How to Properly Charge an RV Lithium Battery

 

Going lithium is all the rage for those who like to boondock in their RV. Not only does the lithium battery offer a more usable battery capacity at 90 percent (compared to 50 percent for lead-acid), but it’s also 50 percent lighter, provides a higher current and voltage output, and charges faster because it can be “bulk” charged up to 97 percent. Unfortunately, when it comes to charging and maintaining a lithium ion battery, there’s a lot of misinformation out there. Due to the limitations associated with the lead-acid battery, age-old habits have become ingrained over time. Many assume that charging a lithium battery is the same as a lead-acid battery, but this isn’t the case. There are some significant differences, as we’ll explain in this article. Indeed, these differences are so profound that it can be a detriment to the lithium battery’s life if not corrected.

All things considered, it’s amazing that the lead-acid battery has existed in the RV marketplace for so long. It charges slowly, stores relatively little power, has a relatively short life span, and needs a full charge to prevent sulfation. It’s also heavier than lithium, is hamstrung by Peukert’s Law (meaning as the rate of discharge increases, the battery’s available capacity decreases), and suffers from voltage sag (at 50 percent the lead-acid voltage typically drops to 11.9 volts). Due to limited space, truck camper battery compartments are typically limited to no more than two group-27 lead-acid batteries. Even though the typical group-27 battery is rated at 100 amp hours, you’ll need two to actually get that much usable capacity because only 50 percent of each battery is usable. Still, two batteries isn’t enough for some. For those needing to run a high amperage load like a microwave, hair dryer, or an induction cooktop this usually means one thing—running a generator. Ugh.

Enter the lithium ion battery. Using one or more lithium iron phosphate (LiFePO4) batteries, you can power the aforementioned loads using an appropriately sized inverter—we use a 3,000 watt pure sine wave model in the Roadrunner. When compared to lead-acid, our 12 volt Expion 360 amp hour LiFePO4 battery puts out as much power as seven 100 amp hour group-27 lead-acid batteries! It does this weighing half as much at only 95 pounds, while taking up only one-third of the space. It also lasts longer than a standard lead-acid/AGM battery—between 2,000 and 5,000 cycles—and doesn’t suffer from voltage sag like lead-acid. Moreover, at a 30 percent state of charge (SOC), our Expion 360 amp hour lithium battery still cranks out a whopping 12.9 volts. That’s why the lithium battery, coupled with a capable solar power system, is such a game-changer for those who like to boondock.

Solar power is an excellent way to keep LiFePO4 batteries charged.

Unfortunately, there are some negatives associated with the lithium ion battery. First, never charge a lithium battery below 32F. Doing so can irreparably damage it. Yes, you can use a lithium battery below 32F you just can’t charge it below this temperature. Fortunately, most of the lithium batteries being built today have a battery monitoring system (BMS) built in to prevent charging below freezing. This is also why many lithium battery owners like to keep their lithium batteries stored inside the camper and not in an compartment outside where they can be exposed to much colder temperatures. Second, the cost for a lithium battery is higher than lead-acid with the cost for a LiFePO4 group-27 ranging anywhere between $700 and $1,000. Even though this price includes the required BMS, it’s still seven to nine times more than a standard wet cell lead-acid battery. Not only that, this higher cost doesn’t take into account the charging devices needed to properly charge a lithium battery, which will add even more up front cost (more about this later).

Fortunately, the high, upfront cost to go lithium can be mitigated by building your own lithium battery bank using 24 volt electric vehicle (EV) lithium oxide manganese (LiMn2O4) battery cells. Fellow reader, Steve Hericks, did just that in an article we featured recently here on Truck Camper Adventure. Steve’s DIY camper, called Maximus, features a massive electrical system centered around a 24 volt, 1,100 amp hour lithium battery bank and a buck converter that converts the 24 volts to 12 volts for some of his loads. He charges this battery with a 950 watt of solar power system and a robust DC-DC alternator charging system. You won’t find a generator anywhere near Steve’s camper. That’s because Steve’s battery, coupled with a 4,000 watt pure sine wave inverter, is large enough to run an air conditioner, a convection microwave, and an induction cooktop.

How to Properly Charge an RV Lithium Battery

Expion360’s brand-new 360 amp hour lithium ion battery.

Battery charging in a truck camper is typically accomplished using solar power, the truck’s alternator, and a 110 volt AC converter-charger powered by either shore power or by a generator. Unlike lead-acid, which needs 14.4 volts to charge, lithium requires 14.6 volts. Since the time and voltage requirements differ by battery chemistry, it’s important to use the correct settings on each charger for the battery type to avoid over-voltage damage and to achieve a proper, full charge. This means all of the battery chargers in your camper should have a LiFePO4 setting or be made specifically for lithium. Another setting simply won’t generate the voltage necessary to properly charge the battery.

With regard to the charging profile used by lithium, there are some important differences. The lead-acid battery requires three main charging stages—bulk, absorption, and float—to properly charge. An equalization and maintenance stage for lead-acid is used to prevent sulfation as well. Not so with lithium, which requires only two stages, a bulk charge up to 95 to 97 percent SOC, and a tiny absorption charge for 10 to 15 minutes for the remaining 3 to 5 percent SOC. Because lithium doesn’t sulfate the additional float and equalization stages aren’t needed.

