Advice Wanted: Charger for 8-12 Cell NiMH Packs

Greetings Everyone

I have been assembling a variety of Mag hotwire mods as gifts for friends and colleagues. All of these flashlights use FM AA battery holders with 8 to 12 cells in series. I would like to provide the recipients of these flashlights a charger that would allow them to charge the pack through the built-in female charging jack.

FM points to the chargers available here. However, I am having a hard time finding a charger on this site that fits the following criteria:

1.) will charge NiMH packs from 8 to 12 cells
2.) will plug into 110 AC
3.) does not require the use of a temperature sensor

This charger here seems to be ideal, but requires that you use the temperature sensor for voltages higher than 12V. This seems like kind of a pain in the butt if you want to leave the battery holder in the flashlight while charging.

So, I am looking for your opinions and advice on simple to use and reliable chargers for charging 8 to 12 cell series NiMH packs.

Thanks in advance for your help.

Cheers,
Battery Guy

I think it depends on how aggressive you want to be regarding charging safety. I have used that exact charger many times without the temp probe, and it always terminated fine. Some people clip off the temp probe wire altogether (not that I recommend that).

The one thing I found using NiMH cells of various types in FM's carriers is that over a certain amount of charge/discharge cycles, the cells would often get significantly out of balance, depending on their quality, age, & position in the series string. Eventually, I found the need for a hobby charger to be able to do an occasional conditioning charge. For most practical purposes, I think this would work.

Edit: Now that Jason's post reminded me of that one, and its price and charge range capability, that is hard to beat....but....the people using it may want something REALLY simple like the link above.

Yeah, it doesn't sound like the people your setting up are going to want a hobby charger to charge these.

What I've thought of doing for NiMH packs similar to this, but have never actually done it, Is find a proper voltage wall wart with the proper connector already on it, that would charge the packs at a 0.1C rate. This would negate the problem Lux mentioned, as whatever kind of charger you use, hobby or otherwise, that charges the pack at a rate higher than 0.1C, the cells will eventually get out of balance.

The only drawback is that charging would take 14-16 hrs. On the other hand, if the users forget and leave the pack/light on charge, it's not going to really hurt anything. Plus, it'd be about as simple and no fuss as you can get.

Dave

Dave, isn't there a pretty firm guideline though to charge NiMH at like 0.5 to 1.0 C rates to get proper termination. I'm thinking (but would need to check Battery University) that only 0.1 C charging NiMH will damage them. There's just no easy answer to proper battery care it seems. I kind of miss the good old early days, when ignorance was (fortunately) bliss.

LuxLuthor said:

Dave, isn't there a pretty firm guideline though to charge NiMH at like 0.5 to 1.0 C rates to get proper termination.

Click to expand...

This is true if you're trying to get a -dV termination with a "smart charger". A 0.1C charge rate however, is the rate that is used for a 14-16hr forming/balancing charge. This rate has been shown to have little, if any ill effect on NiCd, or NiMH cells. GP ran a test where they left NiMH cells on charge for 1 year at this rate, and according to them, the cells suffered minimal damage. I'm not too sure of that myself and personally, try to stop the charge on time, as I don't figure going beyond 16hrs helps any either. At the same time, if someone were to forget they were charging their pack for a day or so, it's not going to hurt anything.

One possible downside to using a 0.1C charge rate for hotwires, is that many suggest that if you are going to discharge NiMH cells at a high rate, you should also charge them at a high rate. Still, for a super simple charging setup, which BG is looking for, this may work out. Before I procured a hobby charger I used to do basically the same thing with a bench PS and never had any problems. I'm just suggesting this as a possible solution.

Dave

LuxLuthor said:

I think it depends on how aggressive you want to be regarding charging safety. I have used that exact charger many times without the temp probe, and it always terminated fine. Some people clip off the temp probe wire altogether (not that I recommend that).

Click to expand...

Oh, that is good to know. In the specifications section it states "You must use temperature sensor to charge 12V or higher NiMH battery pack". I interpreted this as the charger will not work if the temperature probe is not used. Now I interpret this as "we are telling you that you must use the temperature probe to cover our butts in the event that our charger fries your batteries".

LuxLuthor said:

The one thing I found using NiMH cells of various types in FM's carriers is that over a certain amount of charge/discharge cycles, the cells would often get significantly out of balance, depending on their quality, age, & position in the series string. Eventually, I found the need for a hobby charger to be able to do an occasional conditioning charge. For most practical purposes, I think this would work.

Edit: Now that Jason's post reminded me of that one, and its price and charge range capability, that is hard to beat....but....the people using it may want something REALLY simple like the link above.

Click to expand...

