Charge RV Batteries

The fastest way to charge RV batteries is more than just what's supplying the power

You might be expecting me to talk about solar versus generator versus shore power, et cetera. I'll touch on that, but that's not where you need to focus your efforts.

To get the fastest way to charge RV batteries possible, your entire system has to be up to speed (pun intended!). When you see a race car, it's not just the engine that is fancy and powerful, right? Everything from aerodynamics, steering, braking, and safety systems are also enhanced.

This article assumes your RV battery charging systems are working correctly. If your system doesn't seem to be working correctly, check out our article on RV battery charging problems.

How batteries store energy, and what that means about charging

Batteries are really cool and fun to learn about. For now I'll be extremely brief about the basics.

Batteries store and release energy by reversible chemical reactions. Apply a voltage to a battery and some “stuff” changes in the chemicals.

There are various battery technologies available. Each has different specific mechanisms to store and release energy.

The most common types for RV batteries are lead-acid batteries and lithium batteries. You may also hear AGM battery, which is a type of lead-acid battery.

The chemical reactions that occur can only happen so fast. Luckily there are lots of those reactions that occur at the same time, in bulk. There are limits to the speed, however.

The closer your battery is to full charge, the less of these chemical reactions occur simultaneously. This turns into slower charging. Going from 50% to 75% charge (for example) can occur faster than 75% to 100%.

Chemical reaction speed changes with temperature. Cold temperatures mean slower charging, warm temperatures lead to faster charging.

There's a rule of thumb used in engineering that the speed of chemical reactions double with every increase of 10°C. It gets a bit more complicated with batteries because heat is generated during charging.

Some batteries have built-in heaters that use its own battery power to keep it warm. Battery heaters are available to keep your batteries warm in cold weather.

Battery technology impacts charging speed

The type of battery you select makes a big different in the fastest charging speed. When designing or upgrading your RV battery system, make sure to keep this in mind.

There's a reason why LiFePO4+ batteries are the current leader in RV battery technology. While lithium batteries are expensive, they have a great characteristics.

Among their characteristics are the ability to charge very rapidly. As a broad generalization, the amp hour capacity of a LiFePO4 is the same as the maximum charging rate in amps.

Contrast this with a lead-acid battery, where the maximum charging rate (in amps) is more like ⅓ of the amp hour capacity (in amp hours).

Now, I'm very much aware that changing to a lithium battery bank is a very costly endeavor. In addition to the batteries it likely means you will need to update your battery charger for all systems.

Don't worry, there are plenty more tips to the fastest way to charge batteries.

Adding parallel batteries for faster RV battery charging

Remember how I said those chemical reactions happen in bulk, at the same time? That's called parallel processing. You can take this concept to the next level and parallel your batteries.

By adding additional batteries in parallel to your RV, you can work around the slow chemical reaction issue because you're charging two or more batteries at the same time. A battery that can only charge at 20 amps now becomes a battery bank that can charge at 40 amps, 60 amps, or more.

When updating your system or starting from scratch, you can select a large single battery or multiple smaller batteries in parallel to combine your battery bank amp hours.

If using lead acid batteries, make sure you select deep cycle batteries. Starting batteries, which is a standard car battery, are designed for very high discharge rates for a very short time. Deep cycle batteries are designed for lower discharge rates for a long time.

Note: Your battery bank must have the same type of batteries. In other words, you should only parallel batteries of the same chemistry.

Lithium batteries operate at a slightly higher voltage than lead-acid batteries. Combining the two will deplete the lithium batteries as they overcharge the lead acid batteries!

You can combine different battery technologies using a dc-dc charger properly configured. This will account for the voltage differences in the system.

Second note: installing additional batteries to an older battery system can also cause issues. This one isn't as severe as the issue above, but can lead to having less capacity than you think.

It's best to replace all batteries at once, but if you must add to an existing system, just know that there's a chance there will be some parasitic drain in the system. Think of it like installing a leaky faucet.

Third note: there's a myth that you can only parallel batteries of the same capacity. This is not true! You might have a 100ah deep-cycle battery and add a 75ah deep cycle battery. You end up with 175ah of capacity, with no issues.

Keep your wires cool for the fastest possible RV battery charge

I said earlier that batteries should be warm, however, nothing else should be warm. If you've ever jump started a car, or unplugged a high-power heater that had just been running, you may have noticed the wires get warm.

Conductive wires are used to transport electrons, which is the electricity we use. This transfer causes heat. The more we have to “force” high currents through thin wire, the hotter it will get.

The solution is to use thicker wire for high currents, which stay cool. Your RV might not have been designed for high-current applications, such as fast charging.

In our motorhome, the wire connection from the RV power converter to the battery is only 10 gauge wire. 10 gauge wire is well suited to applications under 20 amps, but will warm around 30 amps, and get very hot at higher currents.

This heat is wasted power. Wasted power isn't charging you battery, and it creates fire risk.

Upgrading my wire to 4 gauge wire (lower number wire gauge is thicker) would see the resistance drop significantly, and would generate less heat. This means I could deliver more power to my battery bank, assuming my power delivery device is up to the job, and assuming the battery bank can take in power that fast.

Good connections make for good RV battery charging

We also have to make sure everything is connected very well. Poor connections will get hot and limit the amount of power – the RV battery charging speed - that can be transferred.

Jumper cables have small contact areas that bite in. While this is a good idea to break through corrosion for the temporary connection, it's not an optimal electrical connection and will generate a lot of heat.

We need to ensure that all connections are not corroded, are tight, and have large transfer areas. Old batteries can have corroded battery terminals, which can be cleaned and tightened.

Any new installations, such as solar panels, should use high quality connections and good crimping techniques to terminals.

I recommend purchasing a crimping tool to make your connections. There are some good DIY crimping options available as well. Make sure they have good electrical contact and mechanical strength.

The size of your battery charger can impact RV battery charging speed.

Let's use the example of your RV's power converter. This takes 110V AC power, via shore power or a generator, and turns it into DC power for your battery bank.

A power converter has a maximum capacity. Perhaps you have a 55A power converter. You can't expect any more than 55A from this, of course.

You could upgrade your chargers. In the case of your RV power converter you could opt for a 100A power converter.

This same concept applies to other technology as well. Your wind turbine or solar charge controller could be upgrade to allow a higher maximum current.

Just a reminder, make sure your cables are sized appropriately when upgrading components. As mentioned above, heat is wasted energy and limits your battery charging capacity.

Incoming power can limit your RV battery charging rate

If you want a high power, fast charge to your RV battery, you need to start with high power as well. This will depend on the charging technology.

For shore power connections, use a high amperage line. If you use the RV park's 15a line, your entire RV is limited to about 1800 watts. This is for everything you use, fridge, lights, air conditioning, all of your electrical appliances and devices.

Connecting to a campground's 30a line doubles that available power. This ensures you have enough power to quickly charge your RV's battery system while also using power for other devices.