When most of us research off-grid power in a van or RV, the first question is “how many solar panels do I need to install?” But our vehicles can power our lives too, because they have a built in generator. 

A vehicle’s alternator takes energy from the engine and uses it to charge the vehicle’s battery. A battery-to-battery charger prompts the alternator to generate more energy. That energy is then passed into our house batteries.

The alternator used to be called a generator, and it works in the same way. The mechanics are similar to energy generated by water falling over a hydroelectric dam and spinning wheels along the way. The engine cranks a wheel on the alternator and generates energy.

There are two types of devices we’re looking at: isolators and DC to DC aka Battery to Battery chargers.

We give you background and how-to info so you can make an informed buying decision. Our guides are based on technical research and practical experience building our van on a budget, and now living in our van full-time. If you're already savvy, or just want the recommendations..
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Do You Need Alternator Power?

How do you live your life on the road? The benefit of alternator charging is directly proportional to the time spent driving.

  1. There’s some newer products that make this whole thing a lot easier. Combination charge controller & alternator chargers (like the Renogy one we recommend here) are now available. If you’re designing a new electrical system, we definitely recommend charging from the alternator. You’re going to have a charge controller anyways, and the combo devices are only a bit more expensive. We like Renogy’s DCC50S for this.
  2. Alternator charging can increase the lifespan of your battery. Simply, for battery health you want to charge your battery to 100%. If you solar panels can’t keep up with your demand, the alternator can make up the slack.
  3. Depending on your usage, you might actually be able to skip solar panels altogether. If you drive a lot and don’t consume a lot of electricity. A budget setup for off-grid electricity could be just a battery to battery charger and a 12V battery.

You will also need to consider all the potential issues with the three main charging options:

  • Alternator:The vehicle needs to be running for quite a while to get a meaningful charge from the alternator. A 15-minute drive isn’t going to be much energy. Compared to solar panels, it’s pretty limited. Solar panels are trying to generate energy all day, while the alternator only works when the engine is on.
  • Solar: Cloudy days will happen. Depending where you are for the winter, they might happen a lot.
  • Shore Power: Plug in to an outlet to charge your batteries! No sun or driving necessary. But you do need to be near an outlet (no boondocking).

How to Select the Size of Amps for DC to DC Charger

Next, you need to choose what size (how many amps) charger you are shopping for. We will do some simple math before deciding on the correct charger for each system.

The charger size depends on the capabilities of the battery bank. (If you have more than one battery, use the combined Amp-hours for this comparison). Here’s the rule of thumb:

  • AGM: Charger Amp rating = Battery Ah capacity * 20%
  • Lithium: Charger Amp rating = Battery Ah capacity * 50%
  • So, a 100Ah AGM can use a Charger up to 20A
  • 100Ah Lithium can use a Charger up to 50A

We recommend checking the specification sheet for your batteries. Here’s a screen shot from RELiON’s 100Ah battery specifications:

charger-controller-amp-size-lithium

This battery is recommended to be used with up to a 50A charger. And if we had two of these batteries in parallel, we could use a charger up to 100A.

Still deciding on a battery? We compared AGM vs. Lithium and chose our favorites.

Best DC to DC/Battery to Battery Charger and Isolator for Van and RV

Now that you calculated the amp rating of charger for you (you did that, right?), here are our favorites:

For the best value DC to DC charger for any type of battery:

Renogy DC to DC Battery Charger

renogy-dc-to-dc-charger-van-rv
Best pricing available on a reliable charger from reputable company. Available in 20A, 40A, and 60A sizing

Renogy is known for their good, reasonably priced products

DC to DC chargers work with Lead-Acid and Lithium batteries. Unlike isolators, they will keep your alternator safe and maximize your battery’s lifespan

Wire required from charger to vehicle’s ignition circuit

  or   (get 10% off with our link and promo code)

We recommend DC to DC chargers over isolators because they protect the vehicle’s alternator, and maximize the battery’s lifespan. DC to DC chargers provide a multi-stage charge to the house battery and push more charge current than an isolator.

Renogy’s DC to DC charger packs all the same capabilities as any other manufacturer’s, at a lower price point. One note is that you will need to run a wire from the charger to your vehicle’s ignition circuit. This wire is included with the charger. For a Transit, this is no big deal as the ignition circuit connection is right next to the vehicle battery.

Renogy sells 20A, 40A, and 60A chargers. For a lithium battery, you can use any of them. For an AGM battery, your max charger size is 20% of your battery’s Ah. For example, 100Ah AGM battery should use the 20A charger.

For 10% off this and any other Renogy item, visit them using our link and promo code.  Find item “DCC1212

If you’re designing a new system and need a DC to DC charger AND solar charge controller:

Renogy DCC50S DC-to-DC Charger and MPPT Solar Charge Controller

charge-controller-solar-van-renogyDCC50S
Simplifies and saves money on electrical system by combining components into one

Quality MPPT charge controller

DC to DC chargers work with Lead-Acid and Lithium batteries.

50A charger great for most system, unless you have less than 250Ah of AGM batteries

  or   (get 10% off with our link and promo code)

We’re excited by the new DCC50S Charge Controller MPPT from Renogy. This thing wasn’t out yet when we designed our electrical system in 2018. What’s cool is it’s a combo Solar Charge Controller and DC to DC Vehicle Alternator Charger.

Traditionally the charge controller and B2B charger were two separate pieces of equipment. If designing our system today, we’d definitely buy the Renogy DCC50S. It will even trickle charge your vehicle battery once it senses your house (solar) battery is full.

