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NIMH charging problem....I am cursed!!!!

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NIMH charging problem....I am cursed!!!!

Old 03-29-2004, 09:56 AM
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Jeff396
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Default NIMH charging problem....I am cursed!!!!

Hi all,

I just made the jump to NIMH batteries and am already having problems. I am trying to charge two 1200 mah 4.8 volt batteries and after a second cycle they are only coming up to 830 mah. After the first cycle they hit 790 mah and I expected more of a jump after the second time. Each battery is being charged and cycled by different chargers and are showing identical results. I am currently trying to back to back slow charge at C/10 to see if that makes a difference. The chargers I'm using is a Hobbico Accu Cycle charging at 125 mah and discharging at 250 mah and an ACE Digipulse set at 120 mah to charge and an Ace cycler to cycle it down before putting it back on the Digipulse. Is there anything I could possibly be doing wrong??? I have not had any serious problems with any of these chargers while doing Nicads. Any pointers would be appreciated!

Jeff
Old 03-29-2004, 10:19 AM
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mbba2002
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Default RE: NIMH charging problem....I am cursed!!!!

what brand batteries did you get?? I only use RAYOVAC 1800 nimh and have gotten over 1900 mah after 5 charges. I use a tekin 112c charger. Is your charger set for nimh batteries. Nimh batteries have a different charge pulse rate.
Old 03-29-2004, 10:35 AM
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Jeff396
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Default RE: NIMH charging problem....I am cursed!!!!

Hi,

I got them from www.batteryspace.com but they don't list the brand of the cells. Normally I have gotten Sanyo cells but I've been having some longevity issues with them. These came highly recommended by the guys flying the big 100" planes at my field. Both of the chargers say they are fine for NIMH cells but don't have anywhere to actually change a setting specifically for them. I just figured it was more important not to exceed the C/10 charge rate with the NIMH cells. I did also buy 5 1800 packs for my transmitters and my quarter scale plane that also need to be charged but I was hoping to get the 1200's done first. If I get no better results this time I guess I'll try charging one of the others and see what happens. I truly think I am cursed!!!

Jeff
Old 03-29-2004, 11:50 AM
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Rodney
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Default RE: NIMH charging problem....I am cursed!!!!

You did not say how long you charged them. The 125 Milliampere rate should be fine for these, just charge for at least 14 to 16 hours the first three times. The 250 milliampere discharge rate should also be fine for these to get maximum milliampere/hours out. Is your Accucyle accurate? It may not be discharging down to 1 or 1.1 volts/cell.
Old 03-29-2004, 12:30 PM
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Default RE: NIMH charging problem....I am cursed!!!!

Are you connecting the batteries directly to the AccuCycle, or are you using a standard charge wire plugged into the receiver switch harness through the charge plug?

You need to discharge the battery using as short a wire as possible. That 2 foot long charge wire plus the switch and it's tiny wires will provide a voltage drop at 250 milliamps that signals the AccuCycle that the battery is at 1.1 volts per cell, when in reality it's a lot higher than that. This terminates the discharge prematurely, indicating the battery isn't up to it's rated capacity.

If you are using a short heavy cord between the battery and AccuCycle, then make sure the battery is indeed charging all the way up. Several C/10 charges with discharge cycles are needed to "form" the cells.

Let us all know what you find out.
Old 03-29-2004, 12:34 PM
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Default RE: NIMH charging problem....I am cursed!!!!

ORIGINAL: Rodney

You did not say how long you charged them. The 125 Milliampere rate should be fine for these, just charge for at least 14 to 16 hours the first three times. The 250 milliampere discharge rate should also be fine for these to get maximum milliampere/hours out. Is your Accucyle accurate? It may not be discharging down to 1 or 1.1 volts/cell.
I agree, I came up with 13.44 hours using this formula

Charge time = (battery capacity x 1.4)/charger ma
= (1200x1.4)/125
= 13.44

But the first time you charge it I would charge it longer like they recommend. So Rodney's numbers look pretty good.
Old 03-29-2004, 01:15 PM
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Jeff396
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Default RE: NIMH charging problem....I am cursed!!!!

Both chargers time out at either 14 or 15 hours. I can't remember the exact time. I did verify with a meter that both were charging at the rate they were supposed to be by breaking the positive wire and using a meter. On my meter on the 500ma scale the Digipace is dead on at 120ma and the Accucycly looks to be between 130 and 140ma. Next, when I switch to discharge, I will check that the discharge rates are accurate assuming I can do that in the same manner. I am a little fuzzy on how to tell if they are taking the cells down far enough. Would I have to sit there and wait for the exact moment they switch over and check them with a meter to see what voltage the packs were at?

Jeff
Old 03-29-2004, 01:27 PM
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Default RE: NIMH charging problem....I am cursed!!!!

