How to Connect a Large or Small Inverter to a Battery

Posted on 2/22/2016 by Justin G.

This blog answers questions about which inverters can be powered by 12V DC accessory outlets (cigarette lighter sockets) and which require wiring directly to a battery. In addition to that, we answer the most common questions such as fuse selection, wiring instructions, and general Inverter guidelines.

Skip to a section:
How to Connect the Inverter
Battery Clips - Temporary Connection
Temporarily Connect Inverter to Battery (Battery Clips)
Disconnect Temporary Inverter to Battery Connection (Battery Clips)
Permanent Inverter Installation
Fuse Selection
Wire Routing for Battery to Inverter
Wire Gauge and Lengths
Positive Wire from Inverter to Battery
Positive and Negative Wires from Inverter to Battery
Inverter Grounding Wire
General Instructions for Permanent Installation of Inverters
Variations

 

Making the Decision: How to connect the Inverter

When does a small inverter's power come from a 12V DC outlet and when does that inverter need to be connected to a battery? The basic decision is based on the maximum power the inverter will supply. For most 12V DC outlets, the limit is 15 Amps of DC output (12V x 15A = 180 Watts). That limit is set by the vehicle wiring and the fuse that protects the wiring.

WARNING: NEVER REPLACE A FUSE WITH ONE OF A HIGHER AMPERE RATING – DOING SO CAN DAMAGE WIRING AND CAUSE A FIRE.

Watts are defined as Amps times Volts (W=A x V). In practice, most fuses can withstand periodic over current by 10 percent or, in this case 18 Amps. Furthermore, most 12 volt batteries actually deliver 12.6 to 14 volts while the engine is operating. So, actual watts that can be delivered can be up to 200 watts. Above 200 watts of maximum power output an inverter has to be connected to a battery. This avoids fuses blowing in vehicular electric systems and the subsequent hunt for locating and replacing a blown outlet fuse.

Battery Clips/Clamps - Temporary Connection

Most battery clip cables are not equipped with a fuse. Battery clips are only used for brief temporary connections to a 12 volt battery. As with all connections to a flooded lead-acid car battery, follow the same safety procedures as you would during jump-start procedures presented below.

 

Connect the Positive battery clip to the battery positive terminal. Then connect the negative battery clip to a metal part of the vehicle frame. This sequence prevents a spark from igniting any explosive gasses that may be in the immediate battery area.

 

WARNING – BATTERIES PRODUCE EXPLOSIVE GASES – WEAR SAFETY GLASSES – AVOID SPARKS

Procedure to Temporarily Connect Inverter to Battery (Battery Clips)

1. Make sure the vehicle is parked in a location that does not interfere with traffic.

2. Ensure the vehicle engine is not operating.

3. Open the engine compartment hood.

4. Make sure that the Inverter’s ON/Off switch is set to OFF.

5. Put on safety glasses.

6. Connect the battery clip cables to the Positive and Negative inverter terminals.

7. Place the inverter on a stable surface

8. Connect the Positive battery clip to the battery positive terminal.

9. Connect the negative battery clip to a metal part of the vehicle frame.

10. Connect an appliance cord plug into the inverter or a USB power cord into the inverter.

11. Turn ON the inverter and use the appliance. Note: For brief use of the inverter, it is not necessary to start the engine.

 

Procedure to Disconnect Temporary Inverter to Battery Connection (Battery Clips)

1. Turn OFF the inverter and disconnect any appliance plugs or USB plugs.

2. Disconnect the Negative battery clip from the vehicle frame.

3. Disconnect the Positive battery clip from the Positive battery terminal.

4. Remove the inverter and battery clip cables from the area.

5. Store that inverter and cables for future use.

 

Permanent Inverter Installation

What if you need a more permanent connection of an inverter that supplies more than 200 Watts? There are a few additional requirements for permanent inverter installation.

1. Inverter locations must be dry, cool and dust free. Further, any location must not be in an area that does not have any flammable gas, or battery fumes, or fuel.

2. The inverter On/OFF switch and AC outlets must be accessible.

3. If possible, the inverter should be secured to a stable surface that has several inches clearance for air flow around the inverter.

4. Wiring from inverter to battery must be proper gauge based on maximum power the inverter is to deliver and the lengths of the battery cables.

5. Wiring must be stranded and insulated.

6. A “battery Protection” safety fuse must be installed within one foot of the positive battery terminal. This is a safety requirement and Insurance requirement. The fuse must be a sealed fuse that will not produce a spark if it blows. For small inverters automotive fuses and fuse holders are usually sufficient. Sealed fuses to 50 amps are readily available as MAXI fuses, for example. Battery Protection fuses protect the battery if there is a cable short.

7. If the inverter is being installed in a small vessel, the Battery Protection fuse must be located within 8 inches of the Positive battery terminal.

 

Fuse Selection

Power inverters have an internal fuse or fuses. These protect the inverter from some overloads. The battery protection fuse protects the battery from exploding if there is a cable short. Basically, if an inverter is rated at 400 watts, then minimally, a 40 Amp fuse is sufficient. A quick rule to follow is to divide the AC output watts by 10 and that is the approximate size of battery protection fuse to use. If an inverter is rated at 350 AC watts, a 35 Amp battery protection fuse is required.

Mounting the fuse in an engine compartment requires that the fuse holder and fuse wiring be secured to non-moving parts of the vehicle. This ensures that wires do not become entangled in moving parts of the engine such as the fan, pulleys and belts. Use of plastic wire ties is often used to secure wires and fuse holders to chassis parts. Most automotive aftermarket parts stores carry fuses, wire and fuse holders.

