Product Manual

30
SOLAR BATTERY CHARGING
31
MORNINGSTAR CORPORATION
4.04.0
Float
Float is less affected by temperature changes, but it may also undercharge
or gas too much depending on how much the temperature changes.
The RTS corrects the three charging setpoints noted above by the
following values:
• 12 volt battery: –0.030 volts per ˚C (–0.017 volts per ˚F)
• 24 volt battery: –0.060 volts per ˚C (–0.033 volts per ˚F)
• 48 volt battery: –0.120 volts per ˚C (–0.067 volts per ˚F)
Variations in battery temperature can affect charging, battery capacity, and
battery life. The greater the range of battery temperatures, the greater the
impact on the battery. For example, if the temperature falls to 10˚C (50˚F) this
15˚C (27˚F) change in temperature will change the PWM, equalization and
float setpoints by 1.80V in a 48V system.
If a remote temperature sensor is not used and the temperatures near the
battery are stable and predictable, the PWM absorption setting can be
adjusted using the PC software per the following table:
Temperature 12 Volt 24 Volt 48 Volt
40ºC / 104ºF – 0.45 V – 0.90 V – 1.80 V
35ºC / 95ºF – 0.30 V – 0.60 V – 1.20 V
30ºC / 86ºF – 0.15 V – 0.30 V – 0.60 V
25ºC / 77ºF 0 V 0 V 0 V
20ºC / 68ºF + 0.15 V + 0.30 V + 0.60 V
15ºC / 59ºF + 0.30 V + 0.60 V + 1.20 V
10ºC / 50ºF + 0.45 V + 0.90 V + 1.80 V
5ºC / 41ºF + 0.60 V + 1.20 V + 2.40 V
0ºC / 32ºF + 0.75 V + 1.50 V + 3.00 V
– 5ºC / 23ºF + 0.90 V + 1.80 V + 3.60 V
– 10ºC / 14ºF + 1.05 V + 2.10 V + 4.20 V
– 15ºC / 5ºF + 1.20 V + 2.40 V + 4.80 V
Table 4.3 Temperature Compensation
The need for temperature compensation depends on the temperature
variations, battery type, how the system is used, and other factors. If the
battery appears to be gassing too much or not charging enough, an RTS
can be added at any time after the system has been installed.
See Section
2.3 - Step 4 for installation instructions.
The TriStar will recognize the RTS when the controller is started (powered-up).
4.3.2 Battery Voltage Sense
There can be voltage drops typically up to 3% in the power cables connect ing the
battery to the TriStar. If battery voltage sense wires are not used, the controller will
read a higher voltage at the controller’s terminals than the actual battery voltage
while charging the battery.
Although limited to 3% as the generally accepted wiring standard, this can
result in a 0.43 voltage drop for 14.4V charging (or 1.72V for a 48 volt
nominal system).
These voltage drops will cause some undercharging of the battery. The
controller will begin PWM absorption, or limit equalization, at a lower battery
voltage because the controller measures a higher voltage at the controller’s
terminals than is the actual battery voltage. For example, if the controller is
programmed to start PWM absorption at 14.4V, when the controller “sees”
14.4V at its battery terminals, the true battery voltage would only be 14.1V
if there is a 0.3V drop between the controller and battery.
Two sense wires, sized from 1.0 to 0.25 mm
2
(16 to 24 AWG), can be used for
battery voltage sense. Because these wires carry no current, the voltage at
the TriStar will be identical to the battery voltage. A 2-position terminal is
used for the connection
Note that the battery sense wires will not power the controller, and the sense
wires will not compensate for losses in the power wires between the con-
troller and the battery. The battery sense wires are used to improve the
accuracy of the battery charging.
See Section 2.3 - Step 5 for instructions how to connect the battery sense wires.
4.4 Equalization
Routine equalization cycles are often vital to the performance and life of a
battery — particularly in a solar system. During battery discharge, sulfuric acid is
consumed and soft lead sulfate crystals form on the plates. If the battery remains
in a partially discharged condition, the soft crystals will turn into hard crystals
over time. This process, called “lead sulfation,” causes the crystals to become
harder over time and more difficult to convert back to soft active materials.
Sulfation from chronic undercharging of the battery is the leading cause of
battery failures in solar systems. In addition to reducing the battery capacity,
sulfate build-up is the most common cause of buckling plates and cracked
grids. Deep cycle batteries are particularly susceptible to lead sulfation.
Normal charging of the battery can convert the sulfate back to the soft active
material if the battery is fully recharged. However, a solar battery is seldom
completely recharged, so the soft lead sulfate crystals harden over a period
of time. Only a long controlled overcharge, or equalization, at a higher
voltage can reverse the hardening sulfate crystals.
In addition to slowing or preventing lead sulfation, there are also other
benefits from equalizations of the solar system battery. These include:
Balance the individual cell voltages.
Over time, individual cell voltages can drift apart due to slight differences in
the cells. For example, in a 12 cell (24V) battery, one cell is less efficient in
recharging to a final battery voltage of 28.8 volts (2.4 V/c). Over time, that
cell only reaches 1.85 volts, while the other 11 cells charge to 2.45 volts per