If you have purchased a 100 amp hour battery, you might wonder how long the battery will last. Will it get you through, for example, a long power cut or power your appliances through a weekend away camping?
We have the answer for you in this article where we will share with you the formula for calculating the maximum run time of your battery. We will also explain what amp hour and amp hour ratings are because these terms are important when working out how long your battery will power your devices and appliances.
Table of Contents
Amps And Ampere Hours
It is useful to know the difference between amps and ampere hours. An amp is a unit of electrical current flowing through an electrical circuit. Ampere hours describe how many amps per hour a battery can provide or the size of the battery’s storage capacity.
Ampere hours, or amp hours for short, are used to define how long a battery can provide power at a certain amperage. In theory, a 100 amp battery can power one ampere for one hundred hours. However, in practice, it isn’t as simple as that because most batteries do not draw exactly one amp of current.
An Amp Hour Rating
Amp hour ratings are used to judge the battery’s life and to compare it to other batteries. The rating comes from the amp hours of the battery and the discharge time. The amp hour rating should be displayed along with the other specifications for the battery. If not, then the battery is likely not suited for delivering consistent power.
The amp hour rating gives you an idea of the battery’s capacity. A battery with 100Ah, for example, will last five times as long as a 20 Ah battery in similar conditions. However, other factors, such as the number of charging cycles, also affect how long the battery will last. We will look at these factors in detail later in the article.
Different Representations of Amp Hour Ratings
In this article, we focus on amp hours, but it is useful to know the other ratings used for batteries. The Ah rating is used on large batteries, while for batteries that deliver lower power, the rating is mill-amp hours or mAh.
Some batteries have a C rating. These are usually deep-cycle batteries. These batteries are designed to provide power over a long period and can be regularly deeply discharged.
How to Calculate How Long a 100 Ah Battery Will Last?
When we talk about how long a battery will last, we generally mean how many hours and minutes the battery will provide power to a specific load. Even though the relationship between voltage and current is not constant in appliances such as fridges or TVs, we can estimate how long the battery will last.
Step One
To find out how long your battery will last, you will need to use a formula to work out the watt-hours. Watt-hours (Wh) is used to measure electrical energy consumed at the rate of one watt per hour. The formula to calculate watt hours is Ah x voltage. So you need to know both the Ah and the voltage of your battery.
Let’s clarify how the formula works with an example. If you have a battery with 100Ah and 12 volts, then the calculation looks like this:
100 x 12 = 1,200 so the battery capacity is 1,200Wh
Step Two
Once you have worked out the Wh of your battery, you need to multiply it by the depth of discharge (DoD) limit of your battery. For lead-acid batteries, the default limit is 50% and for lithium 100%. The formula for step two is Wh x DoD and it will give you the AC watts for the battery. So for a lead-acid battery, the calculation would look like this:
1,200 x 50% = 600
Since the DoD of lithium batteries is 100%, there is no need to use this step to work out the running hours of the battery. You can move to step three.
Step Three
For the next step, you need to know the efficiency rate (ER) of the inverter. Most inverters have an efficiency of about 90% to 95%. In our example, we have used a 95% efficiency rating. For step three, the formula is AC Watts x ER = AC Wh. Here is the calculation for our example.
600 x 95% = 570
Step Four
The final step is to work out how long the 570 AC watt hours will last. For this step, you need to know the watts of the appliance or the total watts if you are using the battery to power several appliances.
To work out the running time, you need to divide the AC Wh by the total load you are connecting to the battery. For example, if the total load is 100 watts, you divide by 100, if the total load is 200 watts, you divide by 200. The formula looks like this: AC Wh / Watts = battery run time in hrs.
For example, 570 / 100 = 5.7, this converted into hours and minutes is 5 hours and 42 minutes.
570 / 200 = 2.85, this in hours and minutes is 2 hours and 51 minutes.
How Long Will 100Ah Battery Last For Common appliances?
A fridge that consumes 630 kWh per year can run for 13.3 hours using a deep cycle, lead-acid battery. At a 50% DoD rate, it will run for 8.3 hours. A lithium battery can power it for 15.8 hours. For an example of a 100Ah battery powering a fridge in real life, you can watch this video.
A 100Ah battery can power a TV for ten to fifty hours. The exact time depends on the type of TV as small portable TVs such as those you might have in your RV will require less power than large-screen smart TVs.
If a freezer draws 25-30 amps per day, it can run for about 50 hours with a 100Ah battery. Large freezers, such as one with a 15 cubic feet capacity, will only run for a few hours with 100 amp hours. It would require a 700Ah battery to run for a full 24 hours.
Factors That Affect The Battery Run Time
Earlier in the article, we mentioned that several factors can affect the actual run time of your battery. You can find out what these factors are below.
1. Battery Life Cycle
All rechargeable batteries lose a fraction of their storage capacity with every charge and discharge cycle. Therefore, a battery that has been through a hundred cycles will have a smaller storage capacity than a new battery and power your appliances for a shorter time.
2. Temperature
The run time of your battery can also be affected by the temperature of the environment you are using it in. This is because the temperature affects the chemical reaction that generates the power. In colder temperatures, the efficiency of the battery drops.
For example, at 0°C (32°F) the power may be 50% lower and at -18°C (0°F) it may be down by 65%. Lithium-ion batteries perform better than lead-acid batteries in cold weather and lose less of their capacity.
3. The Age of The Battery
You also need to consider the age of the battery when you are working out how long the 100Ah battery will last. An older battery is likely to have been through more charging cycles, which will have lowered its storage capacity.
4. Type of Your Battery
The type of battery is important when working out the running time. Different batteries have different current storage capacities and depth of discharge rates. We briefly mentioned the difference between lithium and lead-acid batteries earlier. Let’s look at the differences in more detail.
When a 100Ah lead-acid battery is discharged at up to 0.2 current rates, it can withstand up to 900 charging cycles. For a 100Ah lithium battery, the number of cycles is up to 7,000. Certain types of lithium iron phosphate batteries can be discharged fully. This allows them to deliver twice as many Ah compared to a standard lead-acid battery.
Other battery types, such as a gel-cell or an AGM battery, will also give you different running times. You should consider these batteries if you are looking for a battery with a low self-discharge.
5. Discharge Rate
The discharge rate of a battery is the current of amperes at a steady rate that can be taken over a certain time from a battery with a certain capacity. The lower the discharge rate the battery has, the better its capacity is.
Conclusion
We hope you find the formula for calculating the running time of your battery helpful. You can also use it to calculate the running time of batteries with different amp hours. As the examples show, while a 100Ah battery will not allow you to live off-grid for a long time without recharging, it can help power your important appliances during power cuts.
100Ah batteries can also be very useful for storing energy from, for example, your solar panels to use later when the sun has gone down or if the power goes out.
If there is anything else you would like to ask about how long 100 amp hour batteries last, you can write your questions in the comments section.