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Old Feb 03, 2005, 10:09 PM
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Afraid of Quick Field Charger & LiPo's!!

I recently received a Hobbico Quick Field Charger MkII as a gift and have concerns about using it to charge LiPo's. In the instructions it indicates that it can be used for Li-Ion, but not Li-Poly. My guru at my LHS says it CAN be used for Li-Poly as well. I've done a search and I know there are others using this device for LiPo's too.

Since I'm new to electrics and am being extra careful to avoid mistakes with LiPo's, I'd appreciate any advice on this. If it's not 100% safe, I'm not intersted.

Thanks in advance.

Smooth
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Old Feb 03, 2005, 11:23 PM
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Its a great charger but it is made for li/ions(hold lower voltage) so it doesn't complete the charge for li/pos. Some consider this a good thing as you will be farther from overcharging than with a 100% peak charger.......
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Old Feb 04, 2005, 12:03 AM
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good over all charger for the $. Used mine for a year, never any problems ( non caused by charger anyway )

Dan
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Old Feb 04, 2005, 01:23 AM
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It's a very good charger. One and a half years use with no problems. It's true that it only charges to 98% max, but you'll never notice the difference. Don't worry, just make sure that you follow the instructions and keep the power supply above 12 volts.
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Old Feb 04, 2005, 09:58 AM
Southern Pride
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Haralson County GA. USA
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Charges to 4.1 volts per cell is is closer to 80% of full charge than it is to 98% of full charge. The lower 4.1 volts per cell is OK and is easier on your Li Polys.

Charles
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Old Feb 04, 2005, 10:02 AM
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I consistently got 12.4V.

12.4/12.6 = .984 How do you get 80%.
Darrell
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Old Feb 04, 2005, 10:15 AM
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Smooth Spanky's Avatar
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Thanks folks!

Better safe than sorry, right?

Smooth
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Old Feb 04, 2005, 10:26 AM
Southern Pride
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Haralson County GA. USA
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Quote:
Originally Posted by darrell_f
I consistently got 12.4V.

12.4/12.6 = .984 How do you get 80%.
Darrell
Everyone uses what works best for them. I agreed with Bob Boucher Mr. Astro Flight (astrobob) that when you read resting open cell voltage of a Li Poly cell.
3.7=0 % remaining useable capacity
3.8=20%
3.9=40%
4.0=60%
4.1=80%
4.2=100%

12.4/3==4.13

4.1=80%
4.125=85%
4.150=90%
4.175=95%
4.20=100%

So 4.13 volts per cell is much closer to 4.125 (85%) than it is to 98.4%

Charles
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Old Feb 04, 2005, 12:09 PM
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One of my favorite quotes: 74.8% of all statistics are made up on the spot.

On this, I think we are both right.

I think the 98% is an accurate statement of full charge, but I agree with you that the percentage of usable capacity is less as the range of usage capacity goes from max charge (12.6V) to LVC (9V). For me the difference is even less, as I just don’t fly a pack down to 9V. The capacity drop is not a straight line and falls of more rapidly on the top end so the practical difference between charging to 12.4V rather than 12.6 is not that noticeable at the field. For my use, it’s probably means something like 16 minutes fly time verses 18 minutes fly time.

The point is that is really is not necessary to spend over $100 buy a Astro or Trition to get a Li-Poly charger. The Hobbico QFC does a good job at charging 2S and 3S 1P packs to 12.4V and it also provides a big advantage of charging two packs currently. If the extra two minutes of fly time is worth the $$$ and three times as long to charge two packs, then go for the Astro.

Darrell_F
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Old Feb 04, 2005, 12:18 PM
Southern Pride
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Quote:
Originally Posted by darrell_f
One of my favorite quotes: 74.8% of all statistics are made up on the spot.

On this, I think we are both right.

I think the 98% is an accurate statement of full charge, but I agree with you that the percentage of usable capacity is less as the range of usage capacity goes from max charge (12.6V) to LVC (9V). For me the difference is even less, as I just don’t fly a pack down to 9V. The capacity drop is not a straight line and falls of more rapidly on the top end so the practical difference between charging to 12.4V rather than 12.6 is not that noticeable at the field. For my use, it’s probably means something like 16 minutes fly time verses 18 minutes fly time.

The point is that is really is not necessary to spend over $100 buy a Astro or Trition to get a Li-Poly charger. The Hobbico QFC does a good job at charging 2S and 3S 1P packs to 12.4V and it also provides a big advantage of charging two packs currently. If the extra two minutes of fly time is worth the $$$ and three times as long to charge two packs, then go for the Astro.

