Trying to start your car only to hear the dreaded click-click-click of a dead battery is frustrating for any driver. A weak or dead car battery prevents your engine from turning over and starting up. While there could be several culprits, one key factor is whether your battery can deliver enough amps to the starter motor to get the engine cranking. This raises a common question: is 50 amps enough current to start a typical car battery?
What amps are needed to start a car?
The short answer is that 50 amps is generally sufficient to start most car engines. However, the exact amperage needed depends on several factors:
– Battery size – Larger batteries with higher reserve capacity require more amps for starting. Smaller batteries need fewer amps.
– Engine size – Large displacement engines require more amperage to start compared to smaller 4-cylinder engines. This is especially true for diesel engines.
– Temperature – Cranking amp requirements increase in cold weather. Very hot temperatures can also increase the amps needed for starting.
– Battery age and condition – Older, weaker batteries will not deliver their full rated amp capacity, requiring more amps to start the engine.
As a general guideline, a typical car with a 4- or 6-cylinder gas engine requires 100-150 cranking amps on average to start. Most car batteries are rated to deliver 275-600 cold cranking amps when new. So even a battery on the lower end of this range, putting out just 275 peak amps, will have no problem starting your average passenger car.
Larger V8 or diesel engines need upwards of 200-350 amps for starting in cold weather. Big diesel trucks can require 500 amps or more. Very large stationary diesel engines may need 1000 amps or more for cold starting. But for most personal vehicles, a 50 amp battery charger or automotive jump starter pack will provide sufficient current to start the engine if the battery is not totally dead.
How car starting systems work
To understand how many amps it takes to start your engine, you first need to understand what is happening when you turn the key to start your car:
The starter motor
Turning the ignition key activates the starter motor, which is essentially a high-current electric motor. It is powered by the car battery to provide the torque needed to get the engine spinning.
Starter motors have very thick copper wiring to handle the heavy electrical load. The large gauge wires and robust commutator brushes are designed to conduct hundreds of amps from the battery to the starter motor without overheating or causing excessive voltage drop.
Battery supplies surge of power
When you engage the starter, the car battery experiences a huge surge in electrical demand, from virtually no load to hundreds of amps instantaneously. The battery acts like a reserve tank, containing enough stored chemical energy in its lead plates and sulfuric acid electrolyte to meet this massive power demand for a few seconds.
During this surge, the available current is limited only by the battery’s internal resistance and short-term discharge capacity. This reserve power allows it to supply the starter with over 100 amps or more, even if it was just sitting idle beforehand.
Starting process drains battery quickly
Cranking the engine acts like a huge short circuit across the battery terminals. This causes the voltage available to the starter motor to temporarily dip. The battery’s state of charge and condition determines how severely its voltage sags under this heavy load.
A strong, fully charged battery will experience only a slight drop in voltage when delivering maximum current. But a weak or discharged battery can experience a larger voltage drop under starting loads. This reduces the power available to the starter motor.
This burst of current also drains the battery very rapidly. So the engine needs to start and begin generating charging current within a few seconds. Otherwise, the battery will be depleted by the high-current draw of the starter motor. Multiple failed start attempts can leave the battery too drained to restart without a recharge or jump start.
Factors that determine cranking amp requirements
So while a 50 amp battery charger or jump starter may be enough for starting some vehicles, larger engines will need more current. What factors determine exactly how many amps your particular vehicle needs for starting?
Engine size
The most significant variable is the size and number of cylinders in your car’s engine. More cylinders mean higher compression and more friction to overcome when cranking. Higher displacement engines require more torque to start spinning. Some key differences:
– 4-cylinder economy car engines need 75-150 cranking amps on average.
– 6-cylinder family sedans and SUVs need around 150-250 cranking amps.
– Large V8 pickup truck and performance car engines can require 250-350+ cranking amps.
– Big diesel engines in heavy trucks, buses and agricultural equipment often need 400+ cranking amps for cold starts.
