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Old Mar 03, 2004, 11:58 AM
"Hobby Apparatus" Flyer
Mikey C.'s Avatar
Oxford, PA
Joined Aug 2003
1,357 Posts
I don't know. I'd also like to know, though. And oh yeah, mart, sorry about your accident.

-Mike
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Old Mar 03, 2004, 12:05 PM
Just an average RC'er
Jim McPherson's Avatar
Laurel, Maryland USA
Joined Aug 2000
4,291 Posts
Martimer, you didn't mention how many cells your pack was. You have it set for a 3 cell pack. Most hummingbirds I know of use a 2 cell pack. That is very likely why your pack exploded. Unlike what you may have heard before, lithium batteries require that you be very very careful with your charge and discharge rates. You must set your charger to the correct number of cells and correct charge rate. This is important.

Also, you must not discharge the battery all the way till they are dead. Do not go below 3.0 volts per cell (6.0 volts for a 2 cell pack, 9.0 volts for a 3 cell etc.) If you do this you will damage your batteries. There is no reason to discharge lithium cells before you recharge them. Don't do it.

Make CERTAIN that you have the correct cell count before you charge another lithium pack.

I'd highly suggest looking over the Lithium guide at the top of this forum a few times. That should help a bit.

Mike,
The reason your charger only took 1 hour to charge the pack on the first charge is becuase all lithium cells have apx. 1/2 capacity in them when you buy them new.

-Jim
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Old Mar 03, 2004, 12:09 PM
EDF Head
Haldor's Avatar
Stavanger, Norway
Joined Feb 2000
7,999 Posts
Quote:
Originally posted by Mikey C.

I have a few questions. I didn't find the instructions very clear on how to set the amperage. If 1000mAH = 1AH, then if you charge at 1A it should take an hour to charge completely... right? So 2400mAH = 2.4AH should equate to 2.4 hours to charge at 1A, right?
Nope. Lithiums charge differently, in two phases:
CC (Constant Current) and CV (Constant Voltage)

Charging starts with CC until max voltage is reached (~4.2V/cell) then current must taper down to to avoid voltage going beyond max limit. This is the CV phase.

Generally CC phase fill up the battery ~90% and the CV phase the last 10%. The time cycle for each phase is ~50/50.
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Old Mar 03, 2004, 12:56 PM
Registered User
Milton Keynes England (UK)
Joined Dec 2003
184 Posts
Glad you are OK, I too had a exploding pack and fire. Maybe it would be a good idea for the forum to hold a poll about the amount of fires, expolding packs to see how bad the problem maybe..
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Old Mar 03, 2004, 01:03 PM
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USA, MO, O'Fallon
Joined Dec 2003
2,082 Posts
Dave,

I have read the notes already. Perhaps I did not make myself as clear as I should. One of the posts I read recently mentioned charging an not empty pack as a possible problem (overcharging?) and I thought that my charger was supposed to prevent that.

Do you discharge your battteries at all, or do you slap them back ing the charger once the motor starts to slow down? My ESP did not have a low voltage cut out, and so my 'clues' to the end of flight were duration and losing lift.
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Old Mar 03, 2004, 01:13 PM
Space Coast USA
hoppy's Avatar
Space Coast
Joined Oct 2000
20,945 Posts
Martimer,
Was it a 2 or 3 cell pack?
Thanks,
hoppy
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Old Mar 03, 2004, 01:43 PM
Registered User
USA, MO, O'Fallon
Joined Dec 2003
2,082 Posts
Jim takes the prize.

I attempted to charge a 2 cell pack on the 3 cell setting. Operator error coupled with immature technology (the charger really should be able to detect proper voltage for something this important) resulted in a $500 lesson.

CHECK YOUR JUMPERS!!!!!

Man do I feel stupid now. Poor too.

Jim, thank you very much for figuring out what went wrong.
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Old Mar 03, 2004, 01:59 PM
Registered User
Arlington,VA USA
Joined Jan 2001
1,862 Posts
Thats unfortunate. I'm suprised it is not smart.
I really like my Apache 1500 charger.

