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What is a Dual-Purpose RV Battery?

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What is a Dual-Purpose RV Battery?

Most people are aware that automotive batteries are used to power Starting, Lighting and Ignition (SLI), but some vehicles, including Recreational Vehicles (RVs), also rely on batteries to power onboard equipment, accessories and devices. Cranking the battery powers the starter motor to enable the engine in the RV to turn over, but starting batteries are easily rundown and rely on an engine’s alternator to restore the charge. The RV also needs auxiliary power to support house loads, which include appliances and electronic devices, not limited to climate control, entertainment systems, stoves, microwaves, refrigerators, GPS and cell phone chargers.

Many RV enthusiasts spend significant time boondocking (without a grid connection), sometimes in remote or difficult to access locations. It’s times like these where dependence on batteries is high, and durable, reliable batteries really earn their keep. Additionally, RVs that are driven only seasonally or occasionally also must run after being idle and often unmaintained for a significant amount of time. These demands justify a premium product with greater power that also dependably supports house loads, especially when traveling in remote locations. Auxiliary power for house loads requires deep-cycling capability to provide a low, but sustained, current with intense discharges that would deplete a starting battery.

Dual-purpose batteries can produce both the Cold Cranking Amps (CCA) needed to start the engine and the longer and lower discharge rates that are needed for house loads. Most dual-purpose batteries use Absorbed Glass Mat (AGM) technology, in which thin and absorbent fiberglass separators are saturated in electrolyte and compressed tightly between the battery’s positive and negative plates. AGM batteries are sealed to make them leak- and spill-proof, as well as virtually maintenance-free, as they do not require watering or changing.

dual-purpose rv battery
dual-purpose rv battery

Dual Purpose Replacement Batteries

We briefly looked at Dual Purpose batteries in our RV & Marine replacement battery post recently and feel it’s important to get a little more detailed about these types of batteries. One of the biggest misconceptions that we hear is that it’s better to buy a dual purpose battery instead of a deep cycle or starting battery because it does the job of both, it takes up less space and it’s more cost-effective to buy one battery vs two. While there may be some truth in those points overall, it’s not entirely accurate.

Much like the jack-of-all-trades vs master-of-none, the dual purpose battery does ok in its deep cycle role but certainly its runtime will generally be nowhere near as good as a deep cycle battery. So too when comparing dual purpose with a starting battery, in fact there are plenty of occasions where a dual purpose battery won’t actually be able to start a vehicle.

Typically, deep cycle batteries are built to withstand a lot of discharges and recharges but dual purpose batteries don’t do well beyond 50% of usable capacity and also don’t stand up as well in tough conditions, many can be especially prone to overheating especially when used for long periods of time. So these alone should be considerations. If you only take short boat trips on a smaller vessel, the dual purpose may be all you need. A bigger boat on longer trips would probably not be best for a dual purpose battery.

As with all replacement batteries, there are a few key things to look for when selecting a dual purpose replacement battery.

27M-700 Battery
27M-700 Battery

Volts – typically dual purpose batteries are all 12V

CCA or MCA– the cold cranking amps (cca) or marine cranking amps (mca) measurement tells you how many amps will be delivered to the engine in cold temperatures. They are both measured at 0 degrees, the difference between the two is that cca is Fahrenheit and mca is Celsius. Essentially that equates to 0℉ and 32℉. They range from 300 to over 1200.

Amp Hours (Ah) – amp hours give a general idea of the time that the battery can provide power for. Calculating how long your battery will power something for is fairly complex but in general, the bigger the number, the longer the runtime.

Group Number – the group number for dual purpose batteries reflects the physical size of the casing. Typically this will include BCI (Battery Council International) and include a number plus M. That number like 31M will be 13” x 6-13/16” x 9-7/16”.

Reserve Capacity (RC) – similar to Ah in that it’s a way to measure run time, reserve capacity or reserve minutes provide a rating for the number of minutes a battery can sustain a discharge of 25 amps before it will run out. Dual purpose batteries shouldn’t discharge lower that 50% so a reserve capacity of 120 could handle a load for an hour (60 minutes).

