How Long Will A 100 Amp Hour Battery Last?

Ampere hour or amp-hour (Ah) is the unit of measure used to determine the reserve capacity of batteries. In layman’s term, it is the rough estimate of the amount of time a battery can operate on a single full charge.

This unit of measure can also be described as the amount of charge within a battery that will allow one ampere of electric current to flow one hour. It is a metric commonly used in the battery industry, specifically deep cycle type, for comparing batteries.

So, how long will a 100 amp hour battery last? Basing on what is mentioned above and on the definition of reserve capacity. A battery with a rated reserve capacity of 100 Ah will be able to last for 100 hours, delivering a 1 amp electric current.

Theoretically, if it is to provide 100 amperes of current, it will only last for an hour. However, in reality, the amount of time the battery with a rated reserve capacity of 100 Ah will last will depends on the load that it will support since this is only an approximate estimate of the charge capacity of the battery.

The amount of time the 100 Ah rated power cell will also be difficult to determine unless we know the amount of current it will supply. Keep in mind that to calculate for a battery’s capacity, you will have to take the current it supplies and multiply it with the amount of time the battery will be discharged.

To get a better point of view on this issue, let me say that the 100 Ah battery has to power a load rated with 10 amperes since we know that the Battery Capacity (BC) is determined by multiplying the current (I) with time (T).

To determine the battery life, measured in hours is done by dividing the capacity with the current, which will be 100 Ah / 10 amps = 10 hrs. In this case, the 100 amp hour battery will be able to power the load for 10 hours.

Although that may seem to be true, that’s only the way it works in theory. In reality, if you want to determine how long that battery will last. You will have to consider and understand that there are a few significant aspects that affect battery life.

So, the answer on the lifespan of 100 Ah battery will depend on the load it has to support, the type of battery, discharge rate, and the voltage cut off.

Finding your way to this article means that you are trying to understand how batteries and their battery life work. Or maybe you are trying to look for the best deep cycle battery option for an RV or anything.

Regardless of your reasons, if you are here, then you may want to stick for a while to learn more about batteries, power, voltage, and battery life. Essential and informational knowledge that will come in handy in various situations.

Calculating and Measuring Power, Current and Ampere Hours

If you want to measure or calculate work on batteries, battery life of deep-cycle power cells, amperage rating of an appliance, and learn how to convert watts to amperes. I think it will be best if you first take notes of the different formulas that you will use.

Ohm’s Law


Calculating Battery Capacity


Those are the mathematical formulas that you will need when you are to calculate for everything related to batteries and circuits. Make sure to keep them in mind as we will be using all of them as we go along.

Calculating Battery Capacity of Deep Cycle Batteries

Amp Hours or battery’s reserve capacity is determined by the amount of power it is drained augmented by how long it provides the demanded power. If we take a look at it on a mathematical perspective, it will be shown as:

AH = A x T

Given that AH is the capacity in amp-hours, A is the current drawn in amperes or amp, and T is the amount of time the battery is discharged in hours. Since then the question is looking for the duration the battery will last, this mathematical equation will be transformed to:

T = AH / A OR T = AH ÷ A

By looking at the derived mathematical equation. Without any load to be supplied with power, it will come down to the battery’s internal resistance. Due to this, the industry decided to come up with a standard to avoid miscalculations or anything related.

They set an accepted hour rate of 20 hours for deep cycle batteries. This standard 20-hour rate means that a battery is to be discharged up to 10 volts within a 20-hour duration.

Let’s say that a device is rated at 20 amperes, meaning the device will draw a 20 amp current to operate. And then, this device is used for 20 minutes. The amp hours will be determined by this mathematical expression.

AH = 20 (20/60)
AH = 20 (0.333)
AH = 6.67 amp hours

In this case, the amp hours is computed by taking the 20 amp of current and multiplying it with the 20 minutes. This is because it is the entire duration the device was powered by the battery. But in order to compare the batteries in terms of application or any to boost a battery’s reputation, the 6 hour and 100-hour rates are used.

Sample: A twelve-volt RV fridge has a five-amp rating, and it will have to take twenty hours to drain the battery until it has 10.5 volts remaining charge in it. In this scenario, I am assuming that the 100 amp hour power cell is fully charged and brand new. This is how it will look in computation.

AH = A x T
100 Ah = 5 amp x T
T = 100 amp hour / 5 amp
T = 20 hours

If in case you don’t know the amperage rating of the load and you have the wattage. You can get its amperage rating using the Ohm’s, which will be Watts is equal to the voltage multiplied by the current rating.

Here’s how it looks mathematically:

W = 12 volts x 5 amperes
WATTS = 60

Then from here, use the wattage to get the calculate the amp-hour rating, which will yield the five amperage rating. That’s just to show you that using watts to get the amp hours is also possible. For you to see it better, let take a new example where the scenario presented involves the wattage rating.

Let’s say an oven has a required wattage rating of 500, and we are going to get its runtime.

This will be computed as:

500W = 12 volts x AMPERES
AMPERES = 500 / 12
AMPERES = 41.67 amps

Then use the formula for calculating for the ampere-hours will lead to this particular equation.

AH = A x T
100 amp hours = 41.67 amp x T
T = 100 / 41.67
T = 2.4 hours

Now, that 2.4 hours is the time the oven will take to drain the battery before it hits limit. However, this is only in theory since in reality the amperes a load draws from the source increases proportionally, leading to a slower runtime.