Outer casing of the Progressive Dynamics Converter-Charger.

Yet, when it comes to charging, lithium has another important difference—the practice of keeping lithium topped-off or “trickle charged” at 100 percent SOC. You simply shouldn’t do it. This is because lithium can be “stressed” due to ambient temperature changes while in storage. As a matter of fact, the optimum state of charge (SOC) to store a lithium battery for long-term storage is between 40 to 80 percent SOC or when the battery’s open circuit voltage is approximately 13.1 volts. Unlike lead-acid, the monthly self-discharge rate of the lithium battery is a meager 1 to 2 percent and experiences no “health” issues when receiving a partial charge or when being stored less than fully charged. So what’s the bottom line? If you’re keeping your lithium battery topped off for long periods of time at home or at an RV park—stop doing it—you’re probably damaging it.

Keeping your lithium topped-off at 100 percent for long periods of time should be avoided to prolong life.

Like any rechargeable battery, amperage, or the amount of it, is an important factor in how quickly a lithium battery recharges. Like we mentioned earlier, lithium can be charged at a much higher rate compared to lead-acid. This is due to the lower internal resistance of lithium. Indeed, lithium can be “bulk” charged at .8C or 80 percent of the battery capacity (80 amps for a 100 amp hour battery) as opposed to lead-acid, which, due to its higher internal resistance, is limited to a “bulk” charge rate of no more than .3C or 30 percent of the battery capacity (30 amps for a 100 amp hour battery) followed by an absorption phase that can take even longer. This is one reason why some lithium battery owners have installed larger, more powerful AC battery chargers like a 60 or 80 amp model. It’s also why so many are covering their roofs with solar panels.

For smart or standard alternator charging, the lithium battery requires a DC-to-DC charger. Aside from isolating the truck battery from the camper house battery, the DC-DC charger provides the necessary 14.6 volts and charging profile required for LiFePO4 charging. In order to obtain a sufficient charge, however, you’ll also need heavy-gauge wiring like 6 AWG or higher. The standard 10-AWG is too small to pass a sufficient charging voltage in the lengths found in truck campers and RVs. A two- or three-wire Anderson plug or equivalent will also be needed to connect/disconnect the camper from the truck. As for the make of the charger, we recommend any good LiFePO4 DC-DC charger like those made by Expion360, Victron, Redarc, and Renogy. We recommend a minimum charger rating of 25 amps/6 AWG when building your circuit, but higher charge ratings can be achieved by using a second alternator. Steve Hericks’ article on lithium and alternator charging addresses these any other issues in much greater detail.

Truck camper battery charging via your truck’s alternator can easily be accomplished with the right equipment.

Other Considerations

So can you wire a 90 amp hour lithium battery with, say, a 160 amp hour lithium battery made by another manufacturer? You can, but not if they’re different chemistries, meaning you can’t connect a 12 volt LiFePO4 battery with a 24 volt LiMn2O4 battery. Parallel connecting two different size batteries of the same chemistry is fine, however, each will contribute proportionally—not equally—to the load, meaning the 90 amp hour battery will contribute 36 percent of the amperage, while the 160 amp hour battery will contribute 64 percent.

Of course, a good battery monitoring system is a must for anyone who likes to boondock. This is the only way to determine the SOC of your lithium battery. In addition to the SOC, a good battery monitor will also display the battery’s current voltage and the amount of amps being used at present. We use Expion360’s battery meter to monitor the state of our battery, but any battery monitor, like those made by Xantrex, Victron, or Bayite, will do the trick. All of these high-end monitors employ a shunt, a device that measures amperage flowing in and out on the negative side of the battery. The device works to report things in real time, which is what you want when you’re camping off-grid.

Final Thoughts

Going lithium is a very worthwhile investment, but only for those who camp extensively off-grid. If your truck camping experience involves hopping from one RV resort to another, then going lithium would be a total waste of money. You’ll be better off getting a couple of lead-acid AGM batteries to keep the lights on in between stops. However, if boondocking is your modis operandi then going lithium makes a lot of sense. But as we’ve just explained in this article, there are several things to know before you make the investment; otherwise, you may eliminate many of the benefits that the lithium battery provides. As always, consult manufacturer’s directions that came with your battery if you still have any questions or concerns.

Boondocking on Utah’s Hole-in-the-Rock Road.
About Mello Mike 615 Articles
Mello Mike is an Arizona native, author, and the founder of Truck Camper Adventure. He's been RV'ing since 2002, is a certified RVIA Level 1 RV Technician, and has restored several Airstream travel trailers. A communications expert, he retired from the U.S. Navy in 2004 as a CWO3 after 24 years, worked in project management, and now runs Truck Camper Adventure full-time. He also does some RV consulting, repairs, and inspections on the side. He currently rolls in a 2013 Ram 3500 with a 2021 Bundutec Roadrunner truck camper mounted on top.

1 Comment

  1. I prefer to run a generator for the bigger loads until the cost comes down for the batteries and a converter replacement. I have some solar and it is OK but I don’t like the Idea of adding top heavy weight or parking in the sun unless forced.

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