So clearly, the more cylinders and larger the displacement, the higher the current draw during starting.
Battery capacity
The battery’s reserve capacity and state of charge also impacts the amperage needed. A weak or discharged battery will not supply its full rated cranking amp capacity. Trying to start an engine with a depleted battery requires significantly more amps than starting the same vehicle with a fully charged battery.
Temperature
Cranking amp requirements surge in cold weather. Thick, cold oil makes the engine harder to turn over. Cold temperatures thicken battery electrolyte too, increasing internal resistance. This reduces current output. So turn over speed and torque drops, necessitating more cranking amps to compensate in very cold conditions.
Condition of starting system
Starter motors wear out over time, becoming less efficient. Increased friction in the starter itself requires more amps to achieve the same cranking speed. Resistance caused by corroded contacts also reduces voltage available to the starter, increasing the current needed to turn the engine.
Even a partially worn starter motor that is still functioning can draw significantly more amperage to start your engine compared to a new starter.
Battery age and condition
A worn out or old battery has lost capacity and increased internal resistance due to shedding of active material from its lead plates. An old battery will show a higher voltage drop under load compared to a new battery, even if they have the same cranking amp rating. The increased voltage sag lowers power output, so more current is needed to achieve the same cranking torque.
Sulfation, acid stratification and internal corrosion in very old batteries also reduces deliverable current by increasing resistance. So an aging battery needs more amps for starting, even if it was properly maintained. Very old batteries may need 2-3 times more amperage to start compared to a new battery.
Using a jump starter or battery charger
Based on the factors above, a 50 amp battery charger, battery maintainer, or portable jump starter may or may not be enough to start your vehicle. Here are some guidelines for sizing jump starters:
50 amp jump starters
A 50 amp portable jump starter pack is generally sufficient for 4-cylinder engines up to 2.5L in most conditions. It can provide enough power to start moderately sized 6-cylinder engines up to 4L capacity in warm weather. But a 50 amp unit is marginal for sustained cold weather starting of larger engines.
100 amp jump starters
For 6-cylinder family vehicles and small V8 trucks, a 100 amp battery jump starter provides enough power for most situations. Even in cold weather, a 100 amp jump pack has sufficient output for starting average V8 engines up to 6L displacement.
150+ amp heavy duty jump starters
For large truck V8 and diesel engines, a heavy duty jump starter rated for 150 amps or more is recommended. Units putting out 200-300+ amps are ideal for heavy equipment, agricultural and commercial vehicles to ensure reliable starting power even in extremely cold conditions.
Using battery chargers and maintainers
A battery charger or maintainer lacks the high instant power of a jump starter pack. But a 50 amp smart battery charger can “jump start” smaller 4-cylinder and 6-cylinder engines if the battery has enough charge to at least crank slowly. Fully recharging the battery first maximizes your chances of success.
Chargers over 75 amps may successfully jump start larger V8s with some residual charge in the battery. But you run the risk of overheating the charger since cranking puts a very heavy load on these units not designed for instant high-power output. Using battery chargers as makeshift portable jump starters is not recommended.
Other factors affecting car starting power needs
Beyond just engine size, several other factors impact how many amps your car requires for starting the engine:
Compression ratio
Higher compression puts more resistance on the starter motor when cranking. Modern gasoline engines run fairly high compression ratios from 9:1 up to 11:1 or 12:1. High performance engines may be 13:1 or higher. The higher the compression ratio, the more cranking amps required to overcome cylinder pressure.
Displacement and cylinder layout
Not only the total displacement, but how it is divided among the cylinders affects cranking requirements. An inline 6-cylinder engine with 250 cubic inches of displacement takes less current to start than a V8 with the same 250 cubic inch displacement. This is because the V8 has more cylinders generating friction and compression resistance.
Gasoline vs. diesel engines
Diesel compression ratios run 16:1 or higher. The greater cylinder pressure makes diesel engines require more cranking power vs. comparable gas engines. Their sturdier rotating components also increase starting current needs. Expect diesels to require 1.5 to 2 times the cranking amps compared to a gas engine of the same displacement.
