Read the manual thoroughly and understand all of the instructions, cautions, and warnings before using this equipment. If any section of the manual is not understood, contact your nearest authorized dealer, contact Generac Customer Support at 1-888-436-3722 (1-888-GENERAC), or visit www.generac.com with any questions or concerns.

Understanding your home standby generator battery charging system

This article explains how your home standby generator keeps its starting battery charged, how the T1 charging circuit and fuse work, and what charging voltages and currents you can expect from the integrated charging system.

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Environment

This information applies to:

• Generac air cooled 10 to 28 kW home standby generators equipped with:
  • Power Zone 200 control panel
  • Evolution 1 or Evolution 2 control panel
  • Nexus control panel
• Some Generac liquid cooled standby generators
  • See your installation manual or contact a trained service dealer for model-specific behavior.
    
    

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Sections

• Battery charging system at a glance
• How the charging system works
• Charger specifications by control panel
• Troubleshooting: If the battery is not charging
• Common questions
  • Battery charging while running
  • Charging in OFF
  • T1 circuit power draw
  • T1 fuse
  • Charger voltage and current
• Key battery charging facts
• See also

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Battery charging system at a glance

• The starting battery is maintained automatically by an integrated charger in the control panel.
• The charger is typically powered by a 120 VAC charging circuit from the automatic transfer switch (often labeled T1 on many air-cooled home standby installations).
• Charging voltage is temperature compensated.
• The controller uses an 18-hour charge cycle when battery voltage falls below about 12.6 to 12.7 VDC (varies by control panel).
• The battery can charge with the controller OFF, provided the T1 charging circuit remains powered. When utility power is available, T1 is supplied by utility power, so charging can continue even if the generator's MLCB is OFF. During a utility outage, T1 must be supplied by generator output, which requires the generator's MLCB to be ON, the transfer switch to be in the generator position, and the generator to be running under load.
  • On some liquid cooled models, the battery may also be charged directly by the generator’s DC alternator while operating under load.
    
    

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How the charging system works

On generators in this environment with Power Zone 200, Evolution 1, Evolution 2, or Nexus control panels:

• The generator uses an integrated 12 V battery charger built into the control panel.
• The charger is powered by a 120 VAC charging circuit from the automatic transfer switch (often labeled T1 on many air-cooled installations).
• Charging voltage is automatically adjusted based on ambient temperature and battery condition.
• The control panel monitors battery voltage and starts an 18-hour charge cycle when voltage falls below a defined threshold.

Important terms

• T1 is the 120 VAC charger feed from the transfer switch to the generator’s controller/charger.
• The Generator Main Line Circuit Breaker (MLCB) refers to the main breaker located at the generator (not the utility breaker feeding the home, and not the controller AUTO/OFF/MANUAL setting).

Temperature-based charging

Charging voltage changes with temperature. In general, charging voltage decreases as temperature increases to help prevent overcharging.

For Evolution 1, Evolution 2, and Nexus control panels, charging is temperature compensated and commonly reads around:

• High temperature: about 13.5 VDC
• Normal temperature: about 14.1 VDC
• Low temperature: about 14.6 VDC

Power Zone 200 models use a wider temperature-compensated charging range of about 12.2 to 16.0 VDC.

The 18-hour charge cycle

• The controller monitors battery voltage continuously.
• When battery voltage drops below about 12.6 to 12.7 VDC (varies by control panel), the charger starts an 18-hour charge cycle.
• If the engine cranks during the 18-hour period, the 18-hour timer resets.

If the battery is still not within an acceptable range at the end of the cycle, the controller may display a battery/charger message such as “Inspect Battery” (Nexus) or a “Charge Warning” (Evolution 1 / Evolution 2).

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Charger specifications by control panel

Power Zone 200 control panel

• Charging voltage: about 12.2 to 16.0 VDC (automatically adjusted for ambient temperature)
• Maximum current output: 5 A
• Power source: 120 VAC load-side connection via a fuse in the transfer switch (example: fuse F3, varies by ATS model)
• Charge cycle: starts when battery voltage is less than about 12.7 VDC, runs 18 hours
• Requirements: T1 connection plus neutral (often labeled T2 or 00)

Evolution 1 and Evolution 2 control panels

• Charging voltage: about 13.5 to 14.6 VDC (temperature compensated)
• Current draw: approximately 0.05 to 2.5 A (varies by battery condition and temperature)
• Power source: 120 VAC fused T1 connection from the automatic transfer switch
• Charge cycle: starts when battery voltage is less than about 12.6 VDC, runs 18 hours
• Requirements: T1 connection. Most Guardian Series generators with Evolution 2 control panels manufactured after approximately June 2024 require a neutral connection for the battery charging circuit.

Nexus control panel

• Charging voltage: about 13.5 to 14.6 VDC (temperature compensated)
• Operation: charging is controlled through the T1 circuit and follows an 18-hour automatic charge cycle similar to Evolution models
• Indicator: “Inspect Battery” message if voltage is too low at the end of the charge cycle
  
  

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Troubleshooting: If the battery is not charging

If you suspect the starting battery is not staying charged, the most common causes are loss of AC power to the charger (charging circuit), an open/failed fuse in the transfer switch (when equipped), missing neutral on models that require it, or battery/cable issues.

