Determining the correct amp alternator for your vehicle is crucial to keeping your electrical system running properly. The alternator powers all of your vehicle’s electronics and charges the battery while the engine is running. An undersized alternator will be unable to meet the vehicle’s electrical demands, resulting in a drained battery and potential damage to electronic components. A properly sized alternator ensures your battery stays charged and all electrical components function as designed. This article will provide a step-by-step guide on how to determine the proper alternator amperage for your specific vehicle.
What does an alternator do?
The alternator is a crucial component that is driven by a belt connected to the engine. It generates electricity to power the vehicle’s electrical system while the engine is running. Here’s a quick overview of how an alternator works:
- Magnets inside the alternator rotate around copper wire windings, generating alternating current (AC) electricity.
- Diodes inside the alternator convert the AC electricity into direct current (DC) for use in the vehicle.
- The voltage regulator ensures a constant voltage output of around 14 volts.
- The 14 volt DC current powers the vehicle’s electronics and recharges the 12 volt lead-acid battery.
Without a properly working alternator, the vehicle’s battery would quickly drain as it tries to power all of the electronics like lights, ignition system, radio, etc. The battery is only designed to provide power when starting the engine and needs to be recharged while driving.
How to determine your alternator amperage needs
Figuring out the right alternator amperage rating for your specific vehicle involves a few simple steps:
1. Check the factory alternator amp rating
The best place to start is by finding the OEM alternator’s amperage on your vehicle. This rating will be listed in your owner’s manual or can be obtained from an auto parts store by providing your VIN.
Most modern passenger vehicles come equipped with alternators in the 90-150 amp range. Here are some common OEM alternator ratings:
- Compact sedans 70-90 amps
- Midsize sedans 100-120 amps
- Full-size sedans 130-150 amps
- Compact SUVs 100-120 amps
- Full-size SUVs & trucks 130-160 amps
Use your factory alternator amperage as a baseline when determining an appropriate replacement.
2. Consider installed electronics and accessories
The more electronics and powered accessories your vehicle has, the higher the electrical load. Some common electrical add-ons include:
- Aftermarket sound system
- Off-road lights
- Inverter
- Power windows, locks, seats
- Heated seats
- Refrigerator/freezer
You’ll want to choose an alternator that can comfortably handle the total electrical load in your vehicle, especially if you’ve added electronics since it was stock.
3. Determine if you need to upgrade
For completely stock vehicles without added electronics, sticking with the factory alternator amperage rating is usually sufficient. But if you’ve added electrical accessories or notice your headlights dimming at idle, that’s a sign that your alternator may be underpowered for your needs.
Some situations where you may want to upgrade your alternator amp capacity:
- You’ve installed a high-end car audio system
- You frequently use power-hungry accessories like an inverter or fridge
- You need auxiliary power for equipment on a work truck
- Your headlights/electronics dim at idle indicating low voltage
- You want to prepare for future added electronics
As a general guideline, add 20-40 amps to your factory rating if you’ve added more electronics to account for the increased electrical load.
4. Choose an appropriate replacement
Once you’ve determined the target amperage based on your vehicle’s needs, you can start shopping for a suitable replacement alternator. Keep these tips in mind:
- Stick with a quality brand – Denso, Bosch, AC Delco, etc.
- Make sure to get the correct plug and mounting that matches your vehicle
- Shop based on amperage rating, not simply “highest output”
- Higher amp alternators require upgraded wiring and fusing
Installing an incorrectly rated or poor quality alternator can cause issues like:
- Insufficient power generation
- Rapidly draining battery
- Dimming headlights
- Damaging electrical components
- Blown fuses
Take the time to properly determine the ideal amperage rating and purchase a quality unit sized specifically for your vehicle. This will provide reliable performance and prevent electrical issues down the road.
How do I determine my vehicle’s electrical load?
Figuring out your vehicle’s total electrical draw is useful in determining whether your alternator can handle the current setup or if you need to upgrade the amperage capacity. Here is the process for calculating your vehicle’s electrical load:
1. List all electrical components
Make a list of all devices and components in your vehicle that draw electricity. This includes lights, ignition system, safety systems, stereo, accessories, and any added electronics. Some major electrical loads:
- Headlights – 100+ watts each
- Tail lights – 15 watts each
- Dome light – 8-16 watts
- Electric cooling fan – up to 4,000 watts
- Electric fuel pump – 50-100 watts
- Electric power steering – up to 1,000 watts
- Audio system amplifier – 200-600 watts
- Electric seats/windows – up to 100 watts each
- Electric winch – 10,000+ watts
Don’t forget to include any added aftermarket electronics.
2. Determine wattage draw for each component
Reference owner’s manuals or product specs to find the maximum wattage (amps x volts) for each electrical component. For simpler components like lights and motors, you can use general estimates based on typical wattage ratings.