Wire Routing for Battery to Inverter

Wire gauges for small inverter installations are based on the length of the wire, and the maximum amps the wire has to carry. There are two ways to wire to a battery. The first way is to have both positive and negative wires from the inverter be routed all the way to the battery. This method is used aboard vessels that may not have a continuous metal frame that is connected to the Negative terminal of the battery. Often these wires are heavier, so excessive voltage losses do not occur. The most common way to connect the inverter to the battery is used in vehicles that have frames that are electrically connected to the Negative terminal of the battery. This makes any part of the vehicle frame a negative terminal for the battery.

This technique is the way most vehicles in the US are wired. That is, all circuits for starting, lighting, radio and other entertainment appliances all have their negative power terminals connected to the frame. This makes things easy. Only one wire has to be routed all the way to the battery. A short wire from the inverter gets connected to the frame for a secure connection. Another consideration is the type of wire insulation. If the wires are routed near components that get hot like the exhaust system or engine, make sure the insulation is rated for high temperature.

Wire Gauge and Lengths

Determining the correct gauge of wire for powering inverters is a result of resolving the following factors that affect voltage drop in the length of wire under load.
A. Maximum Amps
B. Length of wire
C. Maximum voltage loss in percent (usually not more than 5 percent if the batteries are not simultaneously charging).
Most installations can be up to 15 percent cable loss if the batteries are also being charged. For the inverters described above, the largest capacity inverter is 400 Watts. So let’s calculate the wiring two ways. The first way uses one wire that goes all the way to the Positive battery terminal. The maximum distance for a typical vehicle is 20 feet, including extra wire to route around structures in the vehicle.

Positive Wire from Inverter to Battery

For this discussion, assuming a 400 watt inverter installation, the gauge of wire can be AWG 6. This gauge will be fine for most installations, if the inverter is not continuously outputting 400 watts. Notes: AWG means American Wire Gauge. The lower the AWG number, the heavier the wire.

Positive and Negative Wires from Inverter to Battery

Assuming the same 400 watt inverter installation, the gauge of wire can be AWG 4. This gauge will be fine for most installations, if the inverter is not continuously outputting 400 watts. Why the difference in gauge of wire? This is because the distance is not 20 feet, it is 40 feet from inverter to battery for the Positive wire added to 20 feet to the battery for the Negative wire. The same fuse installation is used to protect the battery as above. So, wherever possible, if the batteries Negative terminal is connected to the vehicle chassis, use the vehicle chassis as the negative wire back to the battery. This saves on wiring and reduces the need for a heavier and more expensive wire gauge.

Inverter Grounding Wire

Some small power inverters with metal enclosures have an extra terminal for a grounding wire. A grounding wire connects the enclosure to a vehicle chassis or to the batteries Negative terminal at the battery itself. Grounding serves two purposes:

1. To suppress inverter noise.

2. To provide an additional negative conductor path to ensure the fuse blows if there is a positive cable short to the chassis.

 

General Instructions for Permanent Installation of Inverters

1. Secure the inverter so it does not change position while the vehicle or vessel is in motion.

2. Connect the Grounding terminal to the vehicle chassis using a short insulated wire. In most cases insulated, stranded, 12 gauge (AWG 12) can be used.

3. Make sure that AC appliances are not plugged into the inverter and the inverter’s power switch is in the OFF position.

4. Make the short (12 inches or less) wire connection to the battery side of the fuse.

5. Secure the fuse holder to a non-moving part of the chassis.

6. Connect the long wire to the inverter side of the fuse but do not connect the far end to the inverter. Cover the far end of the positive wire so that it does not contact any metal.

7. Connect the Positive wire to the fuse.

8. Connect the battery side of the wire to the Positive battery terminal.

9. Make all Negative connections at the inverter, chassis or battery terminal. NOTE: Expect a spark when initial connection of Positive inverter terminal. This is normal.

10. Make the final Positive wire connection to the inverter’s positive terminal.

11. Turn ON the inverter and note that the green LED indicator is lit.

12. Use the inverter.

 

Background

In the world of small power inverters there are those that range from 50 watts of AC output to 400 watts. Most are available as Modified Sine Wave and some as Pure Sine Wave. Some small power inverters are equipped with DC power cords with plugs that can be plugged into a 12 volt vehicle outlet. Some have a cord set that have battery clips identified as Positive (Red color) and Negative (Black color). Some small inverters have two cords supplied; one with a plug and one with battery clips.

12 Volt Outlets

Vehicle outlets are called accessory outlets (cigarette lighter sockets, Lighter receptacle). Most 12 volt outlets in vehicles are protected by a 15 amp fuse. The fuse protects the vehicle wiring so an overload on the outlet blows the fuse, thus preventing that circuit from damage. Most vehicle outlets are controlled through contacts in the ignition switch assembly. Generally, the ignition switch also has an Accessory position that enables power to the outlet even though the engine is not operating. A 15 amp fuse protects the outlet wiring and the contacts in the ignition switch. Some vehicles have accessory outlets that can supply up to 25 Amps. Consult the vehicle owner’s manual for details.

Variations

This discussion provided general considerations and examples about installing smaller power inverters. Of course, if an installation requires lower battery to inverter wire losses, heavier gauge wiring should be used. This is especially true if battery operation must be extended and batteries are not being simultaneously charged. If additional specific installation details are needed – ones not covered by the Users Manuals provided with your inverter, contact:

Wagan Tech Customer Support

510-471-9221 or 800-231-5806

 

See the Power Inverters mentioned in this article:

• Pure Sine Inverters

• Modified Sine Inverters

• Smart AC Inverters (Portable)

• SlimLine Inverters

• ProLine Inverters (Industrial)

 

Post by Justin G. & Jack C.

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