Yeah, the people I make these for really don't want to be dealing with a hobby charger. I had not considered that the cells would go out of balance, but you are absolutely correct. Hmmm....I need to think more about this.

Thanks Lux.

Cheers,
BG

45/70 said:

What I've thought of doing for NiMH packs similar to this, but have never actually done it, Is find a proper voltage wall wart with the proper connector already on it, that would charge the packs at a 0.1C rate. This would negate the problem Lux mentioned, as whatever kind of charger you use, hobby or otherwise, that charges the pack at a rate higher than 0.1C, the cells will eventually get out of balance.

Click to expand...

Dave, you solved my problem! I can provide two chargers with flashlights: one smart charger like this for fast charging when needed, and the other a "dumb" charger that will supply 0.1C or less current for normal charging. This solves the problem that Lux brought up about the cells going out of balance. If the users always want to use the faster smart charger, I can tell them to make sure that they occasionally use the slow dumb charger to rebalance the pack.

Thanks Dave!

Cheers,
BG

45/70 said:

One possible downside to using a 0.1C charge rate for hotwires, is that many suggest that if you are going to discharge NiMH cells at a high rate, you should also charge them at a high rate.

Click to expand...

I have read this a few times on CPF, and quite frankly it has never made sense to me, unless you are discharging the cells hot off the charger, in which case charging at high rate will produce a hotter cell that will perform better at high current loads. Otherwise, I cannot think of any reason why it would make any difference how the cell is charged, as long as it is fully charged.

Do you know if anyone has shown this effect through testing?

Cheers,
BG

Kinda expensive, but you might find one used for cheap on eBay, or elsewhere: Ace DDVC I've had one for years, and it's the only charger I'll use for doing the slow/break-in charge on NiMh and NiCd. Only goes to 10 cells though...

clintb said:

Kinda expensive, but you might find one used for cheap on eBay, or elsewhere: Ace DDVC I've had one for years, and it's the only charger I'll use for doing the slow/break-in charge on NiMh and NiCd. Only goes to 10 cells though...

Click to expand...

Nice, but $$ Ka-Ching

clintb said:

Kinda expensive, but you might find one used for cheap on eBay, or elsewhere: Ace DDVC I've had one for years, and it's the only charger I'll use for doing the slow/break-in charge on NiMh and NiCd. Only goes to 10 cells though...

Click to expand...

Thanks clintb. How does the charger terminate charge? It doesn't say that it uses delta V or a timer, and I don't see a thermocouple. Or do you need to terminate charge manually?

By the way, I am a Mac user and your avatar is really freaking me out!

Cheers,
BG

45/70 said:

What I've thought of doing for NiMH packs similar to this, but have never actually done it, Is find a proper voltage wall wart with the proper connector already on it, that would charge the packs at a 0.1C rate. This would negate the problem Lux mentioned, as whatever kind of charger you use, hobby or otherwise, that charges the pack at a rate higher than 0.1C, the cells will eventually get out of balance.

Click to expand...

Turns out that it is kind of hard to find a wall wart with a 200 mA current limit in the voltage range I am looking for.

After doing some searching, I have come upon this. Voltage is sufficiently high to cover the entire range of battery packs that I would make. The current output is a bit more than 0.1C for an Elite 1700 or an eneloop, but I don't think that it should be too much of a problem. The price is also right.

I think I am going to give this a try. Thanks again Dave for the suggestion.

Cheers,
BG

Battery Guy said:

I think I am going to give this a try. Thanks again Dave for the suggestion.

Click to expand...

If possible, I would err on the side of less than 0.1C. If you go higher than that, you do run the risk of overcharging, excessive heating, premature cell wear etc. I would think with the popularity of wall warts nowadays, that the various parts houses would have a wide range of these. Then again, as I said, I really never looked into it, I just used a bench PS.

As for the fast discharge/fast charge, SilverFox and some others here including myself, I think, were discussing this in a thread a few years ago. I'll never find it, but as I remember, the idea was related to the fact that during charging, the smaller grains charge more easily than larger grains and somehow the faster charge rate is more beneficial for fast discharging. Yeah, I know, I'm not much help here because I can't remember the specifics.:candle:

I know the R/C folks do this, probably because of the resulting hotter cells, as you mentioned and of course, to save time. I think there's more to it than that though. If I have time, I'll look for that thread. As I said, I never had any problem with using a PS. Most of the time I cheated a bit though. I'd charge the packs at 0.5C until they just started to get warm, and then drop the current to 0.1C (or less) for an hour, or so.

Dave

Battery Guy said:

...Otherwise, I cannot think of any reason why it would make any difference how the cell is charged, as long as it is fully charged...

Click to expand...

