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The external cell temperature should not
exceed 45°C during charging and 60° C
during discharging, otherwise the cell will be
damaged, with a permanent loss of capacity.
If the limit values are exceeded for a pro-
tracted period, the cell will be ruined, and
may explode and start a fire.
Variations in capacity
If several cells are assembled to form a bat-
tery which is then discharged at a fairly high
current, the cells will heat up to different
extents as the internal cell or cells are unable
to dissipate heat effectively.
This results in changes to the cells' internal
resistance, which in turn reduces their disch-
arge capacity. The cell concerned will then be
discharged more quickly, with the danger
that it will eventually be discharged below the
permissible final charge voltage of 2.5 Volts.
Considerable differences in capacity can
occur when external temperatures are very
low. For example, if a Li-Poly pack is flown in
an electric helicopter, the front cell will be
greatly cooled by the normal airflow, while
the inner cells become significantly warmer.
The colder cell will lose effective capacity,
with the danger that it will be discharged
below the permissible final discharge vol-
tage.
To avoid the danger of permanent cell
damage we recommend that Li-Poly cells
should only be discharged down to a final
discharge voltage of about 3 Volts.
Storage
Li-Poly cells feature an extremely low rate of
self-discharge (approx. 0.2% per day), and
can therefore be stored for long periods wit-
hout problems. However, before a protracted
period of storage they should be charged up
to about 50 - 80% capacity, and they should
be topped up again after about 4 - 6 months.
Supplementary operating instructions
POWER PEAK ULTIMATE Li
9
Memory effect, cell capacity
Li-Poly cells do not suffer from the memory
(lazy battery) effect, so the standard proce-
dure with NC and NiMH batteries of dischar-
ging before recharging (cycle charging) is not
necessary.
Indeed, it is actually better to avoid dischar-
ging the cells before recharging.
Each charge process reduces the capacity
of the Li-Poly pack slightly, so this would
lead to an unnecessary loss of capacity.
Assembling battery packs
Li-Poly cells can be wired together in series
or parallel in order to increase pack voltage
or capacity, but this does present problems
due to variations in charge voltage and capa-
city.
For this reason it is important to use
selected cells exclusively if they are to be
wired together to form a battery.
Charging battery packs
Integrated charge protection circuit
Each Li-Poly cell usually contains a voltage
monitor module to protect it from overchar-
ging, deep-discharging or excessive cur-
rents.
However, for modelling applications it is
usual to draw high load currents, which
means that the monitor module would very
often be tripped to protect the cells. For this
very reason most battery packs do not
include this feature.
The lack of the protective circuit can gene-
rate problems in the charging of series-wired
Li-Poly cells. As mentioned earlier, individual
cells in the pack may well have slightly diffe-
rent charge states and voltages.
In this case the overall final charge voltage is
not distributed evenly amongst the individual
cells, with the result that cells featuring hig-
her voltage can easily be overcharged.
Order No.
8427 Li
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