Darrell_F
Well with well over 900 LI Poly flights and over 100 discharge tests on my CBA I disagree about how much useable capacity diference there is between 4.1 and 4.2 but it is not woth arguing about.If you are happy that's fine.
Perhaps the best inexpensicve Li Poly charger thsre is is th E Tec Smart Charger Apache 2500 which can be purchased for $45 and will charge 1- 4 Li Poly s to the full 12.6 volts every time. I have two and they are great. I also have 2 Triton, an AF109 and numerious other Ni Cad and Ni MH chargers.

http://www.battlepack.com/LiPolychargers.asp


Charles
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Old Feb 04, 2005, 12:39 PM
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MKII = 80% of 4.2v per cell charge

Darrell_F,

I tested two different MKII chargers and the results were the same, 80% capacity of a 4.20v per cell charge.
If you fly to a higher cutoff voltage and use less of the total pack capacity, percentage of charge for the MKII would be less not more.

The MKII is a nice safe charger for Lipo, but unless there was a revision change(?) it can’t charge a 3-cell pack to 12.4v because it terminates the charge at a lithium-ion safe 4.1v per cell or 12.3v.

This is a test I did for capacity vs. Resting voltage.

72 degree pack and room temp.
The (999mAh) pack was discharged each time at 1C, to a 3v per cell cutoff.
Then I added a fixed percentage of charge, and measured the resting voltage after 12 hours.
Next I discharged the pack and verified the percentage of capacity.
At the end of all testing I charged the pack fully and did a accuracy / repeatability test.
The repeatability of the test was over 98%.

4.20v = 100%
4.03v = 76%
3.86v = 52%
3.83v = 42%
3.79v = 30%
3.70v = 11%
3.6?v = 0%

Capacity below 3.7v "resting" is not usable for flying, it's where the battery voltage dump begins.

Jim

<EDIT>
Recent Capacity@Voltage graphs (testing new packs), are located here.
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Old Feb 04, 2005, 01:15 PM
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hoppy's Avatar
Space Coast
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I'm curious about these claims of resting V vs capacity so....Here's my plan:
Charge the pack to 4.1V/cell. Measure resting voltage. Put it back on charger and take to 4.2V/cell and see how many more mah goes in.
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Old Feb 04, 2005, 01:42 PM
Southern Pride
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Haralson County GA. USA
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Quote:
Originally Posted by hoppy
I'm curious about these claims of resting V vs capacity so....Here's my plan:
Charge the pack to 4.1V/cell. Measure resting voltage. Put it back on charger and take to 4.2V/cell and see how many more mah goes in.
Have fun hoppy. I-A Flyer did a bunch of testing along these lines and I did some myself. Results will vary depending on the capacity and health of the packs but my testing on the upper end between4.15 and 4.2 many of my packs is allways very close to 10%.
I have read reports of where someone was using a Hobbico MK II and they had a friend charge their pack on an AF109 flew the pack for a few minutes then but the pack back on the MK II and it said pack was still full.
Now remember that the AF109 does not charge to a resting voltage of 4.2 but instead closer to 4.15 for most cells (packs).
Charles
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Old Feb 04, 2005, 01:49 PM
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United States, IA, Onawa
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Quote:
Originally Posted by everydayflyer
Results will vary depending on the capacity and health of the packs but my testing on the upper end between4.15 and 4.2 many of my packs is allways very close to 10%.
Yea Charles, I forgot to give the standard disclaimer!
"Questionable results and accuracy, your results may vary!"


hoppy, sounds good I’d love to see more actual data.

You’ll need to do the test though, because the only charger I have that will hold a resting voltage of 12.6v after a 12-hour rest is a board charger, and it doesn’t give mAh added.
Remember though, the MKII has a safety cutoff of 4.1v, the ones I tested had a CV phase of 12.2v. They gave closer to 12.07 @ 12 hours.

MKII charge graph showing voltage.
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Old Feb 08, 2005, 04:17 PM
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Quote:
Originally Posted by hoppy
I'm curious about these claims of resting V vs capacity so....Here's my plan:
Charge the pack to 4.1V/cell. Measure resting voltage. Put it back on charger and take to 4.2V/cell and see how many more mah goes in.
Hope this is close to what you wanted?
I was logging some charging current graphs anyway, so this test was easy.

I think we’re looking at the wrong thing when testing a charger by voltage alone.
Voltage of the CV phase, when combined with the charge termination point determines capacity obtained more accurately.
12.22v CV @ 100mA termination = 80%
12.60v CV @ 100mA termination = 95%
Roughly 15% difference between a 12.2v CV and a 12.60v CV, when judged equally using the same +100mA current termination.

Any charger is 5% below its “personal” (CV phase dependent) capacity potential if it terminates the charge at +100mA instead of 0mA.

And, +100mA is well into the “charge knee” regardless of the chargers CV voltage.
It’s looking like the current termination point is more important than the CV voltage for battery life.

BTW,
I’m thinking a 200-250mA termination (0.2 - 0.25C) would be close to 90% of a chargers capacity potential, and above the “upside-down” charging knee.
I also see how a slight amount of cell voltage balance would occur if the charge were allowed to continue into the knee, as AstroBob hinted(?) in a different post.
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Last edited by IA-Flyer; Feb 10, 2005 at 05:19 PM. Reason: added (0.2 - 0.25C) for accuracy
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