Dual purpose batteries are typically used in boats and RV’s because they both need a battery that provides a high powered charge that helps to start an engine or generator and also provide a lower level discharge to power accessories like lighting, GPS, fish finders, TV’s, trolling motors and the like. Still, before you rush out and look to consolidate your starting and deep cycle batteries, it’s important to do your research to determine if it’s a good fit for your needs as there are some things that a dual purpose battery can’t do or certainly can’t do as well as a specialist battery.

What Makes Deep Cycle RV Batteries Different?

Deep cycle RV batteries are one of the three commonly-available types of RV batteries, but they’re the ones that are most often misunderstood. With this post, we hope to change that, at least to some degree.

An RVer’s access to power is no minor consideration. We all appreciate traveling with the ability to run a number of appliances and electronic devices. While some RVers accomplish this by being tethered to campground power pedestals, other travelers (like us!) prefer to boondock in the forest, in the desert, on the beach, or even in a parking lot on our way to another great destination! To do that, however, we need battery power.

There are many options for RV batteries, but this post is geared toward understanding the popular RV deep-cycle battery.

What Are the 3 Most Common Types of (Non-Lithium) RV Batteries?

The three most common types of RV batteries that are not lithium ion (typically for RV use, Lithium Iron Phosphate) batteries are starting batteries, deep cycle batteries, and marine batteries.

Let’s take a look at each of these three battery types, how they work, and the jobs they’re designed to do.

Starting Batteries

Starting batteries are designed to provide the short but powerful burst of energy required to start an engine and to power the typical small 12V DC loads that a car might have. They provide their maximum power for about one to three seconds.

They typically have a larger number of thinner lead plates, allowing them to discharge in a burst and recharge quickly.

Starting batteries are easily damaged by being drained too low. So, while they can provide high current, they can’t handle deep discharges without suffering permanent damage and/or dramatically decreased lifespan.

Deep Cycle Batteries

Deep cycle batteries are designed to provide a long, steady supply of power rather than short power surges. (This is the key difference!)

These batteries are constructed using a smaller number of thicker lead plates, giving them greater resistance to damage from many cycles of being drained (reasonably) and recharged.

Deep cycle batteries can be drained and recharged many times over. However, it’s best not to deplete a deep cycle battery below a 50% state of charge. For example, if you have a deep cycle battery rated to provide 100 amp-hours of energy, you don’t want to use more than 50% of that energy, or 50 amp-hours.

The big difference between deep cycle batteries and other common batteries is the ability of the deep cycle battery to handle multiple cycles of deep discharge. This is important to the process of providing DC power for RV use.

Marine Batteries

Marine batteries can be starting, deep cycle, or dual-purpose batteries.

A dual-purpose battery is kind of a hybrid between a starting battery and a deep cycle battery. It can provide the strong burst of power needed to start an engine as well as provide more long-term power for onboard accessories due to having slightly thicker lead plates.

But, it’s important to know that not all “marine” batteries are necessarily “deep cycle”.

So, unless you confirm that you’re getting a deep-cycle marine battery, you could end up with one that doesn’t meet your needs for providing off-grid power for any length of time. It would also likely become damaged by continuous cycles of deep discharge.

Should an RV House Battery Have High CCA (Cold Cranking Amps)?

CCA (Cold Cranking Amps) is a measure of a battery’s output capacity for starting an engine. In other words, CCA represents the battery’s ability to provide short, powerful bursts lasting from 1-3 seconds.

You need that power in a short burst to start an engine, but it’s not what you need for a house battery that needs to provide steady, continuous power.

You actually want to avoid batteries advertised for their high CCA because they’re unlikely to have lead plates thick enough to be effective as deep-cycle batteries.

Again, while high CCA may indicate that a battery is great for starting an engine, it’s not so great for providing the long, steady supply of power required by the “house” portion of your RV.

Do Most RVs Come With Deep-Cycle Batteries?

Most RVs come with deep cycle batteries for powering the house portion of the RV. But if you’ve got a motorhome your RV will have two types of batteries.

Let’s use our RV as an example:

Our RV came with a pair of starting batteries for the engine, and a second set of deep cycle batteries for running the house loads.

Can I Replace My Deep Cycle RV Battery Myself?

You can replace your RV’s deep cycle house batteries as long as you’re able to lift them, or have someone who can lift them into position for you. (These batteries are generally between 60-70 pounds apiece.)


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