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Voltcraft ALC 8500 EXPERT Manual

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After you have selected the processing function you wish to use, all the essential parameters for
processing the battery pack are concluded, and are displayed on the screen when you confirm your
selection by pressing the “OK / Menu” button briefly. Now press the “OK / Menu” button again to
initiate the process. The program returns to the main menu; confirm once more with “Return”, and the
screen displays the main window again.
During the process the top half of the screen shows the voltage, current and battery capacity which
can be read off directly; the measured values are constantly updated while the processing progresses.
The screen also shows all the important status information for the corresponding charge channel. If
you wish to halt the program at any time for any reason, simply select the channel in the “Chan-Menu”
and select “Stop”.
13. B. Resist. (Ri measurement function)
Selecting the sub-menu “B. Resist.” from the main menu takes you
to the ALC 8500-2 Expert’s internal battery resistance measurement
function (Fig. 21). Press the “OK / Menu” button briefly to move to the
display window shown in Fig. 22.
In principle the method of measuring internal resistance is extremely
simple: the battery is discharged at a high, carefully defined current,
and the voltage decline compared to the unloaded state is measured.
The internal resistance can now be calculated by dividing the voltage
difference by the load current.
As we are dealing with very small resistances here, the load current
on the battery needs to be as high as possible. However, a continuous
high current would generate a great deal of waste heat, and would
also discharge the battery to a significant extent. These problems can
be avoided by using pulses of current to measure the internal resis-
tance. The current pulses with the ALC 8500-2 Expert are variable
within the range 1 A to 10 A, although we recommend selecting as
high a current as possible, otherwise - bearing in mind the typically
low internal resistance of these batteries - the voltage drop will be
extremely small. Low current pulses only make sense with batteries
which are unable to cope with high peak loads.
Informative results can only be achieved if the voltage is measured
directly at the battery terminals, otherwise voltage fall-off in the measu-
rement cables would seriously falsify the result. To fulfil these requirements, special measurement
cables are used, each wire featuring two spring-loaded probes (see Fig. 1). These probes make reli-
able contact with the terminal caps of the battery (or other desired measurement points). The pulse
of discharge current flows through the broad contacts of the measurement cables, and the second
contact is used to record the measurement directly at the terminal caps of the battery.
If you wish to include the losses caused by cables and connectors in the resistance measurement,
then simply position the probes at the appropriate points. The spring-loaded probes ensure reliable
electrical contact at all four measurement points.
Important: when carrying out the measurement the spring contacts must always be pushed hard
against the contact surfaces of the battery, i.e. as far as they will go. When carrying out comparative
measurements at different cells it is essential to use identical contact surfaces. Even welded solder
tags have a considerable influence on the measured result. Typical values for individual very good
sub-C cells are in the range 4 mOhm to 6 mOhm.
The battery’s internal resistance is responsible for voltage losses in any battery-operated system, but
it is not the only culprit: parasitic transfer resistance, caused by cables and connectors, can have a
considerable influence. In any high-current applications all connectors should exhibit a large contact
area and be a firm, secure fit.
Fig. 21: Battery Ri measure-
ment function
Fig. 22: Setting the pulsed
current for battery Ri measu-
rement
… 20 …


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