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Voltcraft ALC 8500 EXPERT Manual

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4 Charge processes, charge outputs
During the charge process the micro-controller constantly monitors the course of the voltage at each
charge output individually. A series of successive measured values is used to assess the charge
curve. For best possible results from the charge process the ALC 8500-2 Expert constantly monitors
the charge curve for the appropriate battery type with 14-bit accuracy.
Reliable detection of the optimum charge cut-off point is particularly important. With NC and NiMH batte-
ries the charger employs the reliable method of negative voltage difference (peak cut-off) at the end of the
charge curve. Charge currents greater than 0.5 C are recommended, as they generate a pronounced Delta-
V which the charger detects easily. If the unit registers a voltage difference of a few mV in the downward
direction over several measurement cycles at the battery, that channel switches to a trickle charge rate.
The same applies to NiMH batteries, except that the charge curve is shallower than that of NC batte-
ries, and the charger takes this into account. In the case of Lead-acid, Lithium-ion and Lithium-poly-
mer batteries the charge cut-off point is detected according to the current / voltage curve.
Transfer resistances at the terminal clamps can have an adverse effect on the accuracy of the measu-
rement, and for this reason the battery voltage of NC and NiMH batteries is always measured under
zero-current conditions. Batteries which have been stored or deep-discharged tend to provoke pre-
mature charge termination, but the ALC 8500-2 Expert features an additional pre-peak detection
circuit which reliably prevents this occurring.
Where batteries are in a deep-discharged state, the ALC 8500-2 Expert delivers an initial pre-charge
at a reduced current.
Most high-capacity nickel-metal-hydride batteries are very sensitive to overcharging, but this drawback
is balanced by their immunity to the memory effect, which is a common problem with NC batteries.
Long intervals between periods of use, followed directly by recharging (i.e. without first discharging)
are one cause of the memory effect with NC cells; another is constant partial discharges followed by
topping-up. The electrolyte then tends to crystallise out at the electrodes, thereby obstructing the flow
of electrons within the cell. A series of discharge / charge cycles often has the effect of restoring the
full capacity of such packs.
Clearly a charger which only provides a simple charge function is not sufficient for optimum mainte-
nance of any rechargeable battery. The ALC 8500-2 Expert offers various programs for comprehen-
sive battery maintenance, all aimed at maximising useful battery life. As you would expect, all channels
can be programmed to carry out different processes at the same time.
To dissipate waste heat during discharge processes the ALC 8500-2 Expert is equipped with an in-
ternal heat-sink / cooling fan assembly, and a temperature monitor operates constantly at the output
stages to protect the charger from overloading in every situation.
Charge channels 1 and 2 are designed for a charge voltage up to 30 V (corresponding to a nominal
battery voltage of 24 V with NC and NiMH) and maximum output currents of up to 5 A.
The output currents available vary according to the cell count of the connected battery, as they are
limited by the available charge power.
The maximum total charge power for channels 1 and 2 is 40 VA. Please note that the basis for calcu-
lating this figure is not the nominal battery voltage; a higher voltage must be taken into account under
charging conditions. For example, if an output power of 30 VA is drawn for channel 1, the 10 VA is still
available for channel 2. As long as the total power remains below 40 VA, both channels operate si-
multaneously. If this is not the case, the channel whose process was started last has to wait until the
required power is available, i.e. when the charge process of the channel first started is concluded; the
second process then starts automatically.
Charge outputs 3 and 4 are designed to operate at a maximum output voltage of 15 V, corresponding
to a nominal battery voltage of 12 V with NC and NiMH batteries. In this case the maximum possible
charge current is 1 A, shared by the two outputs working simultaneously. For example, if a charge
current of 500 mA is selected for channel 3, then 500 mA is also available for channel 4. However,
channel 4 can supply 800 mA if channel 3 is only delivering 200 mA.
… 8 …


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