SunXtender Technical Manual

Document No. 6-0100 Rev. H
Page 19 of 43
5.4 Charging
Charging Sun Xtender® AGM batteries is a matter of replacing the ampere-hours removed
during discharge plus a little extra to make up for charging inefficiency. The ampere-hour input
necessary for a full recharge depends on the depth of discharge, rate of recharge, and
temperature. Typically, between 102% and 110% of the discharged ampere-hours must be
returned for full recharge. If the recharge is insufficient, the battery’s state of charge will
gradually “walk down” as it is cycled, resulting in sulfation and premature failure.
The recommended method of charging Sun Xtender® AGM batteries is to use a 3-stage
charging profile. In the first stage, a constant current is applied until the voltage reaches a pre-
set limit. The first stage is often called the Bulk charging stage.
In the second stage, the voltage is held constant at the same pre-set limit until the charging
current tapers to a very low value, at which point the battery is fully charged. The second stage
is often called the Absorption charging stage. The battery is considered to be fully charged
when the current drops below 0.5% of the battery’s rated capacity (0.5A for a 100Ah battery).
The absorption stage will typically last 2 – 4 hours before the current reaches this level.
In the third stage, the charging voltage is reduced to a lower value that minimizes the amount of
overcharge, while maintaining the battery at 100% state of charge. This third stage is often
called the Float charging stage.
The following table provides recommended charge voltage settings for Sun Xtender® Batteries:
Table 5-1 Recommended Charge Voltage Settings
Absorption Voltage at 25°C (77°F)
Float Voltage at 25°C (77°F)
Volts per Cell
2.37 - 2.40V
2.20 - 2.23V
For 12V System
14.2 - 14.4V
13.2 - 13.4V
For 24V System
28.4 - 28.8V
26.4 - 26.8V
For 48V System
56.8 – 57.6V
52.8 – 53.6V
The charging volts per cell (VPC) at other temperatures can be determined from the following
equations:
VPC (Absorption) = 0.00004T
2
- 0.006T + 2.510 (where T = °C)
VPC (Float) = 0.00004T
2
- 0.006T + 2.340 (where T = °C), but not less than 2.167 volts per cell.
See Appendix C for a chart of charging voltage versus temperature. Most chargers that have
automatic temperature compensation use a simplified equation with a linear coefficient. The
recommended linear coefficient for Sun Xtender® batteries is 0.004V/cell per degree C
(0.024V/degree C for a 12V battery).
The following table provides recommended absorption times for Sun Xtender® Batteries:
Table 5-2 Recommended Absorption Times
Average Depth of Discharge (DOD)
Absorption Time
Less than 30%
2 hours
30 – 50%
3 hours
More than 50%
4 hours
The absorption time may need to be fine-tuned from these values to assure the batteries
consistently reach full charge per the criteria given above (charging current is less than 0.5% of
battery’s rated capacity).