How Long To Charge A Car Battery At 40 Amps?

Updated October, 2020
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Charging a car battery at 40 amps shouldn't be done for longer than 2 hours. Fast charging car batteries can be dangerous and should be done in short bursts. Failure to do so could result in explosions.

What is an AMP hour anyway?

Amp hours are how many amps an hour a charger can put out. If a battery has 36 amp hours it can output 1 amp for 36 hours, 3 amps for 12 hours, 6 amps for 6 hours, so on and so forth. A 10 amp charger puts out 10 amps an hour to the battery.

How to connect the charger to the battery

Before you start charging make sure you're in a well ventilated area and your battery and charger are compatible. Check your owners manual for more information.

Step 1: Identify the positive and negative terminals. The positive terminal is either red or has a plus sign near it.

Step 2: Disconnect the battery cables and remove the battery. Always disconnect the negative, or ground, first.

Step 3: Clean your battery terminals. Make sure you remove any dirt or corrosion to ensure a proper charge.

Step 4: If required, fill the cells with water. If you have a maintenance free battery you can for go this step.

Step 5: Set the charger to the proper voltage. The proper voltage should be on the battery or in the owners manual.

Step 6: Attach the cables. Attach the positive cable first and then the negative.

Step 7: Plug in the charger. Make sure you're using a grounded outlet without an adapter.

Determining charge duration

If you know how many amp hours your battery has you can calculate how long it's going to take to charge. To calculate that you can use a simple formula. Time= Amp hour/Battery Amps. For these examples we are using a 48 amp battery.

Using 2 amps

Using a 2 amp charger on a 48 amp battery takes 24 hours to charge from empty. The charger puts out 2 amps an hour and has to work for 24 hours to reach 48 amps.

Using 4 amps

Using a 4 amp charger on a 48 amp battery takes 12 hours to charge from empty. The charger puts out 4 amps an hour and has to work for 12 hours to reach 48 amps.

Using 10 amps

Using a 10 amp charger on a 48 amp battery takes take 4.8 hours to charge from empty. The charger puts out 10 amps an hour and has to work for 4.8 hours to reach 48 amps.

Using 12 amps

Using a 12 amp charger on a 48 amp battery takes 4 hours to charge from empty. The charger puts out 12 amps an hour and has to work for 4 hours to reach 48 amps.

Using 40 amps

Using a 40 amp charger on a 48 amp battery takes 1.2 hours to charge from empty. The charger puts out 40 amps an hour and has to work for 1.2 hours to reach 48 amps

Charging at 40 amps is fast, but stay safe!

Charging a car at above 10 amps is considered fast charging. When fast charging make sure to set a timer for when to stop charging. Otherwise your battery may be overcharged and could explode or develop a fault.

What if my battery won't charge?

If your battery isn't charging there could be a few things wrong. It could be as complicated as a chemical issue in the battery or as simple as a faulty terminal. Here are some common issues:

  • The battery is too old and has gone bad
  • Cracks or faults in the casing
  • Corrosion on the terminals

When your battery isn't charging it can be demoralizing. However before you dispose of the battery, try and troubleshoot it yourself. Some battery issues can be solved at home. Troubleshooting yourself can save you money and time.

How to troubleshoot a battery issue and replace

To do some simple troubleshooting on your car battery you need a few tools. A voltmeter, cleaning solution, and a properly sized wrench are the tools you need. The whole troubleshooting process should take about 10 minutes. If troubleshooting doesn't resolve the issue then your battery likely needs to be replaced.

Basic car battery troubleshooting

  1. While charging attach the voltmeter to the battery to make sure the battery can even take a charge.
  1. If the battery isn't taking the charge look for corrosion. This white powder can prevent charging. If there is corrosion scrub it off with a cleaning solution and a toothbrush.
  1. If there is no corrosion, check the connections. If the connections are loose, use the wrench to tighten them.
  1. If none of these restore a charge then it's time to replace your battery.

Replacing the battery

  1. Disconnect the old battery.
  1. Safely remove the old battery and put the new battery in its place.
  1. Connect the new battery to the leads properly and securely.
  1. Properly dispose of the old battery.

When the time arises, is my insurance going to be enough?

When it comes to car insurance, there are several factors to consider beyond monthly cost. A policy that covers you when you need it, has a low deductible, and offers several discounts can be hard to find - which is why we've done the work for you.

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Call an insurance expert
Our team will help you find the lowest insurance prices.
866-996-3156