RV Battery Charging from your Tow Vehicle is a popular and easy thing to do. You simply use the alternator on your tow vehicle to charge your RV’s house batteries. It’s already doing that for the chassis batteries, but most alternators have excess capacity to charge the RV’s house batteries. If it is a self-propelled RV, like a class A, Class B, or Class C, a schoolie, or a van, it may be able to charge your RV’s batteries. You can use the alternator to charge travel trailers and 5th wheels, but there are some special considerations we need to address to do that correctly.
Yes, you can install a larger alternator to provide more capacity for charging the house batteries. We will address that in just a minute. If you would like to support the author, you can use this Amazon affiliate link to visit my store with all the gear you need to alternator charge your RV batteries.
House batteries vs. Chassis batteries
When I say RV batteries, I mean your RV’s house batteries. Those batteries provide power for lights, TVs, microwaves, etc. Note that it is common for lights in RVs to operate on 12-volt DC. It’s an efficiency thing and eliminates conversion losses inherent in the RV’s inverter converting 12 (or 24 volts) DC power to 110-volt AC power. More on that in a bit. I need to make a couple more definitions so you can understand the pitfalls of charging your RV house batteries from the tow vehicle.
The house batteries are usually separate from the chassis batteries on self-propelled RVs. On towables, they almost always have independent house batteries. The chassis batteries are the batteries used to provide power to chassis functions like the starter, headlights, taillights, turn signals, air conditioning fans, etc. They run off your chassis batteries. Everything else in an RV usually runs off your house batteries.
Every battery uses a chemical reaction to store and release electricity. The electricity is stored in the form of electrons in the battery’s electrolytic solution—sulfuric acid in the case of lead-acid batteries and lithium-ion in lithium batteries. I will call this electrical current. An electric current is a stream of charged particles, such as electrons, moving through an electrical conductor (wire). The more current flowing in the wire, the more electrons move through the wire, and the hotter the wire gets because of resistance.
Electric equipment (wire, inverters, batteries, etc.) generates heat when they move or use electrons for some purpose, like converting 12-volt DC to 110-volt AC. A wire gets hot the more electrons flow through it (high current) because of resistance. They can get very hot, catch fire, and burn your RV to the ground. Because of this, we must use the correct type and gauge of wire.
Selecting the correct wire size for charging your RV house batteries from the Tow Vehicle
A wire’s size determines how much current it can carry. Technically, you size wire to provide a certain amount of current at a safe temperature, measured as ampacity. A wire’s ampacity is how much current it can carry over a certain distance at a temperature less than its rating. The longer the length, the larger the wire must be.
You can quickly determine what size wire you need using a chart like this.
To use this chart, locate the amount of current you need across the top. Then, on the left side, find the length of the run. So let us say that we are running 40 amps 17 feet. You’ll need a minimum of 6 gauge wires. However, you can’t just connect the cables to the house battery and call it done. You need to know how much excess current you can siphon off the alternator.
As mentioned, using your tow vehicle alternator to charging your RV batteries has unique requirements. As you can see from the chart, the length of the wire dramatically impacts how much current you can safely carry. So, to charge your towed RV, you need to increase the gauge of the wire because the length is long. Running too much current on an undersized wire is dangerous and can cause a fire.
Determining excess alternator current
To find this value to design your alternator RV battery charging system, you’ll first need to know how big your alternator is. You can usually find this with a Google search. We will assume that you have a 125 amp alternator for this example. We’ll discuss installing a larger alternator shortly.
So you will need a clamp-on current measuring voltmeter. Be sure that it measures DC. You clamp it onto the primary wire *the largest wire on the alternator. Make sure the meter is safely away from any moving parts! Start the engine and get a baseline measurement. It may take a minute or two for the readings to settle because the alternator will be working to recharge the battery from the starter usage. It should only take a couple of minutes. If it takes longer, it may indicate you have a weak starter battery, and you should investigate it.
Once the meter settles down, turn the headlights on bright, turn the heater fan on high, etc. Turn on every electrical device and take another reading. This is the maximum load of the vehicle on the alternator. Subtract this amount from the maximum alternator rating. This gives you how much excess current you have. To extend the alternator’s life, limit the total load to 80 to 90% of the alternator’s maximum load.
My Chevrolet Silverado has a 125 amp alternator. My maximum vehicle load is about 68 amps, giving me 57 amps of excess current. Ninety percent of 125 amps is ~112 amps. Therefore, 112 amps (90% of the alternator’s capacity) minus 68 amps maximum load leaves us ~ 44 amps available as excess. The largest DC-to-DC charge controller I should use is a 45 amp unit.
However, there is more to selecting the proper DC-to-DC charge controller for charging your RV’s batteries from the tow vehicle alternator. You may find very little excess current available, so you might have to install a larger alternator.
Installing a larger alternator is frequently done. It is common for those people with booming car audio systems to have installed a larger alternator. Your favorite auto repair shop or parts store should be able to recommend a larger alternator that will fit your vehicle. We also need to prevent the house batteries from draining the chassis battery. Even though a DC-to-DC charge controller effectively acts as a battery isolator, well-designed systems include battery isolation.
Battery Isolators and Separators
Directly wiring the RV’s house batteries into the chassis electrical system may allow the house batteries to drain power from the chassis battery, leading to a dead chassis battery. When the tow vehicle alternator is Charging Your RV Batteries, we need to control energy flow to and from the house and chassis batteries. Well-designed systems include battery isolation. We do this with a battery isolator or separator. They isolate the chassis batteries from the house batteries but function differently.
A battery isolator is a device that contains a diode. Remember, diodes are electronic components that only allow current to flow one way. Think of a one-way street. Some also balance the amount of current sent to each terminal.
A battery separator is a relay that switches between batteries depending on the battery’s state of charge. We’ll discuss this topic shortly, but let us assume that battery voltage represents its state of charge.
A battery separator monitors the voltage on the chassis and house batteries and can switch between the two. Switching usually depends on the voltage of the connected batteries. Switching sends current from the tow vehicle alternator to the battery pack needing charging. It isolates the two battery packs, preventing one from discharging the other. A DC-to-DC charge controller works like a battery isolator, allowing current flow in one direction. We discuss DC to DC charge controllers below, which are critical for protecting the alternator and batteries from overheating and premature failure.
For more great maintenance ideas, check out these articles on my website:
- Top 10 RV Tire Maintenance Tips
- Spring RV startup and maintenance
- Winter Maintenance on your RV
- How to Sanitize an RV Freshwater Tank
- Electrical Surge Protection for RVs
- RV and Nomadic Travelers Fire Safety
- Top 18 Tips To Prepare Your RV For Storage
Charging your RVs house batteries
There are several ways to charge your RV’s batteries. You can charge them with your inverter/charger, solar panels, an external battery charger, or the alternator on your tow vehicle if so equipped. These methods require careful engineering to prevent damaging equipment or starting a fire. I will simplify the engineering and put it in laypeople’s terms.
It is also important to note how to charge an RV battery to optimize battery life. It varies by chemistry, but it is generally done in three phases: bulk, absorption, and float. In short, during the bulk phase, the maximum current is supplied to the battery until it reaches the absorption voltage. Then, the current gradually decreases until the fully charged voltage is reached. Then, the float cycle starts and maintains the battery fully charged. For reasons I’ll discuss shortly, it is not a good idea to directly connect your tow vehicle alternator to your RV’s batteries because alternators do not charge in phases.
Another thing to know is that the difference between lead-acid and lithium is that lithium batteries limit the length of the absorption cycle. Absorption length varies by manufacturer. You can find it by looking at the specification sheet for the battery. Check out this article on my website about using specification sheets to compare lithium batteries. How to select the best lithium battery for you!
Before using your tow vehicle’s alternator to charge your RV batteries, here are a few final things to know. The most important is the ‘C-rate.’ It’s how we select the correct charging current and compare battery performance. It is also essential to understand when your batteries are not performing like you think they should.
C-rate is used to engineer batteries and charging systems, so it is essential to know when planning how to use the tow vehicle alternator charge your RV house battery. The C-rate measures the rate at which a battery is charged or discharged. The capacity of a battery is generally rated and labeled at 1C. For example, a battery with a 1C Rate means that a fully charged battery rated at 100-amp hours should provide 100-amp hours for one hour. Or a 50-amp hour rated battery will deliver 50 amps for 1 hour, etc.
Along the same lines of thinking, the same 100-amp battery discharging at 0.5C should provide 50 amps for two hours, and at 2C, it should deliver 200 amps for 30 minutes. This would suffice to describe and simplify engineering a battery system in the perfect world. But alas, it is not this simple. And several factors change this simple formula.
Charging the batteries
Keep in mind that the same concept applies to charging. If you supply 100 amps to a 100-amp hour battery with a rating of 1C, the battery theoretically will charge in one hour. Again, if you only supply 50 amps, it should take two hours. And if you provide 200 amps, the battery should charge in ½ hour. Unfortunately, it is not as straightforward as I mentioned. There are several critical factors to understand about batteries, whether charging or discharging, that impact battery performance and lifespan. Excessive heat kills batteries (and alternators, more to follow). Fast charging and discharging shorten battery life and repeated deep discharging shortens battery life. Extreme temperatures impact battery performance and lifespan. And finally, fast discharging also decreases the battery’s capacity because of Peukert’s Law.
In simple English, the faster you discharge the battery, the less capacity it has. Fortunately, the Peukert effect is negligible for most lithium batteries. Lead acid and other battery chemistries are much more susceptible. Another crucial factor when charging or discharging your RV batteries is temperature control.
Extreme temperatures kill batteries and alternators
Excessive heat and cold significantly reduce the energy a battery can provide and shorten its lifespan. Capacity decreases rapidly in batteries at high temperatures. Heavy loads on an alternator for an extended period also shorten its lifespan. You cannot charge lithium batteries at low temperatures. The list of environmental extremes that impact battery life and performance is long! We are particularly interested in the alternator and how temperature affects its performance.
Alternators get hot when charging your RV batteries
Alternators get hot when they make electricity! This impacts the alternator charging of your RV batteries because if you run the alternator at its maximum capacity for long periods, you can damage or destroy it. The alternator in your vehicle was designed to maintain your chassis batteries and provide power to all other chassis-related loads like headlights, signals, fans, etc. They are usually oversized to provide enough power to recharge the chassis batteries if depleted. For example, it’s cold, and you must crank your vehicle for an extended period to get it started. That will drain the battery much deeper than when it is warm and fires up. In that case, the alternator’s job is to recharge the battery and provide the running load.
Fortunately, most of the time, your chassis batteries are fully charged, allowing the alternator’s excess capacity to be used to charge your house batteries. What will kill the alternator is drawing the maximum current over a long time. I cannot over-emphasize the concept that heat kills.
Heat kills alternators
Recall that as the alternator generates electricity, it also generates a lot of heat! The more electricity you make, the hotter the alternator gets. Top this off with the fact that most alternators are located in a hot engine compartment. Excessive heat will damage the electronic voltage regulator and cause the alternator to fail. Let me give you a practical example.
The scenario is this. You have 400-amp hours of lithium batteries with a 1C charge rate. To simplify the math, let’s assume that you have used 75% of the capacity of the batteries, 300-amp hours. You need to supply 300-amp hours of replacement electricity to recharge those batteries. Note that I am ignoring all the other factors impacting charging to simplify this example!
Now, you hit the road, and your alternator starts charging the batteries. Again, we assume your alternator has 100 amps of excess capacity for simplicity’s sake. Theoretically, you could fully charge your house batteries in a 3-hour drive and probably destroy the alternator simultaneously. Why do you ask?
The alternator on your vehicle was not designed to provide that much energy over that amount of time. Remember that the alternator on your vehicle was designed to provide the running load and charge the batteries. Unless your chassis batteries are deeply discharged, it usually only takes a few minutes of running to replace the energy used to start the vehicle. That’s how you kill an alternator when charging your RV’s house batteries—running it under a heavy load for a long time!
Excessive heat in batteries
RV batteries generate heat when charging and discharging, and both work through the exchange of electrons. Remember that you use electrons stored in the battery’s electrolyte solution when discharging the battery to simplify the concept. You force electrons back into the electrolyte solution when you charge the battery. Both of those reactions generate heat. It’s called REDOX or reduction-oxidation, and it is fascinating.
The chemical reactions releasing or absorbing electrons happen faster and faster, generating more heat. So, reactions increase under high charge and high discharge conditions. It takes more chemical reactions to provide or accept more current. Recall that as the battery heats up, it becomes less efficient, requiring even more electrons to meet electric current demand. You end up in a feedback loop that does not end well.
