How Long Does it Take to Charge a Car Battery with a 10 Amp Charger? The Ultimate Guide

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How Long Does it Take to Charge a Car Battery with a 10 Amp Charger

How long does it take to charge a car battery with a 10 amp charger? We explain all!

Charging a car battery with a 10 amp charger will take between 3-10 hours.

But as with most questions of this nature, a better and more accurate answer is ‘it depends’. It depends on the size (capacity) of your battery. And it also depends on the depth of discharge when you begin charging. And the efficiency of the charger you use is also important.

There are formulas that we can use to work out the charge time to charge a battery using a 10 amp charger.

In this article, we will look at how to work out your charge time (formula). This will make it a simple task for you to get an ETA on that battery charge.

We will also show out how to work out the amp-hours of your battery. Because you need this to work out the charge time.

The Formula

As we said, you need the formula to calculate your estimated time.

And here it is…

Ah÷0.8÷10

That’s it. Simple.

So, let us explain what those figures are. It is the amp-hours (Ah) divided by the charger’s efficiency (0.8 or 80%) divided by 10 amps – the amperage of our charger.

And this is how the principle works

Amp-Hours

The Ampere hour (or amp-hour) is a measurement of battery capacity.[1] It measures the amount of time that a battery will stay powered whilst powering an appliance. The way to calculate battery life is amp-hours divided by the amps an appliance is drawing.[2] So, a battery with a capacity of 10 amp-hours will power an appliance drawing a steady 10 amps for one hour.

  • Battery Run Time = Battery Capacity (Ah) / Current (A)
  • Battery Run Time = 100Ah / 100A
  • Battery Run Time = One Hour

Or, to put it another way, a 100Ah battery powering a 100A TV will do so for one hour.

And, using the same principle in reverse, a 10 amp charger will put an average of 10 amp-hours per hour into a battery.

Inefficiencies in charging

The next section of our equation is the 0.8. We include this because the 10 amps per hour going into the battery is an ideal world figure.

Unfortunately for us, we live in the real world and battery charging is an imperfect process.

Most battery chargers work at an average of around 80% efficiency over the course of the charge. So, a 10 amp charger will typically deliver an average of 80% of its 10 amps each hour.

This is because of the three stages of charging. These are bulk, absorption and float.[3] During the bulk charge, charging goes like the clappers. It will reach 10 amp hours per hour. This accounts for around the first 80% of the process. The battery’s ability to receive current then declines as voltage and state of charge rise throughout the last phases of charging.[4]

This means that the charger can no longer force current into the battery at the same rate. So charging slows down to as low as 6 amp-hours per hour. This reduces the overall average current passed into the battery per hour.

There are other things that can adversely affect charging times. These include imperfect battery chemistry, spikes in current and ambient temperature.[5]

So the 10 amp charger should replenish 10 amp Hours per hour, but it will be between 6-10 amp hours per hour. So around 8 amp hours are replenished per hour on average.

Working out the Amp-Hours of the Battery

The other crucial part of the equation on which the rest depends is the amp-hours of the battery. Now if there is a handy sticker on the battery revealing its amp hours that is all well and good. But this is not always the case.

Luckily, if you are in possession of any of the battery’s other vitals, we can use those.

Cold Cranking Amps (CCA)

The first of these is the CCA or Cold Cranking Amps. The CCA is a rating that reveals the capability of the battery to crank the engine on cold mornings. It is the maximum amps a battery can sustain for 30 seconds at 0 degrees Fahrenheit, before the battery voltage becomes useless.

CCA doesn’t convert into amp-hours easily, but here is an approximate way to do it. Divide the CCA by 7.25 to get an approximate measurement of the Ah of the battery.

So, for instance, a battery of 500 CCAs would have approximately 69 amp-hours.

Reserve Capacity (RC)

If you can only get the Reserve Capacity rating of your battery, you can use this to work out the amp-hours as well. The formula is to multiply your RC number by 0.6. So a battery with 123 minutes of reserve capacity will have around 74Ah.[6]

How dead is your Battery

The final piece of the puzzle is to work out how discharged your battery is. We need to calculate the state of charge (SOC). SOC is an expression that describes the present capacity of the battery as a percentage. 100% is a battery with a full charge. 0% is total discharge.

To do so, we need to use a multimeter, voltmeter or wattmeter to measure the battery’s state of charge. Put the positive lead to the positive terminal and the negative to the negative. There should be no draw on the battery while you do this.

Take your reading and use a conversion table to calculate the % charge in your battery. This is known as a state of charge table, like the one below.

So, we can see from the table that a lead acid battery with a 12.1 Volt charge is in fact charged at 50%. So, by using this percentage against the total charge time, we can calculate the charge time for a boost thus:

If we have calculated that a full charge will take 200 minutes, we can calculate that to charge a 50% charged battery will take 100 minutes.

So, 200 x 0.5 = 100, and so on.

SOC 12V Deep Cycle Battery Table

Final Calculation

So, let’s tie all of this together with an example.

Take the calculate the percentage of Ah that needs to be recharged, divide it by 0.8 (the charging efficiency), and then divide it by the number of amps of the charger.

So let’s take our battery with 123 minutes of reserve capacity and multiply that by 0.6 to ascertain the amp-hours

  • RC (minutes) x 0.6 = Ah
  • 123 x 0.6 = 73.8
  • Amp hours = 74 Ah

Great! So now we know the amp-hours.

So we need to test the battery. We test it with a voltmeter and let’s say it comes in at 12.6 Volts. According to our chart, that is 50% charged.

So we can calculate the mp-hours we need using our 74Ah. We take our percentage and divide by 100. 50% ÷ 100 = 0.5. Then we multiply the amp-hours by 0.5 for 50%.

  • Percentage 50% ÷ 100 = 0.5

Then multiply your total amp-hours by that figure.

  • 74Ah x 0.5 = 37 Ah

We now know the amp-hours we need.

So we use our main formula.

  • Time to Charge (h) = Ah÷0.8÷10A
  • Time to Charge (h) = 74Ah÷0.8÷10A
  • Time to Charge (h) = 9.25

So this tells us that our battery will take 9.25 hours to charge to capacity with our 10 amp charger.

What if your charger is not 10 Amps

If your charger is a different rating to 10 amps, remember to tweak the formula to replace the ’10’ with the amperage rating of your charger. Then the formula will calculate the estimated charge time for that battery charger. So, for a 2 amp charger, the formula will be Ah÷0.8÷2A.

Other things to Remember

One of the main things to remember when you charge your battery is that the charger should not be more than 10% of the battery capacity.

So, for instance, a 10 amp charger should not be deployed on a battery that is under 100Ah in total.

This is because using a charger that is too powerful for your battery can damage it. The battery won’t be able to convert the current quickly enough. So it would “charge” very quickly but lose electrolyte due to heat loss.

Conclusion – How Long Does it Take to Charge a Car Battery with a 10 Amp Charge

So there you have it. The full rundown on ‘how long does it take to charge a car battery with a 10 amp charger‘. Remember that you can use the formula revealed here for whatever battery you have, whether it is a golf cart, lawnmower, motorcycle or truck battery.

And you can also customize the formula to meet your needs. So that you can accurately calculate the time that it will take to charge your battery using ANY charger.

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Steve Brown

AUTHOR

Steve is a gadget enthusiast who's always been intrigued by batteries. The founder and editor of Battery Chargers Info, he's assembled a group of like-minded experts to cover every facet of portable power His aim is to help you learn more about your favorite gadgets and their batteries so you can maximize both their performance and their life.

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