The charge characteristics of the 40 Ahr batteries are as follows:
Discharging Characteristics - The 40 Ahr batteries have a nominal voltage of 24 VDC (20 cells at
1.2 VDC/cell). This is the voltage that would be measured with a fully charged battery under a
load equal to its ampere-hour rating, i.e. in this case 40 Ahr. The ‘no-load’ voltage as measured
at the battery terminals approaches 28 VDC. This explains why the batteries will discharge with
DC Ground power connected, particularly if the cart voltage is lower than 28 VDC. Battery
reserves will be depleted at a higher rate, as the cart voltage is lower.
Charging Characteristics - In order to restore batteries to a full state of charge, charging voltage
should be at least 1.4 to 1.6 VDC/cell, or 28 to 32 VDC. This is provided with any DC generator
on line. However, if an external ground power cart has an output voltage of less than 28 VDC, the
batteries cannot be recharged to full capacity, regardless of the time that voltage is applied.
When the DC External power cart output voltage is low, the discharge current from the batteries
is not being supplied to the cart but instead supplied to the aircraft loads. When the batteries are
online, the DC External cart and batteries are connected in parallel allowing current to be con-
sumed by either source. If a low voltage output cart is connected with properly charged batteries
online, as the case is after flight since the engine generators output 28.5 VDC to charge the bat-
teries, the batteries will provide most of the current to the aircraft loads because charged batter-
ies have a low internal impedance. As the batteries discharge, their internal impedance
increases and most of the current to the aircraft loads is provided by the DC External cart. How-
ever by this time a significant portion of the batteries’ reserve can be consumed. Also, it should
be noted that even if the DC External cart and the batteries are at the same potential, there
would be a small current flow from the batteries.
Based on the charge/discharge characteristics of NiCad batteries, it is evident that the external
DC Power cart must have a voltage of at least 28 VDC or battery reserves will be depleted and
cannot be fully restored.
Operators are advised, that if 28 VDC carts are not available or their voltage is lower than 28 vdc,
the batteries should be isolated from the aircraft loads by selecting the BATTERY MASTER,
MAIN, AUX and STBY battery switches OFF while DC External power is connected.
Dash8 - Q400 - Electrical
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