In this case, it has an amp rating which is higher than the 5 amp rating of the first fridge on our example. This type of load will significantly shorten the life of the 100 Ah battery, since deep cycle batteries are not designed for being overloaded, specially in long time intervals. Ideally, we want this to run for twenty hours to establish ideal amp hour needed for it.

Which will lead us to this computation:

AH = A x T
AH = 41.67 x 20
AH = 833.4 amp hours
AH = 833.40 amp hours / 100
AH = 8.33

This will be the ideal amp hours needed to run the load rated at 500 watts for 20 hours without damaging the battery. It would suggest that we have to use the 8100 Ah battery; it will only supply 5.21 amperes since it is the only closer the suggested load for a 100 Ah battery.

That means that, in reality, the amount of battery capacity of a 100 Ah power cell is different from its capacity from a theoretical perspective.

A Look at the Actual Amp Hours We Get from a Battery

After showing you the ways of computing power, battery life, amp-hours of battery, and other related values, you now know that in reality, the capacity of a power cell is not that simple because it is affected by a lot of things.

Significant aspects that we have to think about and we have to consider. Let us learn these factors or facets that affect the ampere-hours we get from a deep cycle battery.

1. The Low Cut-Off Voltage of Batteries

The first thing we have to keep in mind when we are to purchase a battery is that we can not use a power cell 100%.

To understand this, let us take an AGM and wet cell battery, and let’s say that both of them have 12.1 volts remaining in them, which equates to 50% used to charge. This means that the two batteries will still have a 50% charge left, but as you go further of its charge, the voltage it can deliver reduces significantly.

What this indicates is that if both the AGM and wet cell batteries are to be discharged to 30% or lower, the voltage they can deliver will not be enough to power loads with higher current requirements. We also have to keep in mind that using the batteries and draining them too much will weaken their battery plates, shortening their service life.

This will also mean that if a battery is drained to a lower percentage of the remaining charge, the time the battery will take to replace the amount of energy it was discharged will also be longer.

So, subjecting the batteries into heavy discharge loads will not be healthy for the battery and will lead them to their demise. That means we only use a battery for at least 40% of its capacity if we want it to leave longer.

2. The Kind of Battery or the Type of Battery

Battery type is also another aspect that can affect the actual battery capacity we get from a power cell. To have a better understanding of this, let’s go back and take a look at the AGM and wet cell batteries we used in the first aspect.

It is widely accepted that a wet cell battery will have a different voltage output from an AGM battery, as both of them are discharged to lower remaining percentage.

Both of the batteries will have a different rate when receiving charge. AGM batteries are designed to take charge a lot faster than wet cell batteries. This will mean that recovering from a low discharge state will be a lot easier from an AGM type of battery.

In this case, AGM batteries will also last longer since they can repel damage better than wet cell batteries if they are subjected to demanding power requirements.

If we also take a closer look at the example of the AGM and wet cell battery, the capacity they offer will be the same. However, if used in the same scenario, the capacity of the wet cell will be less. That will be based on actual and real usage.

3. Rate of Discharge and Excessive Drainage

We also have to keep in mind that a battery’s capacity significantly decreases as the discharge rate also increases. Let’s take the example on the computations if a 100 Ah battery is subjected to a test where it will be discharged for twenty hours.

If a load or an appliance rated at five amperes is used on the test. The battery will be discharged at five amps an hour; then, the discharging process will be done for twenty hours long.

The example should be specified as 5X20=100 Ah, right? But that is from the theoretical perspective. If that load is to be powered by the battery constantly, and it is exceeding the battery’s quoted discharge rate. The capacity of the battery will have a significant drop.

However, if you are to use two or multiple batteries, each of them will only be discharged with a lesser amount of charge that will less than the quoted discharge rate.

This only means that if a battery is subjected to a constant high discharge rate, its capacity will significantly reduce. This is because the recovery of the battery will take longer, and before it has the chance to fully recover, it will again be discharged.

It puts a lot of strain on the battery plates that will have a quicker pace of deterioration, making them lose the ability to store charge.

4. Voltage Drop – Not Allowing a Fully Recharged Battery

Voltage drop is an inevitable problem that prevents us from having the opportunity to fully use the capacity of a battery. In a lot of cases, batteries are charged and reflect a 100% fully charged result.

However, in these cases, the power cells are only receiving up to 70% of the charge because of voltage drops. This commonly happens when a battery is charged with longer cables, which increases the risk of having a frequent drop in voltage.

We have to bear in mind that electrical wires or cables have an inherent resistance to the effects of the flow of current. When a battery is used on an RV, its alternator will only charge the battery up to 70% because of the inherent resistance of the cables. The longer the cables connected with the battery, the higher the risk of not having a fully charged battery.

Final Thoughts

Now, those are all the things and aspects that you have to understand about a battery’s capacity and how to compute for the various values involved on an electrical circuit. I hope that the information that I imparted you through this article will help you understand the aspects that you have to understand.

If we are to go back, answer the question, the amount of time the 100 Ah battery will last depend on the load that it has to support, its battery type, rate of discharge it will be subjected to, and a few other factors. You can theoretically compute for its ampere-hours, but that wouldn’t be the same in reality.