So you’ve decided to try boondocking, and like many before you, one of your first questions is How long will my battery last boondocking? It’s a pretty straightforward question, but the answer is relatively individualized to your RV and setup. For clarity, this article will assume that the question refers to how long your batteries will last without a charging source like solar or a generator. You will learn how to figure out your total battery capacity and two ways to estimate how long that battery capacity will last before recharging. We’ll also touch on the most popular boondocking recharging methods to increase your boondocking capability.
Finding Your RV’s Battery Capacity
12V DC battery capacity is typically rated in amp-hours abbreviated as Ah. A batterie’s amp-hours can be found either on a sticker on top of the battery or printed on the side. Some RVs have more than one battery, and motorhomes will have a battery devoted to starting the engine and the rest will power the RV. Typically the house battery (the one that powers your RV’s 12V system) will be labeled as a deep cycle battery, while the one used to fire up the engine will be a starter battery.
Starter Batteries VS Deep Cycle
Like the battery that starts your car or truck, starter batteries are designed to release a lot of power fast to turn over and fire an engine. Deep cycle batteries, on the other hand, are designed to output lower amp draws over a long period and to withstand being depleted and recharged regularly. There are different types of deep cycle batteries used in RVs.
Traditional Flooded Lead-Acid (FLA)
This is the type of battery you have to maintain and add water to periodically. It is the least expensive type of battery, and RVs will commonly have either 12V or 6V versions of FLA batteries installed. If they are 6V batteries, they will be wired in a series to produce 12 volts. More on that later. The important thing to note here is that it is recommended to only drain FLA batteries down to 50% capacity to get the most out of the battery life. In other words, if your FLA battery says 100Ah, following best practices results in half or 50Ah of usable power.
Absorbed Glass Matte (AGM) Batteries
These are often referred to as sealed or maintenance-free because they don’t require you to add water. These are also lead-acid batteries, but instead of a free-floating liquid sloshing around, their electrolyte solution is held in an absorbent fiberglass matt. Sort of like holding a soaked sponge to a plate. RVers like AGM batteries because they don’t require much maintenance, and when used properly, they typically last longer and can charge faster than open FLA batteries. Once again, battery manufacturers recommend only discharging these batteries down to 50% before recharging them.
Lithium batteries are the newest kid on the RV battery block. They combine the maintenance-free aspect of AGM with the ability to withstand a much deeper depth of discharge (DOD). They work on completely different chemicals and metals than lead-acid batteries. The most common type of lithium battery found in RVs is lithium iron phosphate. You will see this marketed using the compound’s chemical formula LiFePO4.
According to RELiON, “LiFePO4 batteries can be continually discharged to 100% DOD and there is no long-term effect. However, we recommend you only discharge down to 80% to maintain battery life.”
In other words, if your lithium batteries are 100Ah, they can technically use all 100Ah.
Parallel Wiring VS Series
So you look in your RV battery compartment and find two batteries. To find the total Ah capacity of your rig, you just add them up, right? Well, not exactly. There are two ways to wire batteries into a battery bank. One increases the voltage, while the other increases the capacity.
If you have multiple batteries connected by the positive and negative terminals, that is a series. Typically RV batteries are wired in a series if they are 6V deep cycle batteries because a series doubles the voltage to bring the battery bank to 12V. The Ah rating remains the same, however. To calculate your RV battery capacity, you simply look for the Ah reading on one of the batteries. 6V golf cart batteries typically come in pretty high capacities of over 200Ah. But, remember, if you stick to the 50% rule, only half of the Ah capacity is usable if you plan on maximizing battery life.
If you have two or more 12V batteries connected to your RVs system, they will be wired in a parallel setup. The positive terminals will be connected together, and so will the negative terminals. Wiring batteries in parallel increases the battery bank’s capacity while the voltage remains the same. If you have multiple batteries wired in parallel, they should always be the same voltage and Ah capacity, so multiply the Ah by the number of batteries wired in parallel, and that will give you your total Ah capacity. If they aren’t lithium batteries, remember that you can only use half the total capacity if you want to maximize battery life.
Monitoring Your Batteries While Boondocking
There are several ways to determine how much battery power your RV has left, but the easiest way by far is a battery monitor. A good battery monitor like the Renogy 500A Battery Monitor or the Victron Energy BMV-712 Smart Battery Monitor available on Amazon will tell you your battery’s state of charge, power draw, and how many amp-hours remain.
Finding Out How Long Your RV Battery Will Last
Now that you know where to look for and calculate your total battery capacity, you can start calculating how long your RV battery will last boondocking. You can use two main methods for this. The first requires you to do a little math and guesswork. The other requires much less math and a little more real-world experimentation. If your phone’s calculator is set to scientific mode and there’s a shortcut to it on the home screen, you may fall in love with all the mathematical guesswork of method one. If, however, the previous sentence triggered your fight or flight response or awoke long-forgotten childhood trauma, you can skip to method two.
