Model Forum / Radio Controlled / Air Models / April 2004

It's about 4.1 to 4.2 volts per cell, so that sounds right.

NiMH and NiCd are about 1.4 volts per cell fully charged, so that
sounds right.

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.

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! :)

LiPo Cell Behavior

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

1.

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.

3.

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.

4.

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.