The way I remember it is that:

• SLOW Charging allows LARGE crystals to form which DO NOT 'dissolve / give back their stored energy' quick enough under HIGH current loads.
  .
• FAST Charging produces SMALL crystals which DO give back their energy quick enough under HIGH current loads.

Battery Guy said:

...Do you know if anyone has shown this effect through testing?

Click to expand...

I'm thinking SilverFox (somewhere in the CPF Archives) :thinking: :

SilverFox said:

"What is a good rate to charge my NiMh cells?" and "Is this charger good?" are frequent questions that come up around here. Slow charging has been around forever, and it is used in determining cycle life. However, there is a provision in the testing standard for "Accelerated Test Procedures," when testing for cycle life. The accelerated procedure involves charging and discharging at 1C. The goal is to achieve more than 500 cycles. The accelerated test procedures have a footnote that states that 1C charging should be done for 1 hour, or with appropriate charge termination, as recommended by the manufacturer...

---SNIP---

...When using peak voltage termination, you now need to look beyond termination to determine the best charging rate for your cells. Slow charging produces large crystals, and large crystals produce voltage depression. Once again, ultra slow charging may not give you the best performance. Now we are in an area that is application dependent. I have often heard that you should charge at about the same rate that your application uses the cells. There may be a lot of truth in this...

Click to expand...

There's probably more detail in another one of the threads LINKed in my FAQ, but, 'tidbits' aren't generally well-labeled.

45/70 said:

As for the fast discharge/fast charge, SilverFox and some others here including myself, I think, were discussing this in a thread a few years ago. I'll never find it, but as I remember, the idea was related to the fact that during charging, the smaller grains charge more easily than larger grains and somehow the faster charge rate is more beneficial for fast discharging. Yeah, I know, I'm not much help here because I can't remember the specifics.:candle:

Click to expand...

TakeTheActive said:

The way I remember it is that:

• SLOW Charging allows LARGE crystals to form which DO NOT 'dissolve / give back their stored energy' quick enough under HIGH current loads.
  .
• FAST Charging produces SMALL crystals which DO give back their energy quick enough under HIGH current loads.

Click to expand...

Hi Gents

I think that this explanation has its roots in the nickel-cadmium chemistry, where the metallic cadmium electrode dissolves and forms cadmium hydroxide crystals on discharge, and the opposite occurs during charge. In this chemistry, the charge and discharge rates can have an effect on the size and shape of the cadmium and cadmium hydroxide crystals.

However, in the NiMH system, there is no dissolution and/or precipitation of any materials. The metal hydride alloy is essentially a sponge that holds the hydrogen. Over time, the metal hydride alloy does form a film of corrosion product on the surface, and eventually self-pulverizes, but those changes are permanent and irreversible, and in general thought to degrade both capacity and power performance.

I am really quite intrigued by this idea that charging a NiMH cell faster will improve its high rate discharge performance, but I am still skeptical. I have a few Elite 1700 cells. Help me come up with some simple experiments to show if this is really true or an urban CPF legend.

Cheers!
BG

Hello Battery Guy,

Here is a simple test...

Cycle the cell a few times to mix the chemicals, then charge at 0.1C for 16 hours, let the cell rest for 1 hour, then perform a 5C discharge down to 0.8 volts and record the capacity and mid point voltage. Next charge at 2C, rest for 1 hour, then discharge at 5C.

When you compare the results you would hope to find that the 0.1C charge gave you a little more capacity, but the 2C charge give you higher mid point voltage if there is any truth in this.

Tom

SilverFox said:

Hello Battery Guy,

Here is a simple test...

Cycle the cell a few times to mix the chemicals, then charge at 0.1C for 16 hours, let the cell rest for 1 hour, then perform a 5C discharge down to 0.8 volts and record the capacity and mid point voltage. Next charge at 2C, rest for 1 hour, then discharge at 5C.

When you compare the results you would hope to find that the 0.1C charge gave you a little more capacity, but the 2C charge give you higher mid point voltage if there is any truth in this.

Tom

Click to expand...

Piece of cake. I will post the full 5C (8.5 A) discharge curves from the 0.1C and 2C charges by the end of this week.

I will start a new thread and post the results there. We are way off topic for this thread. The OP might get upset.

Cheers!
BG

Hello Battery Guy,

As you may have noticed, sometimes things get a little side tracked. Eventually we resolve the side issue and get things back on track.

I am sure the OP of this thread will be understanding of this...

Getting back to the original topic, I will add that I have had excellent service from my Schulze charger.

Tom

Battery Guy said:

The OP might get upset.

Click to expand...

I very much look forward to seeing your results, and I say screw the OP if he can't stand our off topic meanderings....he's probably some compulsive geek or something! :nana: