Battery Basics
Part Two
Part Two
battery basics typography
How to keep your 12-volt batteries alive and properly charged
By Mike Sokol / photos by author

atteries are invaluable to anyone who has ever gone “off the grid” in search of adventure — and we’re not talking about those 1.5-volt Type D dry cells you pump into a flashlight to keep from stumbling over tree roots at night. Most of an RV’s electrical system is powered by one or more 12-volt batteries, so knowing all you can about them might save a vacation should you flip a light switch and nothing happens. When last we met, I discussed the differences — from cost to chemistry, energy capacity, maximum charging rate and lifespan — for three popular RV battery types: flooded lead-acid, absorbent glass mat and lithium iron phosphate (for more information, go to In this part two of Battery Basics, I’m going to divide the article up into a few sections of “Do’s and Don’ts” — covering how to charge the various types of RV house batteries properly, what you should do to maintain them during usage and the best ways to store them over the winter.

RV batteries come in three major chemistries
RV batteries come in three major chemistries: absorbent glass matt, lithium iron phosphate, and flooded lead-acid.
Match Your Charger to Your Battery Chemistry
While FLA (flooded lead-acid), AGM (absorbed glass mat) and lithium (lithium iron phosphate or LiFePO4) batteries can be dropped into most any RV and provide nominal 12-volt power, each of them behave differently while charging. Not understanding this can shorten their lifespan considerably and even destroy them in short order.
Inteli-Power 9200 Series with Charge Wizard from Progressive Dynamics
A converter/charger such as this Inteli-Power 9200 Series with Charge Wizard from Progressive Dynamics converts the 120-volts AC from the generator or shore power into 12-volts DC to charge the RV batteries and power 12-volt DC appliances.
Make sure that whatever charger, converter/charger or inverter/charger you’re using has a setting that matches your battery chemistry. Most all of them will be able to charge FLA and AGM batteries if set properly, but if your charger is more than five years old it may not be able to charge modern lithium batteries to a state of charge (SOC) of more than 75%. If you’re upgrading to lithium batteries, you need to review your charger at the same time. Let’s dispel one myth right from the start: The 7-pin trailer wiring harness is not a high-amperage charger.

I am frequently asked why the plug/receptacle between a tow vehicle and trailer doesn’t provide enough charging current to even keep the RV’s residential refrigerator operating without running down the RV battery. Here’s the sad news: You’ll be lucky to get more than 4 or 5 amperes of charging current though that 7-pin connection. That’s little more than a trickle charger — especially when it’s trying to power a residential refrigerator. A typical residential refrigerator that draws 5 amps at 120-volts AC will require 10 times that current from the 12-volt battery. It could easily be drawing 50 amps from your battery while the compressor is running; that will drain a 100 amp-hour (AH) battery totally flat in a few hours of compressor time.

limited ability to charge RV batteries
A 7-way trailer wiring harness has a limited ability to charge RV batteries from the tow vehicle, typically 5 amperes or so maximum.
You can add separate heavy-duty wiring and a quick-disconnect plug between the tow vehicle’s alternator and an RV’s 12-volt batteries, but then you’ll also need to match battery chemistries or add a DC-to-DC converter. These come in various amperage sizes with 25- and 50-amp versions being popular choices. Many will also function as a solar panel charge controller.

Speaking of solar panels: Never try to connect solar panels directly to your RV batteries. While it might take a day or more to fully charge your RV batteries from a modest number of solar panels, there’s nothing to stop the solar panels from overcharging your batteries and destroying them. So always use a solar charge controller that is compatible with your battery chemistry.

3-stage charger from REDARC Electronics
A DC-to-DC charger like this 3-stage charger from REDARC Electronics is used to control charging current between a tow vehicle and RV house battery, as well as solar panels and batteries.
12-volt DC charging outlet
Even though most inverter generators have a 12-volt DC charging outlet, you really should use the 120-volt AC output to connect to the RV shore power cord and allow your RV’s battery charger to do the job.
Flow-Rite watering system
The Flow-Rite watering system allows you to properly fill all the cells in multiple lead-acid batteries at the same time.
You should also never try to directly connect a lithium battery to your vehicle battery or you’ll likely burn up the alternator. This will require some sort of DC-to DC-charge controller as well ( Also, be aware that most lithium batteries have internal surge current limits that make them poor starting batteries — so don’t think you can simply drop in a lithium battery for starting your tow vehicle’s engine. It probably won’t have enough Cold Cranking Amps (CCA) to start a big diesel engine, for example.

Charging RV Batteries from a Portable AC Generator
All this stepping up and stepping down of voltages may seem counterintuitive, but don’t use the 12-volt DC charging outlet on an inverter generator. Not only is it limited in the amount of current it can provide (typically around 7 amperes), it’s also not voltage- or current-regulated. This means it will take a long time to charge your batteries fully — then it will proceed to overcharge them.

The best way to charge RV batteries from a portable AC generator is to simply plug the shore power cord into the generator directly, using the appropriate dogbone adapters. For example, a 2,000-watt generator will easily be able to provide up to 10 amps or so of 120-volts AC, which an RV’s charger will turn into up to 80 to 100 amps of 12-volt DC charging current. And, because you’re now using the charge controller in your converter/inverter/charger, generator power is now matched to the battery voltage and amperage charging requirements.

Battery Maintenance
While a car battery may not require this amount of care, FLA batteries in an RV application are a different animal, because they may go through dozens of deep charge/discharge cycles every month — and every time they’re recharged, they lose a little water vapor. It’s possible, therefore, to have the water level get too low in just a month’s time — and once the water level gets low enough to uncover the plates, the battery is heading for an early death.

While refilling an RV’s FLA batteries with distilled water isn’t the most glamorous job, you can certainly make it a lot easier with a single-point watering system from Flow-Rite ( This allows you to refill all six or more cells at the same time with a single water container. Such a system should be a “must-have” for anyone using FLA batteries, and they’re available for 6- and 12-volt batteries. AGM batteries don’t require the same type of periodic maintenance, so they’re a good choice for battery systems that may need to be unattended for months at a time. However, you still have to be careful not to overcharge them. Lithium batteries require the lowest maintenance of all if properly designed and built. Another big advantage is that they can be disconnected and will self-discharge at less than 1% per month — you can literally disconnect them and leave them without a charger connected to them all winter or even all year. Many lithium battery makers suggest they be discharged down to 50% SOC for winter storage, while others recommend they are completely charged to 100% SOC before storage. Be sure to check with the particular manufacturer to see what it recommends.
a brand of battery maintainer
Battery Tender is a brand of battery maintainer that keeps a battery properly charged for months at a time without overcharging.
allows you to stop parasitic battery drain
Many RVs have a battery disconnect switch that allows you to stop parasitic battery drain while in storage — but often it also disconnects the battery charger or maintainer.
What about FLA and AGM trickle charging?
Well, you don’t want to leave any two-stage charger connected to RV batteries or risk boiling out the water and getting sulfuric acid all over the place. That’s why battery maintainers (or tenders) were created. These are essentially “smart” chargers that will taper the charging current down to the fraction of an ampere (milli-amps) needed to keep RV batteries fully charged through the winter/storage months. If you don’t have a modern charger/converter with this maintenance/tender function, you can get an outboard one from a company like Battery Tender ( that you can clip right onto your battery terminals directly.

Parasitic Current Drain
It’s well worth knowing — and remembering — that even when everything inside of an RV is powered off, everything using 12-volts DC will likely still be drawing a little current while in sleep mode. This is enough parasitic current to drain an RV battery in months or even weeks. That’s why many manufacturers have included a battery ON/OFF switch as part of their 12-volt DC system. However, turning that battery switch to OFF may also disconnect the charger. This is why if you’re plugged into shore power, your battery switch should stay on. If you have a solar charging system, it may or may not be affected by the main battery disconnect, and simple testing with a multimeter can usually answer this. If you’re charging with a Battery Tender, though, then turning the battery switch off is fine.

Should you bring your batteries into the garage over the winter?
Unless you’re worried about thievery, I would say that leaving them in the RV is the safest way to go. Just disconnecting those heavy cables can be dangerous if you’re wearing any jewelry that can short out to the chassis, and the sulfuric acid can wreck your vision if you get it in your eyes. I receive dozens of emails every spring from readers who hooked up the cable incorrectly and reversed the polarity. If you’re lucky, that will only blow the DC fuses on the inverter — but it can destroy some 12-volt DC appliances such as a refrigerator, furnace circuit board and tank monitoring system.

metal jewelry removed
Be sure to remove all metal jewelry and wear eye protection when working around any RV batteries. There’s enough current to weld a wedding ring to a wrench and have the ring turn red-hot while it’s on your finger.
What about the cold? Any battery that’s properly charged will be safe down to the lowest temperatures you can possibly reach in the U.S. Of course, if you allow a FLA battery to discharge down to 0% SOC it can develop ice crystals in the electrolyte and burst at the seams. Then it’s “bye-bye” battery and “hello” checkbook. And yes, lithium batteries should be not be charged when their temperature is lower than 40 degrees F., and most battery management systems will completely prevent charging below freezing. However, many of the newest lithium batteries have built-in electric heaters that will bring the cells up to the proper temperature before allowing them to charge. Some installers will also add little electric blankets around lithium batteries to warm them up. It’s only the charging cycle that is temperature-dependent, however — lithium batteries can be safely discharged even when the thermometer is way below freezing.
Battery Tender
(877) 456-7901

(616) 583-1700

Progressive Dynamics
(269) 781-4241

REDARC Electronics
(704) 247-5150

Mike Sokol
Mike Sokol headshot
Mike Sokol’s lifelong interest in electricity started when he was just 8 years old; by the age of 14, he had taken the U.S. Navy’s course on tube theory. Holder of a mechanical engineering degree and a Master Electrician’s license, he is the moderator of the AC Power & Grounding Forum and has been an active participant in dozens of forums through the years. An accomplished musician who also has presented seminars on surround-sound production and large-sound-system design and operation around the country, he formed No Shock Zone Inc. to fund many projects found on his RVelectricity Facebook group and YouTube channel. He can be reached at: [email protected].