A battery Amp Hour rating (abbreviated as Ah) indicates the amount of current (in Amps) a battery can provide for one hour. The reserve capacity of deep cycle batteries used in RVs (including lithium batteries) is represented in Amp Hours.
For example, a 12 volt RV battery with an Amp Hour rating of 100Ah means the battery can power a continuous 100 Amps of current for one hour before it is fully depleted.
This maximum output over one hour is simply a benchmark. A single hour is just a practical timespan that can be applied to power consumption calculations over time. Typically you won’t draw the maximum rated power from your battery (or batteries).
Lets look at some real world applications where Amp Hours are used to represent reserve capacity of a battery or battery bank.
What is the difference between Watt Hours and Amp Hours?
Watts are used to represent the amount of power consumed (or produced) as a single value. For example if we wanted to figure out how much power is required to run all of our small kitchen appliances, we would add up the power required (in Watts) for each appliance.
Watt Hours are a representation of power over time. This is similar to Amp Hours which represent electrical current over time. The difference is that Watt Hours are easier to work with and preferred by many for that reason. Here’s why.
Watts allow us to represent and compare power production (or consumption) using a single value. Unlike a Volt or an Amp, a Watt is not actually a physical electrical property that can be measured. Watts are a calculation defined by this formula:
Watts = Volts x Amps
You can see in the power formula that Voltage and Current (in Amps) always need to be evaluated together in power calculations. Using Watts simplifies this.
So instead of referring to a standard light bulb as a 0.5A 120VAC light bulb, we just say it’s a 60 Watt bulb. Then we can easily compare it to a 40 Watt bulb and determine that we need a 100 Watt power source to power both.
To convert back and forth between Amp Hours and Watt Hours we use the same formula as above. Just substitute Watts with Watt Hours and Amps with Amp Hours as follows:
Watt Hours = Volts x Amp Hours
Amp Hours = Watt Hours / Volts
I’ve included Watt Hours together with Amp Hours in the following examples to help you better understand the relationship.
Amp Hour Rating of a Battery Bank (Group of Batteries)
Individual batteries are often combined to create a larger battery bank. The reserve capacity of the entire battery bank is also represented in Amp Hours since it behaves as a single large battery.
To calculate the reserve capacity of a battery bank you will need to understand how the individual batteries are connected. Series and Parallel connections are handled differently.
Note: Do not connect batteries with different voltage and Amp Hour ratings. All batteries in a battery bank should have identical ratings.
Batteries Connected in Parallel
The overall Amp Hour rating of the battery bank is calculated by adding the Amp Hour ratings of the batteries together. Output Voltage will be unchanged.
EXAMPLE 1: Two 12V 100Ah batteries connected in parallel
- Battery Bank Reserve Capacity = 100Ah + 100Ah = 200Ah
- Battery Bank Voltage = 12V
- Battery Bank Watt Hours = 2,400Wh
EXAMPLE 2: Four 12 volt 100Ah batteries connected in parallel
- Battery Bank Reserve Capacity = 100Ah + 100Ah + 100Ah + 100Ah = 400Ah
- Battery Bank Voltage = 12V
- Battery Bank Watt Hours = 4,800Wh
Give it a try. What if you connected two 24 volt 100Ah batteries in parallel? What would the output voltage and reserve capacity of the battery bank be?
Batteries Connected in Parallel
With a parallel battery configuration the Amp Hour ratings of the batteries are added together. Output Voltage remains unchanged.
EXAMPLE 1: Two 12 volt 100Ah batteries connected in parallel
- Battery Bank Voltage = 12V
- Battery Bank Reserve Capacity = 100Ah + 100Ah = 200Ah
- Battery Bank Watt Hours = 12V x 200Ah = 2,400Wh
EXAMPLE 2: Four 12V 100Ah batteries connected in parallel
- Battery Bank Voltage = 12V
- Battery Bank Reserve Capacity = 100Ah + 100Ah + 100Ah + 100Ah = 400Ah
- Battery Bank Watt Hours = 12V * 400Ah = 4,800Wh
Give it a try. If you had a battery bank with two 24 volt 100Ah batteries connected in parallel, what would the Output Voltage and Reserve Capacity (in Ah) of the battery bank be?
Batteries Connected in Series
In a series configuration Amp Hours remain the same. The output voltage of the battery bank is calculated by adding up the battery voltage ratings.
EXAMPLE 1: Two 12V 100Ah batteries connected in series
- Battery Bank Voltage = 12V + 12V = 24V
- Battery Bank Reserve Capacity = 100Ah
- Battery Bank Watt Hours = 24V x 100Ah = 2,400Wh
EXAMPLE 2: Four 12V 100Ah batteries connected in series
- Battery Bank Voltage = 12V + 12V + 12V + 12V = 48V
- Battery Bank Reserve Capacity = 100Ah
- Battery Bank Watt Hours = 48V x 100Ah = 4,800Wh
It’s your turn again. If you had two 24 volt 100Ah batteries connected in series, what would the Output Voltage and Reserve Capacity (in Ah) of the battery bank be?
Here’s a common RV battery configuration combining two or four 6 volt deep cycle batteries to create a 12 volt battery bank. Read the article: How to combine six volt batteries to create a 12 volt battery bank
Calculate how long a battery can power your RV – Using Amp Hours?
Each piece of equipment in your RV consumes energy at a different rate. So you’ll first need to calculate (or measure) the total electrical load (in Amps). Then divide the usable battery capacity by the electrical load to get the number of hours.
Length of Time (Hours) = Usable Battery Capacity (Ah) / Total Electrical Load (Ah)
Example: Let’s assume that your RV has a 12V 100Ah battery and a small power inverter to provide AC power. Now imagine you’re watching TV and using your laptop with some lights on.
Question: How long before your batteries dies?
Step 1 – Calculate the electrical load on your 12 volt battery
- Laptop Computer Plugged In (3A @ 120VAC = 360W; 360W / 12V = 30 Amps )
- LED TV – AC Powered (1A @ 120VAC = 120W; 120W / 12V = 10 Amps)
- A few LED lights (1 Amps)
- Other miscellaneous things running (4 Amps)
Total electrical load = 45 Amps per hour @ 12 volts
Step 2 – Estimate the number of hours you can run this load
When we apply the formula to our example, we see that most of your usable battery capacity will be used up after 2 hours watching TV and using your laptop.
100Ah Usable Battery Capacity / 45Ah Electrical Load = 2.2 Hours
Give it a try. If you add a second 100Ah battery to up your battery bank to 200Ah, how long will the battery bank last?
NOTE: For the sake of this basic example we assume that 100% of the rated battery capacity is available. In reality, we use the term usable capacity to indicate the actual battery capacity available. This varies from one battery chemistry to another. For lead acid batteries (including AGM), the usable capacity is only 50% of the total rated capacity. The usable capacity of lithium batteries is between 80-100%.
What To Read Next
Now that you know what Amp Hours are, let’s figure out How Much Battery Capacity You Need.