More current, more heat, less efficient, more reactions, even more heat, much less efficient, requiring more reactions to meet the demand, and the battery gets damaged. I think you get the point, but how do we deal with this when charging from your RV’s alternator? By limiting the amount of current sent to the battery. We do this by placing a device controlling the current amount sent to the batteries.
Current Control and Temperature Monitoring
For example, an RV’s inverter/charger may provide 2000 watts of AC power and 100 amps of DC charging current. This is a typical setup. This means you have up to 2000 watts of power inverted from 12-volt DC to 110-volt AC, and the inverter can send 100 amps to the batteries for charging. Good quality inverters also include temperature monitoring of the battery pack. Depending on the temperature, they will automatically adjust the amount of current sent to the batteries.
Temperature control protects the batteries from overheating and getting damaged. We don’t have this critical feedback when the alternator is charging the batteries, so it is easy to overheat and cause damage to the RV batteries. We have to control this heat by limiting the current sent from the alternator.
How alternators charge
An alternator controls charging with a voltage regulator. They sense the battery’s voltage and send more or less current depending on the voltage. Your typical lead-acid car battery is designed for this type of charge. Deep-cycle lead-acid and lithium batteries are not so it is critical to manage how the alternators energy is used to charge your RVs batteries with the tow vehicle alternator..
The alternator monitors voltage and sends more current, up to the maximum amount it can generate, to reach some set voltage as fast as possible, usually around 14.2. This essentially represents the bulk charge phase we find in battery chargers. With this in mind, I have one more concept to introduce: battery internal resistance.
Battery Internal Resistance
It isn’t easy, but imagine it like this. You are filling a flat tire on your bike with a hand pump. It is easy to add air to the tire when you first start, but it gets harder and harder to put more air in it as the pressure increases. The same thing is true of a battery. When the battery is depleted (low voltage at the battery terminals), it is easy for it to accept a charge. The battery’s internal resistance increases as it charges, making it resist charging.
The regulator will send a charge current in a typical alternator when the voltage drops below 13.5 volts. When the voltage exceeds 14.5 volts, the regulator will stop supplying current. The state of charge of the battery regulates amperage or current. When the battery is weak, its internal resistance is not strong enough to hold back (resist) the current from the alternator trying to recharge it. As the battery reaches full charge, the internal resistance becomes strong enough to oppose current flow from the alternator. Eventually, the amperage output from the alternator will drop to the point where it stops providing current until the battery voltage drops again. The gradual decrease in charging current is similar to the absorption phase found in a battery charger.
State of Charge (SOC)
I want to note that although battery voltage is used to represent the ‘state of charge,’ there is only one way to understand the state of charge using voltage alone. The only way to reliably estimate the battery charge state is to measure the voltage when it is fully charged and not connected to anything! Disconnecting the battery whenever we want to check its’ state of charge is impractical in an RV, so we use a shunt to measure it. It measures the amount of power flowing into and out of the battery pack and better estimates the state of charge than battery voltage alone.
We can better manage our batteries by measuring how much power has been removed or replaced in the battery pack. I like to think of it as a bank account. You have some balance and withdraw a few dollars to buy gas. Okay, at today’s prices, a lot of money. That decreases the account balance, and if you continue to withdraw, the account will eventually reach zero. You recharge the account by making a deposit.
Batteries are Electron Bank Accounts
Batteries are just electron bank accounts. You can use the electrons for power until the voltage reaches the low cutoff point when the battery cannot supply any more electrons. Then, you recharge the battery with electrons, and we measure those electrons as electric current over time, in amp-hours.
To take advantage of measuring all those electrons, we need some smarts. We use amp hours to discuss how much energy has flowed into or out of the batteries. A shunt and smart meter keep track of energy flow to and from the batter. It displays your electron balance in amp-hours (or percentage of capacity left.) They greatly enhance battery management, and I recommend Victron BMS-712 smart battery management system (shown above) or the Renogy 500A Battery monitor with shunt. Both are good systems, but the Victron has more valuable features, in my opinion.
Manufacturers usually list two values for charge rate: the recommended and maximum charging rates. These things seem simple but greatly influence how long the battery will last.
