Calculating Battery Usage

It's always nice to know how long your battery power will last if you are out and about. And this is some quick math to get an estimate.

How Long Will My Battery Last?

Getting a good estimate

It's always nice to know how long your battery power will last if you are out and about. And this is some quick math to get an estimate.

The Basics

Once you have installed a battery voltmeter here are a few helpful tips for using the voltmeter to check your battery life…

  1. If the battery has been charging, then it's important to let the battery set for 2 to 3 hours without a load or charger connected to stabilize before testing. Otherwise, your reading will be high, caused by a phenomenon called "surface charge".
  2. The best time to check your voltage is when you aren’t using any power. You want to make sure you aren’t drawing from the battery when checking the state of your charge.
  3. The switch keeps your LCD panel from drawing from the battery constantly. Remember to switch the voltmeter off after you have checked the battery charge. If you have a momentary switch, you won’t need to worry about this.
  4. Find a convenient place to access your trailer wiring. I’ve seen people put their voltmeters near their converter panels under the dinette or even behind the sink. Just remember… the closer you are to the battery, the less chance you have of something disrupting your reading, and thus it will be more accurate.

The table presented below are approximate charge state of a battery.

Avoid discharging the battery below the 40% level whenever possible.
Voltage State of Charge
12.6+ 100%
12.5 90%
12.42 80%
12.32 70%
12.20 60%
12.06 50%
11.9 40%
11.75 30%
11.58 20%
11.31 10%
10.5 0%

The Batteries

Deep cycle batteries are usually broken down into specific applications such as RV, golf cart,renewable energy, and marine. The major difference between the two application ( automotive and deep cycle ) is that a deep cycle battery has the ability to be deeply discharged and charged many times during its life. An automotive battery cannot withstand more than a few deep discharges before failure. Like all batteries they are designated by their "group", meaning the physical size of the case. The common sizes you will see in RVs from smallest to largest are Group 24, Group 27 and Group 31.

An additional consideration is that Group 31 and golf cart batteries are considered a commercial grade products while the other deep cycle batteries are more of a consumer grade product. Group 31 batteries are commonly used in diesel trucks, fork lifts and other commerial applications. What does this mean? The Group 31 and golf cart batteries are built to a higher standard and can stand up to more abuse than it's Group 24 /27 automotive cousin.

Battery A/H Rating  Weight (lbs) Footprint Sq Inches
Group 24 85 44 11" X 7" 77
Group 27 105 50 13" X 7" 91
Group 31 130 67 13" X 7" 91
Twin 6v GC 225 124 10" X 14" 140

The Math

How much battery capacity or how much solar power capacity will naturally depends on your daily power consumption. To find out the daily power consumption of your appliances, some simple math comes pretty handy.

All you need to do, is use this simple formula: Watts ÷ Volts = Ah (amps)!

The total sum is 216 Wh/per day. Divide this number with 12 V (volts) and you will get Ah (amps) numbers.

So...216 Wh ÷ 12 V = 18 Ah. There you have it. The average daily power consumption using the above numbers is 18 Ah/per day, which means that a 100 Ah auxiliary battery will provide enough RV power for about four to five days without being charged.