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Old 2018.11.26, 07:21 PM   #1
RFick73
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Question Proper care & maintenance of your NiMH AAA's

So, this past weekend I was a little let down by the performance of some of my batteries. I bought the batteries about 1 1/2 years ago & use them typically once to twice a month or so year round. I'd say they have somewhere in the ballpark of ~50 cycles on them & that # is probably a bit high.

I currently charge my batteries in a SkyRC NC2500. The last few times I have purchased 5 sets of batteries, I have run them through the "Break-In" process labelled the batteries with their finished mAh & voltage levels. I then use that information to typically match them by mAh or by the voltage. What's the better way? I guess it depends on what I want. I used to be ALL about run time, but the brushless have an insane run time as it is, so I've been gravitating towards end voltage matching. I usually mark the groups with a letter or number & keep them together. Does any of this really matter?

It was mentioned that my care & maintenance may be the culprit, so, I wanna make sure what I have been doing is correct or change my bad habits.

Now, I swear I've read charging batteries at a lower amp rate is the way to go but is that the case? Maybe that's just for longevity sake? Do higher amp charging rates help with higher discharging rates. I really don't mind getting less cycles out of them if I get better performance, I know there has to be a tradeoff. I typically run my batteries & then store them for about a month or so until the next usage, at that point, because I'm in no hurry to use them, I charge them at .5amps. But seeing I only have 5 good sets & we run for like 8 hours, I typically run them, charge them & potentially use them again later that day.

I was asked how often I discharge my cells. Well, I don't.. Other than when I run them in my car. I knew it was a thing, but I've just never done it outside of my typical running the batteries through a 'Break-In" process every 6 months or so. The charger has a refresh setting, but I figured because I have the time, I would just use the Break-in process.

I typically bring 10 sets of batteries with me to the track. They are all the same but 5 sets are 3 years old, the others are all 1.5 years old. The set of 3 year old ones I really only keep in case others need a set to use or I really run low. I bring 2-3 chargers with me to the track so I can charge what I need how I need to. I try to go through the 5 newer sets & once I get to about the third set toss my original ones back in. I've often let them go at .5 amps or bumped them up to 1 or even 1.2amps if I knew I'd wanna use them sooner. What should I be doing, should I be discharging them first in the charger then charging up at 1-1.2amps? If I'm not using them should I discharge them, let them sit then charge them the day or two before using them? Should I be running them through the "Break-in" every 4-6 months or using the "Refresh" option more often?

What have others found worked for them? Help!
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Old 2018.11.26, 08:37 PM   #2
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Start by doing a full charge, discharge, charge cycle. Then you can start to analyze data.

Does the charger have and adjustable cutoff voltage for discharge? If so, raise it to 1.1v or so, and discharge the cells. It doesn't matter if the cells have loads of capacity at lower voltage, because that will hurt performance to run with the lower voltage.

After the cells finish the discharge, look at the capacity at that voltage, and remove the lowest ones. Then raise the cutoff to 1.2v, and run the discharge again. Remove the highest results and keep them for race packs, the others group for practice packs. This will ensure that you have cells that will maintain the highest voltages for the longest amount of time.
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Old 2018.11.26, 09:00 PM   #3
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Quote:
Originally Posted by EMU View Post
Start by doing a full charge, discharge, charge cycle. Then you can start to analyze data.
What Amperage rating should I charge them at, 1C? They are 900mAh, so .9? What about discharge? I believe the highest the charger can discharge them is at 1amp

Quote:
Originally Posted by EMU View Post
Does the charger have and adjustable cutoff voltage for discharge?
Yes it does, it looks to be set @ .9 currently. What should it be typically set to, after I run the 1.1v & 1.2v test? I like using this method, TY for the idea.
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Old 2018.12.09, 11:41 AM   #4
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Sorry for a late response, I missed the notification of your reply.

750mah cells i charge at 500mA, 800-900mah cells i charge at 700mA.

Discharge should be at the maximum rate that you can discharge at, with a higher cutoff of 1.1v or even 1.2v. I find after a quality run, my cells are usually around 1.15v to 1.2v, so this data is much more useful than capacity of the entire discharge curve.

I don't see an advantage to charging aggressive besides the first minute or two being fast and then a larger drop off in power through the run. A lower charge rate will give a little less initial punch right off the charger, but the cells discharge will remain flatter than an aggressive charge. This will give you a car that feels similar in thr first minute as it does in the last minute of a race.

I don't think that nimh cells really need regular discharge/cycles if you use them consistently. If anything, constant full cycling wears them out a little faster. I only discharge my cells to extract data on their health and to group them into packs. I find that using a higher cut off as explained earlier gives you the more useful data, since capacity at a lower voltage does not help racing.
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Last edited by EMU; 2018.12.09 at 11:44 AM.
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Old 2018.12.15, 12:20 PM   #5
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EMU,

What Charger/Discharger are you using that allows you to set the discharge cut off to 1.1V or 1.2V? Mine only goes up to 1V, I am using the SkyRC NC2500's

I'm guessing I should still get a pretty good baseline by using .9V & 1V?


