doesn't happen is to have the charger automatically discharge the battery to 1 volt per cell, then turn the charger on for 5 hours.

Really Fast Charging
If a temperature monitor is used NiMH batteries can be charged at rates up to 1C (in other words 100% of the battery capacity in amp-hours for 1.5 hours).


If you are using the 1C charge rate, you best keep an eye on your battery! You are probably going to wipe out your battery if you are charging on a regular basis using this method. You have been warned. I have found that even very good chargers can miss the full charge indicators and will continue to charge a full battery.

A quick note on automatic smart chargers for NiMH batteries, they tend to fail to see the small minus delta V bump that indicates a full charge.
The minus delta V bump that is indicative of end-of-charge is much less pronounced in NiMH than NiCad, and it is very temperature dependent. To make matters worse, new NiMH batteries can exhibit bumps in the curve early in the cycle, particularly when cold. Also, NiMH are sensitive to damage on overcharge when the charge rate is over C/10. Since the delta V bump is not always easy to see, slight overcharge is probable.
If you pack is warm or hot off the charger, then you have probably damaged it and reduced it's capacity.

I have never found an advantage to cycling NiMH cells, nor have I ever found any methods that can repair or add capacity back to an overworked NiMH. Seems NiMH only gets worse with use and time. But a well cared for cell should last for years in our planes at or near the rated capacity.

Hope this will be of help to some folks.. Please pass this on to your NiMH flying friends.

-Wayne

p.s. I'm no expert in the field, so please let me know if I made any errors.
Rotozuk is offline Find More Posts by Rotozuk
Last edited by Rotozuk; Nov 27, 2012 at 05:20 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Aug 17, 2012, 09:47 PM
Everything is "only used once"
slope junkie's Avatar
United States, CA, Newport Beach
Joined Jul 2011
123 Posts
Sounds like you are using an old trickle charger as most if not all modern peak chargers detect when the battery is fully charged and stop the charge based on voltage. Trickle charging isnt that good because it keeps charging even when the battery is full by relying on the battery vents to release excessive hydrogen.

1C is a perfectly fine rate for nicads and nickle metals. What people dont know is what rate 1C is or what rate to set their charger:

A 300 mah battery charged @ 1c would be charged at a rate of .3 amps, a 1000mah would be @ 1amp and a 1500 @ 1.5 amp and so on. An completly dead battery charged at 1C will charge in approx 1 hour, no matter the capacity.

Lipos with high C Ratings (discharge rating) can be charged even higher than 1c without any adverse effects because they have such low resistance. My 950 mah 30c thunder power 2S pack is rated for 4.5c if using a smart charger. That's around 4.2 amps!
slope junkie is offline Find More Posts by slope junkie
Last edited by slope junkie; Aug 17, 2012 at 09:56 PM.
Reply With Quote
Old Aug 17, 2012, 10:32 PM
Registered User
flyzwell's Avatar
So Cal
Joined May 2008
2,633 Posts
Good thread Wayne.

I've found a 1C charge on my NIMH batts gets them way too hot.
If I'm in a hurry I'll charge them at half that and they only get a little warm.
Seems to work for me.
flyzwell is offline Find More Posts by flyzwell
Reply With Quote
Old Aug 18, 2012, 12:23 AM
Aloft Hobbies
Rotozuk's Avatar
United States, CA, Novato
Joined Sep 2003
5,554 Posts
The main reason I have brought this up here in the slope section is due to the number of folks I have run into recently that are having major battery issues. One person was charging at over 2C, and was blaming his Spektrum gear for recent failures. His batteries had almost no capacity.

You may get away with 1C on some cells for a while, but you are damaging the cells and reducing their capacity. Thus the statement:
Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10.

I do not use an old trickle charger, I personally use an iCharger 106B, a very good charger. It still fails to detect when NiMH cells reach a full charge. I probably need to fine tune the detect method for my smaller cells. It does have a nice "break in" method for brand new NiHM cells that I like to use.

I think you will find any cells that have been charged at 1C can no longer provide the rated mAH.

Don't believe or trust me, do the research yourself. Heck, read the instructions that came with your charger.

The Alula 300 mAH battery should be charged at .03 amps. That is a small battery, and you want to keep as much capacity as you can. A regular wall wart charger will cook this little battery, my good ol' Futaba wall wart is rated at .1 amps. As mentioned it is hard to charge these little batteries safely as most of our chargers put out too high a charge rate for them, and the smaller cells hate the abuse more then most.