The maximum charge rate is self-explanatory. It is the maximum current (power) you can use when recharging the battery. However, like high rates of discharge, high rates of charge can damage the battery and shorten its life. When a manufacturer provides a recommended charge rate, that rate extends the battery’s life as much as possible. I’ll repeat this another way. It is always better to slow charge a battery than fast charge it!
So you are faced with two challenges when the alternator is charging your RV batteries: controlling the charge rate and keeping the temperature under control. Fortunately, when you control the charge rate, you control the temperature to a certain extent. Because we lack temperature monitoring when the tow vehicle alternator is charging your RV batteries. We err on the side of caution and control how much charge current we send to the batteries with a DC-to-DC charge controller. Here are a couple of scenarios to help you understand how to select a charging rate and method.
You’re dry camping for the weekend and don’t want to run a generator for long periods. In this case, you want to select a highly recommended charge-rate battery. The higher the charge rate, the faster your batteries will charge. The ability to charge at high speed also depends on how much current your inverter/charger can deliver.
Remember that charging at rates above the recommended rate degrades the battery and shortens its life.
In the following scenario, let’s say you depend totally on solar. Your system produces, on average, 25 amps of power (300 watts) available for charging. You can use a battery with a lower recommended charge current. For example, your batteries have a 20-amp recommended charge rate close enough to the 25 amps your system can provide. This minimizes lifespan shortening caused by high charge rates.
One final scenario, add to the example above that you occasionally camp with electrical hookups and would like to charge your batteries when on shore power. Again, you need to balance charge capacity with your expected usage. If you are sitting in the campground using shore power, you are not using power from your batteries. You have all the time in the world to charge your batteries. Remember that the slower you charge or discharge your batteries, the longer they last.
So stop and think about how you will use and charge the battery, and select one that falls within those parameters. You can waste a lot of money picking the wrong battery.
We send a lot of current to the batteries when we fast charge. The battery voltage rises rapidly, but the battery will not completely charge, and the battery will heat up significantly. Recall that as the temperature increases, battery performance degrades, and it will not accept a full charge. This reduces capacity. Another problem, particularly with lithium batteries, is that the manufacturer usually provides two recommended charging currents. When designing your alternator battery charging system, you must pay attention to this because it impacts battery performance and lifespan. In short, don’t fast charge unless the battery is specifically designed for it. Regardless of the charging method, safety requires proper fusing!10% off with RenogSolar10
Fusing when using the tow vehicle alternator to charge your RV batteries
Proper fusing is imperative to protect the alternator and batteries! Remember that hundreds of amps of DC power can be present at some points in the system. This much amperage can kill you, so please exercise proper caution when working with the systems we discuss here.
Selecting the fuse is pretty straightforward. Remember that a fuse’s job is to protect, so it needs to be appropriately sized.
Select a fuse with a rated voltage higher than the circuit’s voltage. Only select DC-rated fuses for DC circuits. Please place them in an area that is easily accessible for future servicing, and be sure to replace the protective cover. The last piece of the puzzle is the DC to DC charge controller.
DC to DC charge controller
When you are using a tow vehicle alternator to charge your RV batteries, you must use a DC-to-DC charge controller in nearly every case. This device controls the current (amps) drawn from the alternator and sent to the batteries. Good ones include the ability to charge in phases and temperature control. Remember to extend battery life; you must charge batteries in stages for best performance, so having this feature is essential.
They act as a shunt because they only allow current flow in one direction, from the alternator to the battery, protecting the chassis battery from being discharged by the house batteries. This affiliate link supports the author by shopping for your DC to DC charge controller. Thank you!
Tow Vehicle Alternator RV battery charging system design
Let us use all this information to design your alternator charging system. We’ll start with standard equipment and do the math for the design.
We have 2x 100 amp-hour lithium batteries at 1C, with each battery a recommended charge rate of 50 amps. Note: on multiple battery systems, you times the recommended charge rate by the number of batteries in the pack. This case gives us a total recommended charge current of 100 amps. The alternator has 55 amps of excess power. We are well under the total recommended charge rate by selecting a 50 amp DC to DC charge controller. A design like this should work well to prolong battery life and still perform excellently.
I hope you found this article on using your tow vehicle alternator for charging your RV batteries useful. I am always open to questions and comments. Please feel free to comment on this article.