Secondly,

After I'm done running the batteries in my car, what should I be doing with them? Allow them to cool before doing anything? Should I be tossing them on a discharge cycle & then fully charge them at the rates you mentioned earlier? If so, what should the typical discharge voltage be when you do this?

I've attached the settings of my charger to this post for your review in case you see anything that I should change.

Thank you again for all your feedback!
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File Type: jpg Screenshot_20181215_101719.jpg (115.1 KB, 6 views)
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Old 2018.12.15, 05:41 PM   #6
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I almost never drive the car until it dumps, so that way I don't worry about polarity reversal in the cells. After I use the cells I place them in my battery box button side down.

If I am going to use them again within the hour, I'll wait a few minutes then charge them up again. If they're done for the night they just stay in the box in the state they are in. I rarely discharge for the sake of cycling the cells unless I haven't used them for a while.

For testing cells, I use a muchmore CTX-D2 discharger. This is REALLY hard on the cells, so ihs only used for analysis. The linear discharge is 5A, and 8 can set the cutoff voltage from 0v to 1.25v I believe. It will give a readout in seconds that each cell discharged and the average voltage of the cell. This was a fairly common discharger for NIMH sub-c packs before lipo took over. It is much too high of a discharge amperage for regular use, so I only use it for analysis and I keep the cutoff high to reduce the wear on the cells. What it does tell me is which cells are not healthy and drop voltage under load quickly.

This data was more useful for brushed modified racing, now that the current requirements of brushless are considerably less, I have been thinking about swapping out the resistors to reduce the amperage of the discharge to closer to 2A or even 1A. While it doesn't tell me internal resistance directly, the voltage drop under load indicates the amount of resistance in the cell.

I wouldn't really recommend to someone to purchase one, as it does shorten the life of the cells, but it can help finding the best cells for a race out of a bunch fairly quickly. When only looking at the useful part of the data in the initial discharge of the cycle. My typical discharge will only be around 350-500mah at most before cutoff.
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Old 2018.12.16, 11:40 AM   #7
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EMU,

You mentioned that the Muchmore CTX-D2 Discharger may be a bit too aggressive. Other than modding it to play nice, which one's would you suggest?

While we're on the topic, which chargers do you suggest?
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Old 2018.12.16, 06:01 PM   #8
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Unless you are really looking for hundredths of a second out of the batteries, you don't really need to go out of your way to discharge and analyze the cells. I used to spend a lot of time, effort and money on cells and analysis of them to make race packs only for championship level events. Otherwise, I just charged up my cells and raced for club level, and excluded the poor performing cells.

Much of the RC world has moved away from NIMH, and for the most part only mini-z sticks with it. The motors in this scale run best at 5v. There aren't many options for analysis outside of a turbo matcher 4/35/8, which is still big money even though it is a retired product. The CTX-D2 is aggressive, but was a good way to pull the weakest cells when you have a huge lot to go through to find the best ones.

Now, I use two Opus chargers from amazon, which cost around $25-30 each. It's very similar to the lacrosse bc-700 or bc-900, but the cells don't overheat as much because there is more space between them. The skyrc chargers are very good, and provide good data analysis, but are pretty costly.

The opus chargers give me everything that I need to go racing on nimh cells now, can charge at 400/600/800/1000mA for 4 cells, so good for a range of cell capacities. 750mah cells I charge at 600mA, 800+mah cells I charge at 800mA. The cells come off warm and punchy.

Im currently using eneloop aaa cells for racing, which are a little down on punch, but consistent. I don't discharge them, or even do any analysis beyond looking at the numbers on my chargers when they charge. Without having world championship level racing going on, I don't feel that such analysis and time expenditure to get the top 10% of my cells is worth my time.
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Old 2018.12.16, 09:33 PM   #9
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Are you using chargers that charge each sell separately or are they charged in series as a four cell pack? I only use the single cell chargers for the last year. I find that it is much better that way. I agree with EMU on his posts.

I made a discharger just to see what kind of voltage my cells have at a 2 amp discharge rate, I don't discharge the cells down with it to store the batteries only to single out the bad cells.


After I finish a run I just pull the batteries out of the car and put them in the battery case until the next day I race. I only run the cells once a race day. The second time they never run as well as they do on the first run of the day.
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Old 2018.12.16, 09:34 PM   #10
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I would also take the trickle charge off.
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Old 2018.12.17, 06:47 PM   #11
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For what its worth in a short post. I charge my Peak 900's at 1.5 amps with a Hitec X4 in 4 PN charging trays 4 sets at a time. Never matched my cells, I just run what came in the package. I discharge on the PN dischargers between charges. The second run on a charged/run/discharged/recharged set is the best.

I store my batteries discharged to .9 v each, usually discharging within a day or 2 of racing. I've had great luck with this method for racing.

I break in new sets by cycling them at .9 amps charge and 1 amp discharge for up to 5 cycles.

I replace my 12 sets of batteries about every 2 years at the same time. Probably dont need to but I can so what the heck.
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