As an example I had a customer that bought the Alula 300 mAH battery and after a couple of charges he wanted to return it as it was only giving him about 100 mAH. Turns out he was charging at 1C for extended periods.

I don't know where everyone got the 1C charge rate in their heads, but it is wrong. 1/10 C or .1 C is what you want if you have the time. I try to charge all of my batteries the night before if I can. For my bigger batteries (AA or larger) I just use the old wall warts, they work great!

Hope this might help some of you out!

-Wayne
Rotozuk is offline Find More Posts by Rotozuk
Reply With Quote
Old Aug 18, 2012, 01:20 AM
She's Flying Again
fly-by-night's Avatar
Newport Beach, CA
Joined Oct 2008
341 Posts
HiMh

Wayne,
For my money your correct, this is maybe were they have searched?

http://batteryuniversity.com/

But I also use wall chargers along with my main charger and everything has lasted at least 4+ year or more with discharges within 4%. I have found that the most critical point is the brake in - most important for long lasting batteries. Don't forget that both elements Heat and Cold take a toll on batteries, you can lose 1% per day on average, so they say?

Steve


Quote:
Originally Posted by Rotozuk View Post
The main reason I have brought this up here in the slope section is due to the number of folks I have run into recently that are having major battery issues. One person was charging at over 2C, and was blaming his Spektrum gear for recent failures. His batteries had almost no capacity.

You may get away with 1C on some cells for a while, but you are damaging the cells and reducing their capacity. Thus the statement:
Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10.

I do not use an old trickle charger, I personally use an iCharger 106B, a very good charger. It still fails to detect when NiMH cells reach a full charge. I probably need to fine tune the detect method for my smaller cells. It does have a nice "break in" method for brand new NiHM cells that I like to use.

I think you will find any cells that have been charged at 1C can no longer provide the rated mAH.

Don't believe or trust me, do the research yourself. Heck, read the instructions that came with your charger.

The Alula 300 mAH battery should be charged at .03 amps. That is a small battery, and you want to keep as much capacity as you can. A regular wall wart charger will cook this little battery, my good ol' Futaba wall wart is rated at .1 amps. As mentioned it is hard to charge these little batteries safely as most of our chargers put out too high a charge rate for them, and the smaller cells hate the abuse more then most.

As an example I had a customer that bought the Alula 300 mAH battery and after a couple of charges he wanted to return it as it was only giving him about 100 mAH. Turns out he was charging at 1C for extended periods.

I don't know where everyone got the 1C charge rate in their heads, but it is wrong. 1/10 C or .1 C is what you want if you have the time. I try to charge all of my batteries the night before if I can. For my bigger batteries (AA or larger) I just use the old wall warts, they work great!

Hope this might help some of you out!

-Wayne
fly-by-night is offline Find More Posts by fly-by-night
Reply With Quote
Old Aug 18, 2012, 01:26 AM
When the wind blows
African's Avatar
South Africa, WC, Cape Town
Joined Jul 2012
458 Posts
Thanks Wayne for the great info . I have had plenty of my ni-mh batt not keep there stated mah . The cells are rated as 1800 and most of the time we get only half of that. Most of us , especially me just put in any high amount to get them charged ASAP to get to the slope ! Yes and they do get hot. Let's say there are perfectly charged and I do not go fly for a month , do I charge them again , and if so at what rate? I still have some ni-cad batt which are all most 20 years old and they still work reasonably well ! Unfortunate'ly they are getting less and less available .
Anton
African is online now Find More Posts by African
Reply With Quote
Old Aug 18, 2012, 02:06 AM
Aloft Hobbies
Rotozuk's Avatar
United States, CA, Novato
Joined Sep 2003
5,554 Posts
Quote:
Originally Posted by African View Post
Thanks Wayne for the great info . I have had plenty of my ni-mh batt not keep there stated mah . The cells are rated as 1800 and most of the time we get only half of that. Most of us , especially me just put in any high amount to get them charged ASAP to get to the slope ! Yes and they do get hot. Let's say there are perfectly charged and I do not go fly for a month , do I charge them again , and if so at what rate? I still have some ni-cad batt which are all most 20 years old and they still work reasonably well ! Unfortunate'ly they are getting less and less available .
Anton
Most NiMH cells loose about 10% of their charge a month. (I hear different figures on this discharge rate. Seems different cells have different numbers.) So, yeah it is a good idea to top them off if you are planning for a LONG flight time. But most slope planes usually have rather large capacity these days, so that top off charge may not be needed. But if I'm flying a fully molded plane with 6 or 8 digital servos in it, I'm plugging it into a wall wart the night before.