Method 1: The Math
To calculate how many amp-hours you are using, you must first determine how many amps the devices you want to run draw. If you can pick up the device and look at it, you will most likely find the power consumption information on a sticker or stamped into the plastic casing of the appliance. Devices that run on 12 volts will typically list the number of amps they pull, which makes calculating easy. The formula looks like the following:
(Device Amp Draw) X (Hours You Intend To Run Device Per Day) = (Estimated Ah Used By Device Per Day)
Sometimes, a device’s power consumption will be listed in watts. To find the amps used by a 12V device from watts, you will need the following formula:
Watts / Volts = Amps (It may help to remember “West Virginia” W / V=A)
Recalling from middle school algebra, you may have already figured out that you can rearrange that equation to find out the wattage of a device if you know the amperage.
Watts = Volts X Amps
From Watts To Amps To Amp-Hours
Let’s take a 12V water pump that uses 50W of power when running, and determine how many amp-hours per day a boondocker might use.
50W / 12V = about 4 Amps
Daily Use Estimate: (5 min for dishes) + (10 min for showers) = 0.25 hours
4 Amps X 0.25 Hours = 1 Amp-Hour(s) per day
Calculating Amp-Hours For 120V AC Appliances
If you have an inverter, you can run some of your AC appliances from your batteries. An inverter does two main things.
- It converts the DC (direct current) from your batteries into AC (alternating current).
- An inverter also steps up the voltage from 12V to around 110V so that you can run your 110-120VAC appliances.
There are a couple of key things to remember when calculating amperage draws from AC devices through an inverter.
- If a 110-120V device’s sticker lists its amperage, that amp draw is for the voltage stated on the sticker. A 160W AC fan will draw around 1.4 amps at 120 volts (160W / 120V ≈ 1.4), but the same fan will draw closer to 13 amps from a 12V battery source depending on its state of charge (160W / 12.6V ≈ 12.7 amps).
- It costs energy in the form of heat for the inverter to work. In other words, you will get a power loss of up to 15% when using an inverter. You must add this power loss into your calculations.
Note: A full lead-acid battery will put out between 12.7 and 13 volts when full, and lithium batteries will read up to 14 volts.
Pulling together the above information gets you the formula for calculating a 12V amp draw through an inverter.
First, find the 12V amp draw using the wattage listed on the device sticker (160W in our example).
160W / 12.6V = 12.7A
Next account for the 15% energy loss of the inverter by dividing the amperage by 0.85.
12.7amps / 0.85 ≈ 15 amps total
Adding It All Up
To help you estimate, approximate amp draws for common 12V DC appliances are listed in Table 1 below. Now that you know how to find your total battery capacity in amp-hours and calculate amp-hours from your various appliances, you are ready to figure out how long your RV battery will last. All you have to do is add up all of the estimated amp-hours for each appliance you intend to run per day and divide your total battery capacity by the estimated daily Ah usage.
Say, for example, I estimated my daily amp-hour total was 40Ah, and my rig has a 200Ah battery bank.
Lead Acid Batteries
Keeping with the 50% rule mentioned above, I have 100 usable Ah when using lead-acid batteries.
(200Ah / 2) / 40Ah per day = 2.5 days until batteries are at 50% capacity.
If using lithium, you can use the entire Ah rating.
200Ah / 40Ah per day = 5 days on lithium
Table 1: Approximate 12V RV Appliance Amp Draws
|Estimated Amps @12V
|Propane Furnace 16k BTU
|Propane Furnace 25k BTU
|Range Hood Fan
|Absorption Fridge, DC Mode
|Absorption Fridge, LP Mode
|12V Outlet, Socket
|Portable Electric AC/DC Fridge/Freezer
Method 2: The Real World Test
If you read method one and thought it seemed a bit involved, you’re not alone. Fortunately, there is another way to estimate how long your batteries will last boondocking. By testing it at a campground. The simple steps are as follows.
- Find out your total battery capacity.
- Head to an RV park or campground for a couple of days.
- After making sure your battery is full, keep your RV unplugged from the park’s power and pretend you are boondocking. Make sure to power devices and appliances as if you were actually boondocking.
- Keep an eye on your battery monitor periodically to ensure you don’t drain your batteries further than you should. After a full day and night, note how many Ah you have left according to your battery monitor.
- Simple subtraction will tell you how many Ah you used in a 24-hour period. You can then divide the total battery capacity in Ah by the Ah used in a day to find out how many days you would last on a single charge.
- Plug your rig into the RV park when done and recharge your batteries.
If you want a closer estimate, try doing the above test for multiple days and then calculating an average daily Ah usage.
Ways To Extend Your Boondocking Trip
If you’ve run the numbers and feel your battery capacity doesn’t buy you as much boondocking time as you’d like. The following video has some useful information on getting power and recharging your batteries off the grid to extend your boondocking time.
Do you have an awesome boondocking power-saving tip you’d like to share? Tell us about it in the comments.