• Verify your unit has access to utility power and is installed per the installation manual.
• If your controller is displaying a battery/charger message (for example “Inspect Battery” or “Charge Warning”), allow time for the charge cycle to complete and follow your manual’s guidance.
• For Evolution 2 units that require it, confirm a neutral is present for the charging circuit (a trained service dealer can verify proper wiring).
• Inspect battery cables/terminals for looseness or corrosion and correct as needed (follow safety instructions in your owner’s manual).
• If the charging circuit is suspected, a trained service dealer should verify proper AC power on the charging feed (often labeled T1 on many air-cooled installations) and inspect protection devices in the transfer switch.

Note: For safety, do not open the automatic transfer switch (ATS). Transfer switches can contain live voltage. If you need the charging circuit or transfer-switch protection inspected, contact a trained service dealer. Use the dealer locator or see How Can I Find a Generac Service Dealer in My Area?

 

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Common questions

Is my home standby generator’s battery automatically charged when the generator is running?

When utility power is available

• The built-in charger uses household power to maintain the battery.
• The charger is powered by the ATS charging circuit (often labeled T1 on many air-cooled installations).
• With utility power is present, the charger supplies power to the controller and maintains the battery.

During a power outage, after the generator takes over

• In many installations, after power transfer, the charger continues to receive AC power through the same charging circuit and can continue maintaining the battery.
• On some liquid cooled models, the battery may also be charged directly by the generator’s DC alternator while operating under load.

During the weekly exercise

• In most cases, the weekly exercise does not transfer power to the home, and the battery continues to charge from utility power if it is available.
• Some models support an exercise mode that transfers power. In that case, the battery can charge from generator power during the exercise.

Will my battery still charge if the home standby generator is in the OFF position?

Whether the battery charges while the controller is OFF (and/or the generator’s Main Line Circuit Breaker MLCB is OFF) depends on whether the transfer switch is being powered by utility, or by the generator during an outage. 

When utility power is available 
The T1 circuit is powered from utility through the transfer switch, and the charger can maintain the battery even if the controller is OFF and/or the generator’s MLCB is OFF. 

During an outage (generator supplying power to the transfer switch)
The T1 circuit must be powered from the generator output. For the battery to charge in this condition:

• The generator MLCB must be ON.

• The transfer switch must be in the generator position.

• The generator must be running under load.

If these conditions are not met, the T1 circuit will not be powered, and the charger will not operate.

Additional note for some liquid-cooled models
During an outage, some liquid-cooled generators charge directly from the generator DC alternator output. These models still require that the generator MLCB is ON, the transfer switch is in the generator position, and the generator is running under load for the battery to charge. 

If the T1 circuit is not powered (loss of source power, open circuit, blown fuse where equipped, loose/incorrect wiring, etc.), the charger will not operate and the battery may discharge over time.

Does the T1 battery charging circuit require a constant power draw?

The charging circuit is intended to have a constant 120 VAC supply available. The actual power draw on that circuit varies based on battery state of charge and ambient temperature.

What is the T1 fuse in my transfer switch?

On many home standby installations, the generator’s built-in battery charger is powered by a 120 VAC charging circuit from the automatic transfer switch (often labeled T1). When equipped, a fuse in the transfer switch protects this charging circuit.

DANGER: Generac does NOT recommend untrained end users attempt to service fuses in transfer switches. Transfer switches can contain live voltage even when the generator is not running.

• If the charging circuit is interrupted (for example, an open/disconnected circuit or a blown fuse when equipped), the battery may not stay charged and battery/charger warnings may occur.
• Fuse location and rating vary by transfer switch model. A trained service dealer can verify proper power and protection on the charging circuit.

For transfer-switch-specific details (including fuse location by model), see What is the T1 fuse in my transfer switch?.

If you need a trained service provider, use the dealer locator.

What is the amperage and voltage of the battery charger integrated into the control panel?

See Charger specifications by control panel above. Charging voltage and current vary based on control panel type, ambient temperature, and battery condition.

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Key battery charging facts

• The charging system is automatic and not user adjustable.
• The controller starts an 18-hour charge cycle when battery voltage drops below about 12.6 to 12.7 VDC (varies by control panel).
• If the engine cranks during that period, the 18-hour timer resets.
• When utility power is available, the charger can maintain the battery as long as the T1 circuit is powered (even if the controller is OFF and/or the generator's MLCB is OFF).
• During an outage, T1 power may be supplied from the generator output; this requires the generator's MLCB to be ON. In these scenarios, the generator is charging its own battery.
• The charging circuit is intended to have constant 120 VAC available; delivered power varies with temperature and battery state of charge.
• A blown fuse (when equipped), an open/disconnected charging circuit, a missing neutral on models that require it, or wiring issues can prevent charging and may lead to battery/charger warnings.
  
  

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See also

• How Can I Find a Generac Service Dealer in My Area?
• What is the T1 fuse in my transfer switch?
• Find manuals for your specific model: product lookup page
• Find a trained service provider: dealer locator