The exception to the above would be the Sanyo Eneloop style batteries. They are designed to hold their charge, and will not need a recharge if fully charged within the last year or so. (Claims 85% of charge at one year.)

When you charge at C/10 or lower you should not damage your battery unless you leave it on for a VERY long time. (Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10.)

If you wanted to keep a NiMH fully charged all the time (like an emergency flashlight) I think you would charge them at something around C/33. (That is based on my fading memory, might wan to double check that!) Not really a good idea to use NiMH for such a use.. NiCad is probably better suited to that abuse.

NiCads seem to last a very long time. NiMH does not seem to have as long a life, but they do have much better capacity.

-Wayne
Rotozuk is offline Find More Posts by Rotozuk
Reply With Quote
Old Aug 18, 2012, 03:33 AM
Sane til the lift starts!
kpeevyhouse's Avatar
Laguna Beach, CA
Joined Jun 2008
10,189 Posts
Some thoughts:

The idea of charging at 1C comes from a general rule of thumb for LiPo batteries. Unfortunately, people have gotten it into their heads that this applies to ALL battery types.

The more cells in the pack, the lower the charge rate should be. (i.e. a transmitter pack with 8 cells). Transmitter packs get hot pretty quickly.

NiCads are much more resilient to overcharging and using higher charge rates than NiMH.


My "fast-charging" rates for a 4-cell 2200mah AA size NiMH is around 0.7 amps. This is what I use to top off a partially discharged battery when I'm in a hurry. I NEVER charge that high when the battery is low, because it will get hotter the longer it's charging. If it's going for 15 minutes, it won't get warm at that rate. But if it's going for an hour at that rate, it'll start getting hot. My "normal" charge rate for such a pack would be around 0.4amps.

I don't have any specific formula to figure out my charge rates. It's more of an art: something that I do by "feel", knowing roughly how much charge is left in a pack, the size of the pack, how much of a hurry I'm in, etc. I have a pretty good sense of how quickly I can charge any given pack without having it get too warm. That's really my only criteria... to charge as fast as I can without the pack getting too warm.

Bottom line: monitor the packs throughout the entire charge cycle. The longer it's charging, the more often you should check the temperature. If it starts getting hot, then shut it down let it cool and continue at a lower rate.


My $.02
kpeevyhouse is offline Find More Posts by kpeevyhouse
RCG Plus Member
Reply With Quote
Old Aug 18, 2012, 10:27 AM
It's not going to build itself
TRISME's Avatar
Aliso Viejo,CA
Joined Aug 2005
6,040 Posts
I've had some nimh packs die long before I thought they should have, and it's probably from cooking them on my way to the slope.
TRISME is offline Find More Posts by TRISME
Reply With Quote
Old Aug 18, 2012, 10:39 AM
Dunetop Flyer
jcarstan's Avatar
Cape Cod, Mass., USA
Joined Dec 2002
777 Posts
Good info. I too have had many packs die all too soon or not charge to full capacity. Now I just have to figure out where I placed the now dead batteries in my foamies and how I wired them to the switch.
jcarstan is offline Find More Posts by jcarstan
Reply With Quote
Old Aug 18, 2012, 11:00 AM
Registered User
Mike Furcolow's Avatar
Paonia, Colorado
Joined Sep 2004
2,417 Posts
Great timing for this post! I just bought a quality charger and want to know how to use it properly. I'm carefully reading through the manual just now....

Anyone care to post a general "quick charge" rate that they use for a 4-cell, AA Nimh, 4.8V, 2000mah receiver pack? Kpeeveyhouse in the post above suggests a quick charge rate of 0.7amps. Sounds reasonable to me...

How about a quick charge rate for the 8-cell 9.6V transmitter NiMh battery?
Mike Furcolow is offline Find More Posts by Mike Furcolow
Last edited by Mike Furcolow; Aug 18, 2012 at 11:08 AM.
Reply With Quote
Old Aug 18, 2012, 11:22 AM
Registered User
LI, New York, USA
Joined Mar 2003
23,928 Posts
Good discusion and quite timely for me.

First, as I understand it there are two classes of NiMh cells, higher resistance which must be charged more slowly, and lower resisitance which can take higher charge rates.

