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that level established by the Plant Float Set-Point (fsp) or Boost Set-Point (bsp), dependent on the
highest temperature monitored by the QS873A VT-Probes located at the plant batteries. Thermal
Compensation should be used in a plant containing "sealed" or valve regulated "maintenance free"
batteries. Note that Thermal Compensation is automatically enabled if the system controller detects
the presence of a VT-Probe. Refer to the Installation Instructions for more details on wiring and
configuring this feature.
Thermal Compensation lowers plant voltage from the fsp for monitored battery temperatures which
are above the ideal temperature established during configuration as the Battery Thermal Slope
Nominal Temperature (ntt). (The items in parenthesis are the user configurable points referred to in
the graph shown below.) Lowering the plant voltage helps to keep the batteries at their optimum state
of charge while protecting them from thermal runaway. Thermal runaway is a complex sealed battery
phenomenon where, for one or more of a number of reasons, one or more cells in a string are unable to
dissipate the internal heat generated by their charging current and experience an increase in internal
temperature. By lowering the float voltage as cell temperature increases, the float current is lowered to
a point where this destructive behavior can be avoided. If a cell failure is imminent and the cell
temperature continues to rise above the threshold configured for Battery Thermal Step Temperature
(stp), plant voltage drops in a single step to a level which keeps the remaining cells in the string from
overcharging and being damaged. Refer to Figure 6-2 for a graphical view of Battery Thermal
Compensation and the relationship of its various set points.
The QS840A can also increase plant voltage above that set by the fsp or bsp for colder environments,
again seeking to keep batteries at their optimum charge state. Batteries will lose capacity as the
battery temperature drops below their optimal operating temperature. Increasing the plant voltage for
decreases in temperature causes more current to flow into the batteries. This results in electrolysis of
the water in the batteries. Since this reaction is exothermic, it also serves to keep batteries warm. This
feature results in an increase in plant voltage, and is required to be enabled during controller
configuration.
Temperature
Compensation
Voltage Adjustment
(volts per cell)
lsp* (ntt - ltt)
0
rve = 0, Disabled
(No Low Temp
Compensation)
usp* (utt - ntt)
usp* (utt - ntt) – 0.17
Default Settings
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ltt - Lower Thermal Limit (-5 to +20)
rve - Low Temperature Compensation Enable (rve=1, Enabled)
lsp - Lower Temperature Slope Slope (1mV to 5mV / Cell / °C)
5
25
Cell Temperature (⁰C)
Figure B-1: Slope Thermal Compensation
ntt - Battery Thermal Slope Nominal Temperature
(15 to 30)
usp - Upper Temperature Slope (1mV to 5mV / Cell / °C)
utt - Upper Temperature Temperature (30 to 55)
thr -Battery Thermal Alarm Threshold (30 to 85)
65
45
55
spt-Battery Thermal Step
Temperature (45 to 85)
2
75
1
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