Li Poly, fully charged voltage?

| I have 3 Kokam 1500 mAh, 11.1 V 3S batteries. When they were fully | charged, each battery measured at 12.44 Volts.

It's about 4.1 to 4.2 volts per cell, so that sounds right. | As I recall, my NiMH 10 celled 12 V batteries were measured at | around 14 plus Volts when they were fully charged.

NiMH and NiCd are about 1.4 volts per cell fully charged, so that sounds right. | I am wondering why the difference if Li Poly gave longer flight | times?

LiPoly and NiMH cells are totally different, apples and oranges. LiPoly cells have more in common with your Lead-acid or Gel-cell batteries than your NiCd or NiMH batteries.

| Or explain?

LiPoly cells give longer flight times because that LiPoly pack you have is probably half the size/weight of a 10 cell 1500 mAh NiMH pack. That's really about all there is to it -- more energy for a given size/weight. A large part of how LiPoly stores more energy is because each cell has a lot more voltage than NiCd or NiMH.

I put a 3 cell LiPoly 2200 mAh iRate pack into my Combat wings XE2 with an Astroflight 020 motor and 6x4 prop on it. It will now go vertical, and makes a lot of noise while doing so! :)

Doug,

I have not yet flown with these batteries, but it sounds exciting. When my planes were equiped with 10 celled NiMH batteries they weighed about 56 oz. and they went vertical.

Now with the 3S3P Li Poly batteries, they weighed about 46 oz., a reduction of about 10 oz. I expect a lot from these batteries. If only the winds here would calm down...

It's about 4.1 to 4.2 volts per cell, so that sounds right. Your explanations made a lot of sense, except I think the Li Poly cells are 3.7 volts each. That's why 3 x 3.7 = 11.1 volts? Am I correct?

Thanks, Wan

LiPo Cell Behavior

Lithium cell behavior during charge and discharge can be confusing. consider the following attributes of Lithium cells

2. The resting voltage of a cell will accurately indicate its state of charge.

3.7 volts means 10% or less remains in the cell.

4.1 volts means 80 to 85% charge.

4.2 volts means 100% rated charge.

4.3 volts means about 15% over charge. This is the maximum resting voltage over which permanent damage may occur. Resting voltage means no charge going in or out for at least 10 seconds.

2. When discharging the internal resistance of the cells will cause the terminal voltage to decrease. the more current the lower the voltage at 5C discharge a low voltage cut off of 3 volts will usually result in a recovery to between 3.5 and 3.7 volts after 10 seconds indicating that the cell has been depleted to between 5% and 10% of its rated capacity. In my opinion over discharging is the cause of most battery failures.

4. When charging a cell in good condition at a 1C rate the terminal voltage will rise between 10 to 50 millivolts depending on the state of charge. This delta voltage is caused by the internal resistance of cell and is highest at the begining and end of the charge cycle and lowest in the middle. If a cell were almost fully charged so that it had a resting voltage of 4.2 volts one can expect to see a terminal voltage of perhaps 4.25 volts at the cell terminals when charged at a 1C rate. The 109 charger does not measure the voltage at the cell terminals but about 12 inches away inside the charger. so there is a slighly higher reading during the charge. That is one reason why we stop charging very often during the charging cycle to measure the resting voltage.

6. If a cell has been discharged too hard or too long it can exibit a higher internal resistance, for this reason one we have a built in cut off if we detect a large change in voltage over a short time period. If you see this happen you need to lower the charge current and closely monitor the next charge to make sure the cells have not been previously damaged by over discharge.

Thanks to Doug and Joe,

I now realize that Li Poly cells can be charged to about 4.1 to 4.2 volts level. (about 12.44V divided by 3 cell = about 4.1 to 4.2 volts per cell)

I just begun using Li Poly indoors with a sub ounce plane. Now I'm replacing NiMH with Li Poly for outdoors flying. I have much to learn about batteries for flight.

After resting for a couple days, connected in parallel, 3S3P, the voltage has dropped to 4.12 V from the freshly charged voltage of

12.44 V. I've read that Li Poly batteries lose only about 1 % per 3 months of storage. Why such a big lost of capacity in such a short time? Am I using the correct terms?

Wan

Its not actually losing anything. The moment the battery peaks, it'll read say,

10.5volts (my Nimhs as an example). after a minute of resting, its down its 9.6. Nothing is being lost, just the charge is equalizing out through the whole cell, instead of being densely packed around the poles. Once your lipo rests a few minutes to equalize, its voltage will stay the same for a long time.

say, 10.5volts

for a long

Thanks Mike. I have another question. Would I have to top the batteries off before using them?

You gentlemen have been very patient. Now I understand. This doesn't mean I may not be back with more questions.

Wan

...

| > Once your lipo rests a few minutes to equalize, its voltage will | > stay the same for a long time.

Well, approximately the same.

I was a bit confused by a drop from 12.44 to 4.1 volts, but I'm guessing that you're just giving the total voltage and then the voltage for just one cell. In any event, I wouldn't worry about it.

As another poster mentioned, the voltage of a LiPoly cell will tell you how charged it is. This is totally different from a NiCd or NiMH cell, which may read 1.4 volts with no load and being almost completely empty.

| Would I have to top the batteries off before using them?

Not at all. Just keep 'em charged, and fly when you wish.

You actually don't need to top off NiMH or NiCd cells either, but doing so does give you a little better performance. Of course, what gives you the best performance is charging from almost empty to full at 1C or so, then immediately going and flying.

But none of that is needed for LiPoly, which is another advantage for them.

| You gentlemen have been very patient. Now I understand. This doesn't | mean I may not be back with more questions.

That doesn't mean that we'll be patient when you come back!

(Just kidding. We probably will be.)

Wan, Bear in mind that the major difference between a Nicd/NiMH charger and a lipoly charger is this: The Nicd/NiMH charger puts out a no-load voltage higher than the battery's terminal voltage. The lipoly charger's no-load voltage is clamped to, and cannot exceed, the battery's terminal voltage. So it's easy to see why using a Nicd/NiMH charger on lipolys would run them above rated voltage, destroying them. Bill(oc)

A drop from 12.44 ( 4.15 volts/cell) to 4.12 volts (you did mean the whole pack voltage, not just one cell) indicates something is very wrong.

Red,

It takes you to find my error. It was just a mistake. What I should have said in my post #5 was; there was a drop from 12.44 V to 12.12 V. I did mean the voltage is for the pack of 3 cells. Does this sound better? Thanks, Wan

Much better!!!

My packs read about 11v after a flight - they are only halfway flat at that. I think around 9v is the dangerously low off load level. Probably best to stick to 10.5v. Off load. On load the voltage can go well below that. Even a full pack can barely struggle over 10v under rated load.

Red,

Both of those readings were made under no load and for about 2 seconds.

Wan