3. Add up total watts
With the wattage figures for each component, add up all electrical loads to get your vehicle’s estimated total wattage/amperage.
4. Factor in battery recharging
In addition to powering the vehicle’s electrical components, the alternator needs capacity to recharge the battery. To account for recharging, add an extra 25% to your total watts/amp draw calculation.
5. Convert watts to amps
Vehicle electrical systems run at 12 volts, so divide your total watts by 12 to determine the amperage capacity required.
This full calculation will provide an accurate estimate of your vehicle’s electrical load and whether your current alternator can handle it or if you need an upgrade. Be sure to perform testing under realistic driving conditions and worst-case scenarios to see if your electrical system can keep up with demand.
How to test your alternator
Once your new alternator is installed, you’ll want to perform testing to make sure it is functioning properly and meeting your vehicle’s electrical needs. Here are a few simple methods of testing alternator output:
Voltmeter testing
Using a handheld voltmeter, you can measure voltage at the battery terminals and alternator itself:
- Battery voltage engine off – Should measure 12.4-12.6 volts
- Battery voltage engine running – Should measure 13.5-14.5 volts
- Alternator output terminals – Should measure battery voltage or higher (13.5-14.5 volts)
Lower readings can indicate a faulty alternator not properly charging the system.
Electrical load test
Turn on all major electrical accessories such as headlights, blower motor fan, stereo, heated seats, etc. Rev the engine to around 2,000 RPM for several minutes. Turn off or disconnect any components that shut off indicating insufficient electrical power. If the alternator maintains output without any shutoffs, it is producing sufficient amperage.
No-load vs under-load testing
Testing alternator amperage output with electrical components turned off will show the no-load maximum output. Testing again with components on simulates an under-load condition, which is a more realistic indicator of the alternator’s charging capability. Make sure to test both ways.
Voltage drop test
Measuring voltage at the alternator terminal and again at the battery terminal checks for a drop in voltage along the electrical path indicating insufficient alternator output or undersized wiring that can’t deliver full output. With a properly working high-output alternator, you should have less than 0.5 volt drop along the charging path.
Use these simple methods above to confirm your new alternator installation provides adequate charging performance to match your vehicle’s electrical loads and keep its battery charged.
Signs of a failing alternator
Some key warning signs that indicate an aging or malfunctioning alternator may be failing:
- Dimming headlights – As alternator output decreases, headlights will be noticeably dimmer or fluctuate in brightness as it struggles to maintain voltage.
- Electrical component shutoff – Radios/electronics may turn off or malfunction as voltage drops.
- Battery not charging – Voltmeter testing showing the battery is not being recharged up to 14+ volts while driving signals a failed alternator.
- Fault lights – The battery or charging system warning lights coming on indicates a detected problem recharging the battery.
- Swollen battery case – A bulge in the battery case can signal it has been overcharged due to a malfunctioning voltage regulator.
Troubleshooting should begin with checking for loose wiring connections, frayed belts, and corrosion that could affect alternator performance before assuming complete component failure. But if testing confirms your alternator is not properly charging, it will need to be replaced.
FAQs
Here are answers to some frequently asked questions about determining alternator amperage:
Is a higher output alternator better?
Not necessarily. Oversizing your alternator substantially beyond your vehicle’s electrical demands offers little benefit. The key is matching the alternator amp capacity to the existing electrical load to ensure proper system performance.
What gauge wiring for high amp alternator?
Upgrading your charging wires allows your alternator to deliver full rated output:
- 70-90 amp alternator – 10 gauge wire minimum
- 100-120 amp alternator – 8 gauge wire minimum
- 130-150 amp alternator – 6 gauge wire minimum
- 160+ amp alternator – 4 gauge wire minimum
Can I put a larger alternator on my vehicle?
Yes, you can install a larger amperage alternator as an upgrade, but you need to ensure it matches your vehicle for proper mounting, pulley alignment, and electrical connections. Larger gauge wiring upgrades may be needed to handle increased output.
What happens if alternator output is too low?
An underpowered alternator will be unable to meet your vehicle’s electrical demands. This will cause issues like dimming lights, dying battery, and potential damage to electronics from low voltage.
What happens if alternator output is too high?
An oversized alternator can overcharge your vehicle’s 12V system leading to premature battery failure, damaged electronics, and excessive heat buildup. Make sure to size your alternator appropriately.
Conclusion
Choosing the optimal alternator amperage comes down to matching output to your vehicle’s electrical loads while allowing capacity for battery charging. Follow these guidelines:
- Check your factory alternator rating as a starting point
- Determine amp draw of all electrical components
- Account for any added electronics and accessories
- Increase amperage if your system is underpowered
- Test thoroughly to confirm sufficient output
With a properly rated high-quality alternator matched to your vehicle, you’ll maintain reliable electrical operation and prevent issues caused by an underpowered or overworked charging system.
