elv 8500-2 Expert Operating Instructions Manual page 10

However, these transfer resistances generally remain
unchanged relative to each other. For this reason it
is always worthwhile carrying out an optimisation
process in any high-current application. This invol-
ves eliminating unnecessary connectors, and using
short cables of generous cross-section wherever
possible. All connectors should exhibit a large con-
tact area and be a firm, secure fit. In principle the
method of measuring internal resistance is extre-
mely 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 then be calculated by dividing
the voltage difference by the load current.
In practice the process is not quite so simple: on the one hand the voltage differences are very small
- in the Millivolts range - and on the other hand the charger has to be capable of absorbing the high
discharge current and the dissipated power, even when the duration of the current pulse is brief. A
further difficulty is the fact that informative results can only be achieved if the voltage is measured
directly at the battery terminals, otherwise voltage fall-off in the measurement cables would seriously
falsify the result. To fulfil these requirements, special measurement cables (optional) are used, each
wire featuring two spring-loaded probes (Fig. 1). These probes make reliable 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 measu-
rement 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 note:
It is in the nature of the battery Ri measurement process that it is impossible to provide reversed
polarity protection. Please take care, as connecting a battery with reversed polarity can cause dama-
ge.
Fig. 1: Special measuring cables with spring-
loaded probes
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