How Long Will a 100 amp Hour Battery Last – Calculate Yourself

How long does a 100 Amp Hour battery last? It’s a question most people bump into before buying a new battery.

Well, how long a battery will last depend on multiple factors such as the type of battery you are using, its age, and the appliances you are powering with it.

Calculating the runtime of a battery is no rocket science, but it needs a bit of preciseness. Here’s how you can determine the runtime of your battery.

AMPs and AMP Hour: Is There Any Difference?

Most people confuse AMPs with AMP Hour or AH, but these similar-sounding terms have a lot of differences. AMPs is a unit used to measure the electrical current, whereas AMP Hour is the current storing capacity of a battery.

AMP Hours

Technicians use AH or AMP hours to determine the electrical current consumed or produced. Also, it is the capacity of a device to consume or produce current.

Here’s an example to help you understand the term better:

A starter battery powered motor that draws 400 amps for 20 seconds uses 400X 20/3,600 = 2.2 AH of the total capacity. The runtime of any device depends on its capacity, and such as in this case, the capacity of the battery is 100 AH.


Amp is a term used to determine the current flowing through any circuit. You can even say Amps is an essential reading required to calculate the AH or any device.

This is the basic breakdown of Amps and AMP-hour. Curious to know more about the core difference between Amps and AH? Check out this article and get more help in calculating the runtime of a 100AH battery.

How Long Will a 100AH Battery Last?

AH or Amp Hour X Voltage = Watt Hour.

The above-mentioned formula will help you convert amp-hour to watt-hour and calculate the Watt-hour of a battery, but that figure will be more theoretical and less practical.

To get the exact or at least an approximation next to the exact hours, you should calculate the runtime of a battery keeping multiple things in mind.

The formula doesn’t cover the type of battery you are using, its low-cut voltage point, and the discharge rate of the battery.

These things are subjected to change with the age of the battery. Below mentioned are some essential things you should keep in mind:

Type of Battery

The type of battery has a significant impact on the Amp Hours you get from it. In simple words, the battery type impacts the current storage, and that affects the total hours of power the appliances get with it.

Lead-acid 100AH Battery

A lead-acid battery is able to withstand around 200-900 discharging/charging cycles when getting discharged at 0.05-0.2C rates up to 100-50% Depth of Discharge or DoD.

Lithium 100AH Battery

A lithium 100AH battery can withstand up to 2000-7000 discharging/charging cycles when getting discharged at 0.1-0.3C rates up to 100-50% Depth of Discharge or DoD.

A 100AH lithium battery has better cycles than a lead-acid battery, so you get more Amp-hours irrespective of the AMP-hour rating of the battery.

Furthermore, a LiFePO4 or Lithium iron phosphate battery can discharge up to 0% capacity, which delivers twice as Amp-hours as a regular lead-acid battery.

Note: It is not a good idea to drain the LiFePO4 battery up to 0% capacity, as it can hamper its current storage capacity.

You can go for Absorbent Glass Mat (AGM) or gel-cell batteries if you want a lighter and low self-discharge battery.

What Happens If You Deplete the Battery Below a Certain Level?

If you deplete the battery below a level, then it may downgrade the lifespan of the battery, or its current storing capacity will get low with time.

Use a low voltage cut off point that protects the battery from getting drained completely.

Parasitic Draw: A Silent Battery Drainer

Parasitic draw is the current consumed by the appliances even when they are off. Some appliances have components that consume a 12V charge from the battery continuously.

However, it is normal but can have a significant impact on the Amp-hours of the battery. Even the tiny LED indicator draws power/energy continuously from the battery.

Voltage Drop

Voltage drop is a common problem with any type of battery. Even if the battery is displaying a 100% full charge, there may be less charge in the cells. This usually happens when the battery is connected using long cables, as these cables increase the risk of voltage drops.

If the battery has less charge available, it will directly reflect in the Amp-hours it will deliver.

Now let’s get back to the runtime of a 100AH battery. To calculate the battery lifespan in hours, you can use this formula:

Battery Life = Amp Hours / Current in Amperes.

Or you can even use this battery life calculator by putting in the Battery Capacity and current.

Again, the reading you will get will be theoretical and not practical. It wouldn’t be wrong to say that you cannot calculate the exact runtime of a 100AH battery or any battery.

Some devices may draw 5 Amps constantly, whereas some will draw more or less depending on their requirements. When the current discharging from the battery isn’t constant, it can be challenging to calculate its runtime.

So, there is no accurate answer for how long will a 100AH battery last if you have multiple devices connected with it. But if you use it with a particular device, then you can calculate the total hours you will get power from a battery using the above-mentioned formula or by using the battery life calculator.

Is There a Way to Maximize Battery’s Performance and Capacity?

Yes, you can enhance the battery’s performance to ensure that it stores more supply current, and as a result, you get more power/energy from it.

Here are some faster ways to implement right away:

Use a Low-Cut Voltage Device

When any battery is drained below a certain level, its performance and efficiency also drain gradually. To prevent the battery from draining below that level, use a low-cut voltage device.

Check out this YouTube video to know how a low-cut voltage battery protector circuit works.

Make Sure the Battery is Charged Fully

The total hours of power you get from any battery depends on how much juice it contains. Make sure to check that the battery is topped up fully to power the appliances for longer.

Use the Right Type of Battery

Using the right type of battery is another essential thing to do for better runtime. LiFePO4 batteries are highly popular among RV owners as they have high charge/discharge cycles, and the battery can even discharge up to 0%.

Frequently Asked Questions:

Q: Can I use two 100AH batteries for more runtime?

A: Yes, you can connect two batteries to get more Amp-hours to run the connected appliances for longer. There are two possible ways to connect two batteries, including: in series and in parallel.

When used in parallel, you will get more Amp-hours from the batteries. However, the output voltage will remain the same. A parallel connection is good if you want to power a single device from the batteries or inverters.

On the other hand, a series connection will double the voltage, but the Amp-hour rating will not change.

To use the batteries in parallel, connect their positive terminal with positive and negative with the negative terminal.

Now connect the inverter to the negative terminal of one battery and to the positive terminal of the other battery.

To connect the batteries in series, connect the positive terminal of one battery to the other battery’s negative terminal. Do the same with the two leftover terminals to make the battery connected.

Link the inverter to the negative terminal of one battery and the positive terminal of the other battery.

Note: It is advised to use new batteries of the same rating for series and parallel connection. Don’t intermix old and new batteries, as it may damage the connected appliances.

Q: How long does a 100AH battery take to charge?

A: It depends on the battery’s condition, such as how flat it is and the charger you are using to recharge it. Usually, a 100AH battery takes around 10 hours if recharged with a 10 AMP charger.

Always use certified chargers to charge the batteries safely.

Final Thoughts

Now you can quickly figure out how long a 100AH battery will last! As a lot of numbers are involved, it can be a bit challenging. Read the article and know about the essential factors that greatly impact the battery’s runtime.

Lastly, you will be able to calculate a theoretical figure only as to how long a battery will last depend on its type, rate of discharge, and other factors listed in the article.

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