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Radio - Servo - Gyro - Gov - Batt > NiMH Discharge Question
 
 
Tom Berg
Senior Heliman
Location: Clovis, California

Hello All,

I have read a bunch after doing a NiMH search. I have a question about the NiMH batteries after a short discharge and then trying a short charge (less than 20 hours). Then, after about 40 minutes of use, the Rx only "cycled" for an 8 mAh discharge and switched to auto charge on the cycler.

The question is, DO I HAVE A BAD PACK?? or was the cycler (an AcuCycle) possible giving me a false discharge reading??
-->>

Here is the background:

I have 1700 NiMH rx pack. I cycled it new and had approximately 800 mAh discharge after a 15 hour charge at 125 mA. I called Radical RC and he said that NiMH would need about 20 hours at a 100 mA rate.

I did the "full" charge of 20 hours and got 1775 mAh of the pack. So I started using the pack. Figuring it was just fine.

Now here is the question/problem.

I have been flying during the afternoon and putting about 1 hour on the heli and then charging for about 7 hours at night. I figured that 7 hours charge would keep the batteries topped off (right).

Yesterday I finished about 40 minutes of time on the helicopter Tx/Rx in the afternoon. I went home and put both on the cycler. I went to T-ball with my son and came back to find the Rx had started to charge after only 8 mAh!!!

I figured it was a glitch, so I started the battery discharge again. The second time was about 75 mAh! (after about 1 hour of 125mA charging) Meanwhile, the Tx is going strong and went to about 1125 mAh off of the used 1700 pack.

--->>
I do not know if the NiMH "short" discharge and "short" charge cycles I was doing caused a problem. Do I have to charge the pack for 20 hours each time I want to know there is a "full" charge.

For now I am putting my 1500 NiCd pack back in the heli and going to the fun fly this weekend. I will cycle the NiMH pack a couple of more times to see what happens. I just do not have time to finish this 20 hour charge, discharge the pack for about 6 hours and then start another 20 cycle before tomorrow.

Thanks for any insight.
05-09-2002 Over year old.
HOMEPAGE  
 
 
RC-CAM
Senior Heliman
Location: USA

I doubt YOU hurt your pack.

As you know, the standard Accu-Cycle is a timed charger (fixed 15 hours). It offers 25/50/125 mA charge currents for your rx pack.

You did not harm your pack by your shallow charges, but it is not a wise thing to do if you want to utilize the full 1700mAH pack capacity. My math suggests that you are 25% short on charge time.

At 125mA, you should plan on a 20 hour charge if the pack is fully discharged. This means that when you use the discharge/cycle feature, it will take more than one recharge to fully recover. Be prepared to press the charge button twice.

Even if your pack is not fully exhausted after flying, just plan on performing a full 20 hour charge. At your C/14 charge rate there will no cause for alarm if you over extend the charge times.

During cycle testing, be sure the cell count switch is set correctly. Use the 250mA discharge current. The 500mA rate will introduce errors since the Hobbico charger leads are wimpy (low gauge) and higher discharge currents can introduce a voltage drop. This affects the measurement. I built my own leads to correct this issue.

Also, if you will be making measurements that matter then connect the charger DIRECTLY to the battery. For accurate capacity readings, do NOT connect the Accu-Cycle through the Rx's On/Off switch charge jack. That is a fine place for casual charging, but not for serious measurements.

Lastly, I have noticed that some Accu-Cycle chargers under report capacity by about 5%. A good calibration will correct this. Also, brand new NiMH packs can take several cycles before they report their real capacity. Keep that in mind.

If you do not get decent measurements after a careful recharge then something is wrong. It could be a bad pack, screwy charger, or simply incorrect charger settings.

Good luck.
05-09-2002 Over year old.
HOMEPAGE  
 
 
Rotary Flyer
Senior Heliman
Location: North UK

Discharging (cycling) of NiMH batteries is not required after every session. They are supposed to memory proof, so you only need to top `em up after a days flying. the occasional cycle should be ok, but not every time, and this is supposed to apply to NiCD packs as well. Over cycling is bad, not good. Or have I been misled?
05-09-2002 Over year old.
 
 
RC-CAM
Senior Heliman
Location: USA

You must be thinking of "battery discharging"

"Cycling" is used to accurately measure the true mAH capacity of a pack. It requires a battery test system (a "cycler"). It is done a few times a season to help detect common problems before they become serious. It is best to keep a written log of the mAH readings to help watch for a failure trend.

I have found seemingly good cells that were on their last legs using this method. I can also determine when a well seasoned pack is ready for retirement. I cycle my packs once a month during a busy flying season, an act that is not at all detrimental to a healthy battery and totally pain free for me.