Anyone who has flown electric airplanes using NiMh packs is very familiar with the 1C to 1.5C typical for NiMh motor packs. These are typically 2/3A to 4/5A for parkflyers. In fact the new 1500 5 cell NiMh packs I purchased with my new Supra, which appear to be 2/3A motor pack cells, are labeled to be charged at up to 2 amps. I typically charge them at 1/10 to 1/2 C but might give them a quick charge at the field at 1 amp if they need to be topped up.

When I was using NiMh for my parkflyers I used to charge them at 1C to 1.5C and discharge them at 10C and get hundreds of cycles on them with very little loss of capaity. But those were motor packs.

When I have purchased AA or AAA or N cell receiver packs, they have been labeled to be charged for 14 to 16 hours at 1/10 C as a "set-up" charge, then 1/10 C thereafter. But those were not motor packs, they were sold as receiver packs.

What are your thoughts on this?


Now, a question. Does anyone have a chart for NiMh cells that shows %charge capacity based on voltage?

I just purchased this Hyperian battery checker.
http://media.hyperion.hk/dn/sentry/

Among its features is to indicate the approximate charge level. So I used it to check the charge state of a 4 cell receiver pack. It said the pack was at 5 volts, but indicated that it was only at about 5% charge capacity. That surprised me. At 5V I would have expected it to be at 60% or more.

I usually see NiMh packs come of the charger at about 5.4 volts and drop to about 5.2V after a few hours, which I associate with a fully charged pack, about 1.3V per cell. My Hyperian is suggesting that at 1.25V/cell they are 95% used. Seems odd for a pack with a 4.8V nominal voltage.

So, does anyone have a table for NiMh packs that relates voltage to charge state?
aeajr is online now Find More Posts by aeajr
Reply With Quote
Old Aug 18, 2012, 11:28 AM
Where's the wind?
the_canuck's Avatar
United States, CO, Denver
Joined Jun 2004
5,811 Posts
It you are using Eneloop cells, anything up to 1C is okay to use. For the first few charges its best to charge at most 0.5C. NIMH cells are pretty robust, especially in the larger sizes. For smaller cells be more conservative as there is less surface area and they can overhead quickly which can cause damage and shorten the life of the pack.

Buy quality NIMH cells as well. Some of the off brand stuff works okay at first but just doesn't hold up over time no matter how delicately you charge them. I stick with Sanyo cells now after learning that lesson early on in my battery pack building days.

During my research of off brand cells, I found out many of the off brands are seconds from the same factories that produce Energizer, etc. They re brand the rejects and sell them much cheaper.

Andrew
www.canuckengineering.com
the_canuck is offline Find More Posts by the_canuck
Reply With Quote
Old Aug 18, 2012, 12:00 PM
Aloft Hobbies
Rotozuk's Avatar
United States, CA, Novato
Joined Sep 2003
5,554 Posts
I think a better question is HOW FAST DO YOU NEED TO CHARGE A PACK?

If you have the time, always charge at C/10 or less and you will not have any worries.

If you do not have a lot of time, then you can charge faster, but with caution. I generally try to keep it to C/5 or so. We are almost always topping off our batteries, not fully charging a dead pack. Typically an AA pack for a typical sloper probably only needs a couple of hundred mAH added back ot the pack.. This is based on most of us having several planes in the fleet, and flying for an hour or less with 2 to 4 servos.

-Wayne
Rotozuk is offline Find More Posts by Rotozuk
Reply With Quote
Old Aug 18, 2012, 12:26 PM
Registered User
Rick T.'s Avatar
United States, CA, Trabuco Canyon
Joined Aug 2008
1,357 Posts
Help on purchasing a battery charger

Great thread Wayne.

On my way to return a Hobby people Tazr battery charger, looking for opinion on what type to buy. Batteries right now are all NIMH and NICD, looking for something easy to program and not cost an arm and a leg.

Need advise

Rick
Rick T. is offline Find More Posts by Rick T.
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion How to measure mah left on nimh batteries babanan Batteries and Chargers 1 Aug 15, 2012 07:07 PM
Discussion How to charge my nimh-battery with Imax B610 AC babanan Batteries and Chargers 1 Aug 01, 2012 07:58 PM
Help! Need Help Charging NiMH batteries with B6AC Charger bobepine Batteries and Chargers 8 Jun 16, 2012 04:19 AM
Question Nimh battery charging in Tx Bob A Batteries and Chargers 12 Mar 07, 2012 02:07 PM
Discussion Select how much mAh to charge 1100mAh NiMH battery? congcon97 Batteries and Chargers 7 Mar 03, 2012 07:47 AM