Engine condition
Worn piston rings, valve guides, cylinder bores and bearings increase friction and compression leakage, making the engine harder to turn over. Poor compression from these worn components will increase the cranking amperage needed to start the engine.
Oil viscosity
Thicker oil causes more drag in the bearings and cylinder walls. Multigrade oils thicken significantly at low temperatures. Even 5W-20 oil can turn viscous enough in extreme cold to make an engine harder to crank, upping the starting current requirement.
RPMs needed for starting
The cranking speed needed to start varies by engine design. Smaller displacement high performance engines often need higher RPM from the starter to generate enough compression for ignition. The additional cranking speed requires more battery amps compared to a truck engine that can start at a slower cranking rate.
How to measure actual starting current draw
Wondering exactly how many amps your particular vehicle needs for starting? You can measure the current draw directly using a simple clamp meter:
Use a DC clamp ammeter
– Place the jaws of the meter over one of the starter cables or battery terminals.
– Crank the engine and observe the maximum current reading on the meter display.
– Repeat 2-3 times and average the results.
Test in cold weather
Perform your test in cold conditions, ideally below 40°F, when amp draw will be highest. Testing in warm weather may underestimate the actual current required in winter.
Check high current capacity
Make sure your clamp meter can handle at least 400-600 amps to avoid over-ranging the sensor. Units designed for measuring automotive and heavy duty electrical systems are best for this test.
Fully charge battery first
Always start with a fully charged battery to get the most accurate current draw baseline. Testing with a discharged battery underestimates the true cold cranking amp requirement.
Average several results
Crank the engine 2-3 times in succession when testing. Average the peak current numbers, since the first 1-2 crank cycles may draw slightly more amps than subsequent cycles due to initial oil viscosity.
Tips for maximizing starting power
If your vehicle is having trouble starting or needs an excessive number of amps, here are some tips for optimizing your starting system:
Check battery and cables
Make sure battery posts and cable connections are clean and tight. Loose connections cause voltage drop and reduced power to the starter.
Load test battery
Load test the battery using a carbon pile load tester or digital battery analyzer. This checks the battery’s actual condition under a heavy electrical load. Replace if weak or heavily sulfated.
Clean battery terminals
Over time corrosion on the battery terminals inhibits current flow. Remove terminals and clean with a wire brush or baking soda solution.
Check starter motor
If your engine cranks slowly and needs excessive amps to start, have your starter tested. A weak or high resistance starter will draw more current.
Use a lower viscosity oil
Switch to lighter viscosity oil like 5W-20 or 0W-20 to reduce drag in cold temperatures. flows easier when cold.
Install a battery warmer
A battery warmer pad attached below the battery keeps the electrolyte warm for better cold weather starting.
Upgrade to a higher capacity battery
If your battery is more than 3 years old, upgrading to a new battery with higher cold cranking amps provides more starting power.
Key takeaways
– For most passenger vehicles, 50-100 amps is sufficient to start the engine under normal conditions. Larger engines and diesels require more starting current.
– Several factors affect how many amps are needed – engine size, compression ratio, battery condition, temperature, etc. Measure current draw directly for the most accurate amperage requirement.
– Upgrade to a higher capacity battery or install a battery warmer for better cold weather starting performance. A heavy duty jump starter may be needed to start large diesel engines and equipment.
Conclusion
While 50 amps is enough to start many average-sized car engines under normal conditions, cold weather, weak batteries, and large displacement V8 or diesel motors often require 100 amps or more for reliable starting. Measuring your engine’s actual current draw using an ammeter provides the definitive answer to how many amps your car needs. With the proper battery capacity, cables, and maintained starting system components, most vehicles will start reliably with just 50-100 cold cranking amps available. But for cold weather starting or marginal components, the higher current of a heavy duty jump starter may be required to get your engine spinning and running properly.