Brand new packs are cycled so that the user can establish a baseline mAH value. Sometimes several cycles are needed to obtain a good reference. This also is a time where C/10 charges are initially needed to help "form" the cells on brand new packs.

When the cycling has a useful purpose, it is not a bad thing.
05-09-2002 Over year old.
HOMEPAGE  
 
 
Tom Berg
Senior Heliman
Location: Clovis, California

RC-Cam, Thanks.

I will be cycling the NiMH battery tonight. I just finished the 15 hour charge at lunch and switched all my batteries over to the multi-charger. I will let them go until tonight which will give about 20 to 24 hours of charge (depending on what time I get to work on the heli).

I will still put the 1500 NiCd pack back in the heli for this weekend. I would rather have some extra weight in the heli than have the batteries go dead while flying.

It is a very good point about cycling the batteries through the switch. I did just pick up the charge jack and plug it in. Maybe the battery will test out better direct on the cycler.

Thanks again.

Tom
05-09-2002 Over year old.
HOMEPAGE  
 
 
Twobeers
Key Veteran
Location: Toronto, Canada

NiMi batteries should be slow charged and discharged in a normal fashion (ie:not fast discharge) at least 3 times before being put into service. After which it is fine to fast charge them at a reasonable rate. Cycling is not necessary with NiMi's. They don't normally have a tendency of developing a memory problem as severely as NiCad's do.
I made myself a little doohicky for doing this. 2-22ohm-2 watt resistors and a 9V jumbo Led in parallel. This gives a consistent current draw of approximately .436-.440 amps. I discharge them till the Led goes out. I could not find a 10 or 11 ohm 5 watt resistor at my local electronics part supplier thats why I used 2 22ohm resistors.
05-09-2002 Over year old.
 
 
Tom Berg
Senior Heliman
Location: Clovis, California

Hello guys.

I did a 24 hour charge - 15 hours at 125 mA and 9 hours at 100 mA on the 1700 Rx pack.

I discharged the pack and my accu cycle read 1530 mAh, which is 90% of the capacity. Even with 5% error, that would only be a true 1600 out of 1700 which is just 94%.

This shows that it does not appear to be a "bad" pack, but since it is less than 1 year old, I whish it was over 100%.

I will do the suggested extra cycles and watch the readings. I will also check my timer again. I think the last time I checked the AccuCycle was off by about 3%, and that is not too bad.

If this pack does not go back up to 100% capacity, I will probably not put it back into the heli.

I'll keep you all posted.
05-10-2002 Over year old.
HOMEPAGE  
 
 
RC-CAM
Senior Heliman
Location: USA

Expecting 100% capacity from a pack is not totally fair, but certainly something to hope for. However, if the pack is new and you are getting 90% of capacity, then I would not be alarmed. In the battery industry, if a pack is within 80% of its rated value it is considered a keeper.

There are several reasons for this, such as the statistical nature of battery production, measurement accuracy, test equipment tolerance, environmental conditions, etc.

For example, the AccuCycle's end-of-charge voltage is factory set to 1.05V per cell. This is very typical of R/C cycle systems. However, some of the battery makers are now using 1.00V/cell in determining their battery's capacity. A tiny difference such as this make a big difference in the measured results. For sure, it will cause your readings to appear a bit low.

BTW, if you have an accurate ammeter, put it in series with one of the charge leads. During the discharge cycle, measure the load current. It should be 250mA, +/- 5mA. I'll bet yours is off a bit. Charge current accuracy is less of an issue, but it should be within 10% to confirm that the unit is not goofy.

For some battery life info, check out this page on Red's site: http://www.rcbatteryclinic.com/life.html

05-10-2002 Over year old.
HOMEPAGE  
 
 
Tom Berg
Senior Heliman
Location: Clovis, California

Thanks for the great information.

Here is what I have after another full cycle.

1st = 1530 mAh; 2nd = 1526 mAh. Timer was off a little (10 minutes was actually 10 min, 12.5 seconds) therefore, the timer was approximately 97.959% of true. With this reading, I would be at about 1560 mAh on the pack.

Still not at 100%, but you do mention that is a lot to expect. I think my last test will be to fully charge and cycle the 1500 mAh NiCd pack that is on the helicopter now.

If the NiCd readings show about 90% capacity, I will probably go back to the NiMH pack. If the pack shows 100% or better, than the NiMH will probably go into a plank.

I'll let you guys know how the race turns out.

Blue Skies.
05-13-2002 Over year old.
HOMEPAGE  
 
 
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Radio - Servo - Gyro - Gov - Batt > NiMH Discharge Question
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