The solar and alternator capabilities of the DCC50S have standard specs that match those from any stand-alone equipment. It can function with up to 660W of solar panels (that’s a lot!), and any vehicle alternator.

There has been some confusion on the specs, so we confirmed with Renogy. It has 50A total current capacity. This is not mutually exclusive – for example, if there is no solar coming in, then it can pass up to 50A from the alternator. Vice versa if you aren’t driving and there is lots of sun, 50A can come in from the solar. This is a good design because the device switches its 50A capacity between whatever charging source is working for you at the moment.

The only functionality it’s missing is an LCD screen or Bluetooth to display charging levels. Fortunately, most of these stats are displayed on the battery monitor you’ll want to be getting to monitor your battery levels and health.

For 10% off this and any other Renogy item, visit them using our link and promo code.  Find item “DCC50S”

Tight budget? You can charge with an isolator if willing to take the risks:

Keyline 140A Smart Isolator

battery-isolator-van-rv-best
Cheaper than DC to DC chargers. Easy install.

For this willing to accept the downsides, this isolator works as intended

Won’t provide as much current as DC to DC charger

Will reduce lifespan of lead-acid batteries. Doesn’t work with lithium.

Isolators were the tried and true alternator charging method for many years. But with the advent of “smart” alternators, they are a lot less effective. We’ve got a comparison isolator vs. DC to DC charger farther down in this article.

This isolator will not work with a lithium battery. For a lithium battery, you’d need Battleborn’s specialized isolator. But at that specialized price, you might as well get a DC to DC charger.

To summarize, we would only recommend isolators in this situation: older vehicle with AGM/lead acid battery, and willing to reduce your battery’s lifespan.

The Other Alternator Chargers:

Sterling Power BB1230 Battery to Battery Charger – Sterling makes another reliable DC to DC/B2B charger. Simple reason we haven’t recommended this one – it’s no better than Renogy’s option, but it’s more expensive. The unit is also pretty loud.

Victron Orion 12/12/18 DC to DC Charger – Victron is another off-grid juggernaut brand. We would certainly trust this product, but see no reason to select it over the Renogy option. It offers no features that Renogy doesn’t offer.

Battleborn Lithium Isolation Manager – For a lithium battery, you need a specialized isolator. But at that specialized price, you might as well get a DC to DC charger.

Charging with Isolator vs. Battery-to-Battery/DC-to-DC

There are two types of devices you can use to charge from the alternator.

Isolator

The vehicle and house battery are combined into one, but your house battery is “isolated” from the vehicle battery. A “smart VSR isolator” ensures you don’t drain your vehicle battery. The isolator diverts energy to your house battery only when the vehicle battery is at an adequate voltage.

Isolators are simple devices and don’t function well with newer vehicles with “smart alternators.” Most newer vehicles have implemented smart alternator technology because they improve gas mileage. In short, smart alternators only do as much as necessary to charge the vehicle’s battery, and no more. Further reading: https://www.redarc.com.au/alternator-vs-fixed-alternato

Smart alternators produce minimal extra energy. For that reason isolators have minimal extra current they can pass to our house battery. So for Transits, Promasters, and newer Sprinters, isolators won’t charge the house batteries as much as a DC to DC charger. Here’s the other thing: smart alternators sometimes push huge current through the isolator. In those cases, we’d be charging the battery at a rate that exceeds the battery’s acceptable charging rate.

For AGM batteries: Isolators do not provide a full multi-stage charge to your house battery. This significantly reduces the battery’s life because a full charge is necessary. On the flip side, if you are getting solar and/or shore power as well, those sources that make up the slack and charge it to 100%.

For Lithium batteries: You need to buy a Lithium specific isolator. Unfortunately it’s barely cheaper than the B2B options so you should just get one of those.

Battery-to-Battery/DC-to-DC Charger

DC-to-DC chargers (aka “Battery to battery” chargers) do more than just pass through current. B2B chargers boost the current and buffer the cycling of smart alternators. B2B chargers are “set it and forget it.” Once installed, it will automatically switch on and off based on your vehicle and house battery voltage.

The current limiter in B2B chargers prevents overworking alternators.

They also deliver a multi-stage charge to house batteries, which maximizes the battery’s lifespan.

So what do we do, isolator or DC to DC/B2B charger?

For us the choice is easy, DC to DC/B2B charger for 90% of electrical systems. We would only recommend isolators in this situation: older vehicle with AGM/lead acid battery, and willing to reduce battery lifespan.

How to Wire a DC to DC/Battery to Battery Charger

The wiring of a standard B2B charger is pretty simple. Take a look at this image from Renogy. Essentially, positive and negative to vehicle battery, and positive and negative to house battery.

b2b-alternator-charger-wiring-diagram-van-rv-solar

Be sure to add fuses or circuit breakers near both batteries. These should be sized about 20% greater than the current. For example, with Renogy’s 60A charger you should get a 70A breaker.

We recommend circuit breakers so you can always turn the current flow on or off. Additionally, if there is an electrical short you would not have to replace the fuse, you just flip the circuit breaker back on.

Renogy’s B2B chargers and  also require a wire to the ignition. For the Ford Transit, the ignition circuit is located near the battery under the driver’s seat:

Transit-alternator-charging

Wiring Renogy’s DCC50S charger is a little different because it is a combo charge controller and charger.

We sincerely hope this information is helpful on your build journey!

This post contains affiliate links that may earn us a commission if a product is purchased. But, we always strive for the reuse and repurpose of materials – so we encourage searching for a local used option before buying from our links.

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