I am hooking both of the batterys directly to the chargers with no switch. However, the wires I have running from the chargers are more like 4 or 5 ft long. Being that the Digipace charges 6 packs at a time it's hard to get 3 planes in that small of an area. Could that really account for that large of a problem? I haven't noticed any problem with my Nicad packs using the same equipment.

I just plugged in the two transmitter packs. Of course since I'm limited to 150mah charge rate on the Ace Digipace it will take two sessions to get them up to full charge as they are 1800 mah packs. I'm hoping that this double charging session on the 1200's does the trick. This will be their 3rd cycle down.

Jeff
Old 03-29-2004, 01:28 PM
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Default RE: NIMH charging problem....I am cursed!!!!

ORIGINAL: Jeff396

Would I have to sit there and wait for the exact moment they switch over and check them with a meter to see what voltage the packs were at?

Jeff
If it transfers over to charge automatically I think you are correct. You could put a voltmeter in parallel with the charger/discharge and monitor the voltage every now and then. It's supposed to switch over around 1 to 1.1 volts per cell (4 - 4.4 volts on a 4 cell 4.8 pack).
Old 03-29-2004, 05:13 PM
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Default RE: NIMH charging problem....I am cursed!!!!

I'll try that.......thanks!

Jeff
Old 03-30-2004, 12:57 PM
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Default RE: NIMH charging problem....I am cursed!!!!

Well, both of the batteries came up an additional 90 mah to just over 900 mah. This was the third cycle. I guess as long as I'm getting an improvement I will continue. I did hook the meter up in parallel with the charger on the Accucycle. By the time I noticed it had switched over, which couldn't have been more than 10 minutes or so, the battery was already up to 5 volts. That does make me wonder if it's going down all the way to 4.0 - 4.4 volts. As I said, the strange thing is that both chargers are giving results that are pretty close. Is there something in the NIMH makeup that would make it end its cycle prematurely as opposed to Nicads? I have never had this much of a problem with any of my Nicad......and I've had a bunch.


Jeff
Old 04-05-2004, 02:25 PM
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Default RE: NIMH charging problem....I am cursed!!!!

Do yourself a favor and try a friends NiMH capable charger. Then do a search here on RCU for "Accucycle". Some people have had terrible experiences with them. Mine is sitting in a box. It's a replacement, of a repair, of an original. It lies so bad, I wouldn't sell it or give it away for fear that it will cause a crash. I went on to Sirius and Triton chargers with repeatable, good results.
Old 04-05-2004, 07:17 PM
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Default RE: NIMH charging problem....I am cursed!!!!

I would go ahead and use them for a time and then cycle them in about 2-3 months to see if they are at capacity
Old 04-06-2004, 03:03 PM
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Default RE: NIMH charging problem....I am cursed!!!!

Cycling is bad for NiMh cells. The cyclers can discharge the packs below the safe level for the cells and then you will have reduced the pack capacity. (you can ruin the cells in one excessively deep discharge)

Some of the modern NiMh rated cyclers won't as deeply discharge to prevent this problem.

The result is... the discharge to higher voltage gives a lower capacity reading on the cycle.

Cycle testing NiMh is not as good an idea as with NiCds. Go with an on-board voltmeter (the appx. $15 Voltwatch style things) and when the meter reads down 2 lights from max (not moving the servos)... its time to recharge. If you note a significant change in how much flying time you get (with the same plane and flying style...) you need to replace the batteries.
Old 04-06-2004, 11:07 PM
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Default RE: NIMH charging problem....I am cursed!!!!

Hi Jeff..

I doubt you are having a problem with either your cycler or your charger. Rather, I suspect you'll find that if you try using NiMH Sanyo or Panasonic cell in your packs and then run the same charge and cycle routines you'll find the packs hitting the rated numbers.

There's been some very substandard chinese sourced cells creeping into the hobby lately... price should not be the main concern for a R/C pack. Little issues like construction materials, quality control and construction proceedures all are reflected in the price of the cell. And you get what you pay for... in more ways than one. It's not just capacity that 'off' in some of these brands.. were finding internal press-fit construction, cheezy plate insulators, alloy impurities.. not good stuff for an airplane. Barely adequite for a mp3 player.

FWIW.. not all cells are the same quality; if the base building blocks of your pack are sub-standard... well, lets just say I wouldn't recommend a cell from the company that made yours for anything more robust than a labelmaker with an extension cord.

Good luck!
Old 04-09-2004, 03:29 PM
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Default RE: NIMH charging problem....I am cursed!!!!

I had trouble with the chinese made nicads (850mAh AA) in my Panasonic phones. I bought 3 cordless phones last year and since then, the two that get used the most have had the batteries go already. That's only a few months. Since Radio Shack couldn't come up with replacements (under the ext warranty) I ended up soldering my own, and noticed a huge difference in the run time of the phones.

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