Very simple. Has the following features:
Jumper selectable
1-4 cells
250,700,1200, and 1500ma charge rates (1C)
Auto-trickle down for last phase of charge (puts more Mah into cells)
Low-voltage destination-battery detect (This would have saved you a destroyed pack and scary situation)
High-voltage destination-battery detect (will not charge an already-charged pack)
Low/High voltage source 12V battery check (will not run down your charger ba
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Old Mar 03, 2004, 02:00 PM
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blueprints's Avatar
Joined Feb 2004
214 Posts
Other than e voltage difference between a 2 cell n 3 cell LiPo. Would it also mean a 3 cell LiPo last longer than a 2cell? or does it solely got to do with the mAH only. THanx.
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Old Mar 03, 2004, 02:33 PM
Brunswick, Ohio
BrunswickOH's Avatar
Joined Aug 2003
5,736 Posts
Sorry about your accident and I'm glad nobody was hurt. Again we have a lipo problem that can not be blamed on the batteries.
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Old Mar 03, 2004, 05:44 PM
Registered User
Walled Lake, MI, USA
Joined Feb 2000
11,182 Posts
Martimer, I've been so focused on trying to make sure that you understood the fundamentals of Li-polys before using them again that I forgot to say that I am sorry about your loss and pleased that no one was injured. This is really the most important thing to be concerned about.

I'm glad you've read the sticky and understand a lot of the basics about Li cells. This kind of knowledge will help you from making some mistakes. But you will still make mistakes, just like the rest of us, as we humans just can't seem to nail that elusive perfection thing.

You may have picked up the idea about fully discharging Li packs by reading a message about someone putting a fully charged 3-cell pack on an automatic charger, and the automatic charger interpreting the pack as a partially charged 4-cell pack. This is one of the dangers of some automatic chargers. This does not mean that you should try to discharge a Li pack as far as you can. Never do that!

It's always best to ask lots of questions as you are doing now. There are many people here who have a lot of knowledge about Li cells that can help you with anything that you don't fully understand.

Blueprints, if you have 1000mAh cells, you can connect as many of them in series as you want, and you will still have a 1000mAh pack. The only thing that happens as you add cells in series is that the voltage increases with the addition of each cell. With a 1000mAh pack, you should be able to draw 1000mA (or 1A) for an hour. If the cells are connected in parallel, the voltage remains the same but the overall capacity of the pack increases, i.e. 1000mAh + 1000mAh + 1000mAh = 3000mAh for three Li cells in parallel. If you connect three 3-cell series packs in parallel (3s3p), you will have three times the voltage and three times the capacity.
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Old Mar 03, 2004, 06:41 PM
"Hobby Apparatus" Flyer
Mikey C.'s Avatar
Oxford, PA
Joined Aug 2003
1,357 Posts
Quote:
Mike,
The reason your charger only took 1 hour to charge the pack on the first charge is becuase all lithium cells have apx. 1/2 capacity in them when you buy them new.

-Jim
Sweet. I did not know that but it makes sense now why they charged so fast. Thanks.

-Mike
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Old Mar 03, 2004, 08:43 PM
Space Coast USA
hoppy's Avatar
Space Coast
Joined Oct 2000
20,945 Posts
Quote:
Originally posted by Dave Hederich
Blueprints, if you have 1000mAh cells, you can connect as many of them in series as you want, and you will still have a 1000mAh pack. The only thing that happens as you add cells in series is that the voltage increases with the addition of each cell. With a 1000mAh pack, you should be able to draw 1000mA (or 1A) for an hour.
There is a difference in flight times between a 2s and a 3s pack.
Consider this.
1. The plane needs 50W to fly right.
2. Cells are lipo 1000mah
3. 2s pack yields 7.4V nominally. To obtain 50W, the pack needs to supply 6.76A.
4. 1000mah/6760ma = 0.147hr x 60min/hr = 8.88 min flying time.
5. 3s pack yields 11.1V nominally. To obtain 50W to fly right, pack needs to only supply 4.5A.
6. 1000mah/4500ma = 0.22hr x 60min/hr = 13.3 min flying time.

Cell capacity still is 1000mah but flying time increases 33%, whcih just happens to relate to the increase in the number of 1000mah cells.
hoppy
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Old Mar 03, 2004, 08:54 PM
Registered User
Walled Lake, MI, USA
Joined Feb 2000
11,182 Posts
Hoppy, I figured he wanted the simple version. But since you got into the advanced course, would it not be true that the added weight of the third cell would have an effect on the flying time so that it would not be increased by a full 33%?

And that's not even getting into the space-time continuum factor .....
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Old Mar 03, 2004, 10:16 PM
Space Coast USA
hoppy's Avatar
Space Coast
Joined Oct 2000
20,945 Posts
Ok, make it 32.9% LOL

hoppy
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