Three Charging Stages Are All That’s Needed

WFCO’s automatic three-stage converters handle every charging need for the RV while extending the battery’s life. Well-maintained batteries should never need more than WFCO’s two-stage (Normal and Trickle) charging modes. Our Fast Charge third stage (Bulk) is for the rare times a battery needs extra power charging.

 The WFCO Normal (Absorption) Mode alone is a powerhouse capable of charging a fully discharged battery in under three hours at 13.6 V. When the RV is used, the battery is readily charged in Normal Mode.

However, suppose the converter detects no significant 3-Stage-chargingvariation in the current draw for approximately 44 continuous hours. In that case, our Trickle (Float) Mode automatically kicks in and brings the battery back to full charge. Maintaining complete charging helps prevent stratification of the battery’s fluids.

For those cases when the battery charge drops significantly due to prolonged disuse, poor maintenance, or very heavy system overloads, our 14.4 V Fast Charge (Bulk) Mode kicks in. Our Fast Charge delivers a maximum four-hour charge to prevent battery damage and extend the battery’s life.

So be assured that whenever you install any WFCO converter or power center, your customers get the best battery charging care available.



WFCO Converters – Theory of Operation

WFCO Converters of every style have become the favored brand for power conversion and electric distribution in the RV industry. They provide RV owners with an efficient and cost-effective method to use an ac power source and provide power to dc components inside the RV while charging accessory batteries simultaneously.

Basic Operation
RVs are frequently sold with at least one 12 V dc accessory battery installed. This battery is usually a deep-cycle battery that could sustain a slower power drain. As soon as the RV is connected to ac power, the converter begins changing the battery as needed while at the same time providing 12 VDS power to loads such as lights, radios, and refrigerators. RV owners find this practical when loads such as lights, radios, and refrigerators without being connected to ac power or running the motorhome engine.

When the RV is connected to ac power, users frequently use the lights, refrigerators, fans, and other electronics as they would in their homes. RV users also expect the battery to be fully charged when they want to disconnect from power and move the RV or when they are dry camping and turn off their generator.

WFCO converters are designed to fulfill these needs and expectations by providing three charging stages: absorption, bulk, and float modes.

Absorption Mode is the default or regular operation, providing an output of 13.6 volts dc. Because RVs today are designed with converters sized to provide ample dc output power for all dc loads in everyday usage, an RV will rarely require anything other than absorption mode.

When a WFCO converter is connected to a battery in absorption mode, power is available for charging that battery whenever the converter output is greater than the voltage level of the battery. If the battery is at or near fully charged, the current draw from the converter to the battery may be minimal. On the other hand, if the battery were to be fully discharged, the current draw from the converter to the battery may be pretty high.

Testing has shown that a completely discharged battery (11.9 V dc) connected to a WFCO converter in absorption mode with an output of 13.6 V dc and having a 20-amp lighting load connected to the converter will charge the battery to its fully charged level of 12.7 V dc in fewer than three hours. Adding more dc loads will lessen the current amount and lengthen the time required to charge the battery. Batteries with damaged cells will also require additional time to charge and may never reach a full charge voltage.

Because of the relationship between voltage and amperage, once the converter reaches its maximumly rated operating current level, any increase in the dc load will start to decrease the voltage output level. When the output from the converter reaches a preset level, the converter will go into bulk mode.

Bulk Mode is designed to charge a significantly discharged battery in less time than absorption mode. The microprocessor in WFCO converters continuously monitors the dc line voltage. When the microprocessor detects the preset voltage level, it will boost the converter voltage to 14.4 V dc. The increased voltage will help the battery charge faster while still providing power to the dc appliances in the RV.

In bulk mode, observing the 14.4 V dc output may not be possible because of the voltage-current relationship. To measure the 14.4 VDC, reduce some dc loads while monitoring the voltage at the converter output. As the DC loads are removed, the voltage will begin to climb until 14.4 V dc (nominal) is shown on the meter.

WFCO converters are designed to drop out of bulk mode when the total amperage draw from the converter reaches a preset point, indicating the battery is charged. As the battery continues to charge, the current drawn by the battery will gradually decrease. If the amperage draw stays above the preset point, the converter will remain in bulk mode for four hours. These features have been implemented to protect and extend the battery’s life.

Float Mode is the third stage of converter operation. This mode is designed to provide a trickle charge to the battery. If the converter observes no significant variations in the current draw for approximately 44 continuous hours, it will drop the. The converter’s output is from 13.6 V to 13.2 V. This lower voltage will keep the battery charged while the RV is not in use. This also helps preserve the battery’s life while keeping it charged and ready to use. A change in dc current will cause the converter to exit float mode and return to the default, normal, or absorption mode.


WFCO converters are designed to keep the RV safe and, in some cases, prevent irreparable damage to the converter.

Automatic Cooling Fan – The microprocessor in the converter monitors the current drawn by the appliances and battery and increases the fan speed as the current draw increases. This cools the converter components as required by the load.

Over Temperature Protection – If the internal temperature of the converter exceeds a critical point and the fan cannot. Cool the unit down. It will shut down. This protects the unit from the excessive heat that may damage sensitive components. The unit will restart once the temperature inside the unit is again low enough.

Short-Circuit Protection – In a short circuit in the RV, the WFCO converter will drop the voltage output to zero volts. The converter will resume normal operation if the short circuit condition is removed and no other fault conditions are detected. However, short circuit conditions are dangerous, and an RV will require inspection by a qualified service technician.

Reverse Battery and Overload Fuse Protection – WFCO converters include replaceable fuses for protection from conditions that can permanently damage the converter. These fuses will blow and protect the converter if the battery is misconnected or if the converter experiences an overload condition. Before replacing the fuses, check to ensure the polarity connection is correct, and turn off as many dc loads as possible. Disconnect from power, replace the fuses, and then reconnect to power.


Voltmeters or digital multi-meters are great tools for measuring the voltage at different points in the system. Here are a few simple steps to follow in testing a converter.

  • If a battery is installed, disconnect one of the cables, so the battery is out of the circuit.
  • Turn off all the dc loads in the coach (lights, etc.), so there is no load on the converter.
  • Turn the converter OFF by throwing the converter breaker in the load center, unplugging the shore cord, and leaving it OFF for at least one minute.
  • Turn the converter back ON and measure the dc voltage at the NEG and VCC lugs on the dc fuse board in the power center or the NEG and POS terminals on a deck mount converter. The reading should be approximately 13.6 V dc.
  • One by one, start turning dc loads back ON and monitor the voltage at the lugs on the fuse board or the terminals on the deck mount. The voltage should remain stable but may drop slightly as loads are applied.
  • After completing the testing, reconnect the battery cable removed in the first step.


Batteries charge more quickly when no other connected loads compete for DC current. If the batteries take too long to charge, consider turning off as many DC loads as possible. More power will be available for the battery to charge.

The following styles of batteries are approved for all WFCO converters and power centers:

  • Sealed lead-acid batteries for automotive engines starting
  • Flooded lead-acid batteries for deep-cycle applications
  • Sealed AGM batteries for deep-cycle applications

Do not charge GEL cell batteries unless you use a WFCO WF-6800 Series power converter and have selected the GEL switch position at the back of the unit. No other WFCO converters are designed to charge GEL cell batteries.

Do not charge any batteries other than those listed above with WFCO converters. If you are unsure if the WFCO converter is compatible with a particular type of battery, please call your RV dealer or the battery manufacturer for assistance.

For any WFCO resources, tools, and information on our products, email with your request, or click here to download the WFCO Training Catalog. To learn more about our products through video, click here.