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G4WYW > TECH 09.04.08 13:00l 212 Lines 8093 Bytes #999 (0) @ WW
BID : 030284G4WYW
Read: GUEST
Subj: Charging Nicads part 4 of 4
Path: IZ3LSV<IW2OHX<IQ0LT<IK2XDE<F5GOV<F4BWT<DK0WUE<GB7FCR
Sent: 080409/1014Z @:GB7FCR.#16.GBR.EU #:1554 [Blackpool] FBB-7.03a $:030284G4W
From: G4WYW@GB7FCR.#16.GBR.EU
To : TECH@WW
Part 4 of 4
ICS1700 TERMINATION METHODS.
The ICS1700 uses eight different techniques to determine when to end the
charge, employing voltage monitoring, temperature sensing and timer cutoff
methods.
1) The primary method monitors the linear regression slope for the battery
voltage as outlined previously. The charge is terminated and a
maintenace
mode begins when the slope reaches a derived cutoff threshold. The
derived
threshold is continuously calculated from the start of the charge
routine.
Several requirements must be met; however, before the cutoff threshold
can
be correctly computed to avoid premature termination caused by
anomalies
in the different cells. Due to the sensitivity of the linear regression
slope to the voltage curve, small variations in the voltage curve can
cause large swings in the linear regression slope. This sensitivity to
the
battery voltage is especially useful in monitoring unmatched cells with
different charge peaks.
2) Several unmatched cells may cause a battery charge curve to never reach
a
voltage peak. This condition occurs if some cells reach their
individual
peaks while others are still charging. A second method of charge
termination
watches for a few successive calculations of a negative linear
regression
slope which would indicate a diminishing voltage curve. The ICS1700
then
discontinues charging and places these cells on maintenance.
3) If a charge is applied to a fully charged battery, the cell voltage
will
rise very rapidly since any applied energy bacomes plate surface charge
only and does not add any appreciable energy to the stored charge in
the
battery. The ICS1700 detects this rapid increase in voltage and quickly
shuts off the charge routine to avoid overcharging. The cells are then
placed on maintenance charge.
4) Overcharged batteries which have a charge immediately reapplied to them
will show a sharp drop in cell voltage as a result of the change in the
cell impedance. The voltage drop triggers a sharp decrease in the
linear
regression slope and will cause the ICS1700 to quickly drop into a low
current maintenance mode.
5) After several months of disuse some nickel-cadmium cells may exhibit a
high impedance condition. This high impedance condition may not be
noticeable until the battery is under full charge. Continued charging
of
these cells will create high temperatures and pressures in a battery
that
may have been weakened previously. The ICS1700 watches for these high
impedance batteries and terminates the charge if the no-load voltage
runs
too high. It is recommended that these cells be replaced. The ICS1700
must
be reset before charging another battery.
6) A battery sometimes contains cells that may be shorted internally due
to
dentrite growth or damaged separators. The ICS1700 attempts to detect
these
batteries before a long term high current charge is applied. A normal
will
jump to about 1.3 volts in response to an applied current. A shorted
battery
will remain below a preset threshold, indicating previously damaged
cells.
These cells should be replaced. The charger must be reset before
another
battery can be charged.
7) Failsafe termination methods include a deadman timer which terminates
charge
after a preset amount of time in the unlikely event that the cell
voltage
curve never reaches a peak voltage nor descends in slope. The timer
will
timeout after a certain period based on the selected charge rate. The
charger must be reset before charging can continue.
8) The final method utilises an end-user supplied thermal switch which
will
open circuit if the battery becomes too warm. Shut off temperatures for
nickel-cadmium batteries are about 45 deg.C to 50 deg.C to avoid
unnecessary venting. It is strongly recommended that a thermal switch
be
used when charging nickel-cadmium batteries at high rates.
Short circuit or open circuit charger contacts are detected before a
full
charge is applied to avoid harming the charging circuitry. A high
current
charge pulse is applied to the contacts with the resulting voltage
compared
to preset thresholds. If the voltage never rises or rises too high, a
contact fault is presumed. The ICS1700 assumes that a battery has been
misplaced against the contacts, and will reattempt to start a charge.
At
the end of this period the charger will quit trying and have to be
reset
for further use.
INTERFACING TO EXTERNAL CIRCUITRY.
The ICS1700 requires some external components to control the clock rate
and
provide an indicator display. The chip must be interfaced to an external
power source that will provide the constant current required to charge a
battery pack as well as a circuit that will provide a negative current
discharge pulse.
The charge and discharge signals are active high, TTL compatible signals.
In
addition to being TTL compatible, the CMOS outputs are capable of sourcing
current which can add flexibility when interfacing to other circuitry. A
high
on the charge signal indicates that the constant current supply should be
activated. A high on the discharge signal indicates that the discharger
should
be activated.
LED indicators can be connected to the device to display the charge mode
and
any fault conditions. The device has three outputs for driving external
indicators. The three indicator outputs have open drains and are designed
to be
used with LED indicators. Each output can sink over 20mA which requires
the
use of an external current limiting resistor. The three indicator signals
denote battery fault, charge mode and over temperature.
The battery fault indicator is activated whenever a battery with low
charge
slope, or a high impedance is detected. Either one of these faults
indicates
a defective battery. A low charge slope failure indicates that the battery
is
not accepting charge normally. It is detected by a very slow rise in
battery
voltage during the first twenty seconds of charge. The high impedance
failure
is detects by very high charging voltage during the first twenty seconds
of
charge.
The battery fault indicator is also activated when a poor contact between
the
charger and the battery is detected. It may also activate if the charge
terminals are short circuited or if a battery pack has several shorted
cells.
In the event that a contact fault is detected, the controller will retest
two
times a second for a good contact for ten seconds. If the contact fault is
not
cleared during the initial ten seconds of charging, the controller will
enter
a quit mode and can only be restarted by momentarily opening the over
temperature switch or by momentarily grounding the reset pin.
The charge mode indicator is activated continuously during charging. When
the
controller enters maintenance mode the signal is pulsed on and off at a
one
half second rate.
The over temperature indicator is activated whenever the over temperature
switch opens. This indicates that excessively high temperatures have
occurred
in the battery pack. The over temperature signal also issues a reset
command
to the microprocessor. If a fault condition occurs, it can be cleared by
opening the over temperature line connected to the temperature switch
contained
within the battery pack.
***************************************************************************
****
This data has been prepared for transmission over the packet radio network
by Steve, G1XOW (Nottingham), permission from the author has been
obtained.
Note from G8AMG @ GB7LWB
Steve, then offered his address for furthur info, since the original file
is
7 years old, this is probably out of date, so I have not included it here.
This last paragraph is probably not valid now with the changes to the
advert
rules announced in April 2001.
(SYSOPS PLEASE NOTE:- This is not an advert in any way, I have no)
(connection with the manufacturer of this device, and cannot! supply)
(the said device.)
73 - Mel, G4WYW
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