How-to Charge a Camper Van Electrical System with the Alternator

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In a Camper Van, you’ve already got a generator built-in. It’s called your alternator. Your vehicles battery powers all of the electrical devices that your vehicle needs on a day to day basis. Spark plugs, ECU, Fuel Pump, Air conditioner blower, radio, and much more. The alternator’s job is to keep that battery full.

Now, you’re converting a van into a camper van and plan on installing ‘house’ batteries to run all of your household stuff like, lights, fans, refrigerator, computer and whatever else you’ve got going on.

For most people, their goal is to power these ‘house’ batteries off of solar, which is great! But when the weather gets cloudy and the charge gets low, it’s nice to be able to charge your batteries on a road trip from point a to point b.

A quick disclaimer:

Your stock vehicle alternator isn’t necessarily specifically designed to handle the huge loads that it takes to charge a huge house battery bank. This can over-tax your alternator and shorten it’s life. Our campervan and hundreds of others are operating on this same system, but it’s just something I need to make you aware of.

There are 2 ways to charge your house batteries with your alternator that we’re going to cover: Isolator, and battery to battery charger.

Quick note before we get started.  This is just one part of an overarching “How to Install a DIY Camper Van Electrical System” series.  If you’ve just stumbled on this article directly without seeing that, there are likely some things we’ve already covered.  If you want to check out that step by step guide, you can do that here: https://www.explorist.life/diy-campervan-solar

Also, we have interactive solar wiring diagrams that are a complete, A to Z solution for teaching you exactly what parts go where, what size wires to use, fuse size recommendations, wire lug sizes, and all kind of other stuff to help save you time and frustration.  You can check that out here: https://www.explorist.life/solarwiringdiagrams/

Charging DIY Camper House Batteries with an Isolator

An isolator simply connects your house battery bank to your starting battery. When you turn off your engine, the isolator, “isolates” the two battery banks so that when you use your lights, fans, refrigerator and such, you won’t kill your starting battery, leaving you stranded.

The isolator is simply a pass-through of power and it largely unregulated for the most part in terms of the voltage and amperage that gets sent to the house batteries. The voltage and amperage going to the house batteries is dependent on the voltage regulator and amperage output of your alternator.

Pros of Charging Camper House Batteries with an Isolator:

  • It can charge at a rapid rate, over 100 amps in certain cases.
  • Isolators are less expensive than battery to battery chargers
  • Isolators have the capability to jump-start a weak starting battery in emergency scenarios.

Cons of Charging Camper Batteries with an Isolator:

  • A lack of data makes knowing at what rate your vehicle will charge your house batteries nearly impossible. Making planning difficult.
  • Unregulated power tends to make people nervous during the learning phase.

Charging DIY Camper Batteries with a B2B (Battery to Battery) Charger

A battery to battery charger takes the voltage and amperage that is being put out by the vehicle alternator and regulates it down to a preset and constant amount.

Pros of Charging Camper House Batteries with a B2B (Battery to Battery Charger.

  • Can regulate amperage making system planning easier
  • Can deliver a specific ‘charging profile’
  • Ultimately a bit ‘safer’ than an Isolator

Cons of Charging Camper House Batteries with a B2B (Battery to Battery Charger.

Battery Isolator vs B2B (Battery to Battery) Charger

Which is better? A battery Isolator or a B2B charger? A lot of resources tend to overexplain this with technical engineering jargon. My dad, when somebody was overexplaining something to him, would always say…

“Spare me the labor, just give me the baby”

So, “here’s the baby”: I asked Sean Nichols from Battle Born Batteries which was better for their Lithium Batteries regarding a BIM (Isolator) vs B2B Charger. He said:

“I like the BIM (isolator). It is simpler, quieter and takes up less space. Some people want the B2B charger, which we sell that too, but it does creates a lot more heat”

So there ya go. BUT! He also said in a prior email that:

“We don’t recommend that you charge at a rate more than 50% of your bank capacity.   The battery will handle 100 amps (of charging) also but it will shorten the life-span of the battery.”

In our 2007 Sprinter with stock alternator, we typically see amps near the 100 amps mark into our solar batteries. Since this is well under the 50% mark of our 600 amp hour bank capacity, we are safe. If you only had a single 100 amp battery in your solar bank, that option may not be a good idea. Without further adieu, in a “Spare me the labor, just give me the baby” fashion:

My Recommendation: Isolator vs B2B (Battery to Battery) Charger

If you have a 100 amp battery bank, charge via the alternator using a sub 50-Amp B2B Charger

If you have a 200 amp battery bank, charge via the alternator using a sub 100-Amp B2B Charger. UNLESS your alternator is smaller than 140 amps, you could, in that case, use an Isolator.

If you have 300 amp hours or more in your battery bank: Choose the isolator UNLESS being able to feed your batteries a regulated amperage is important to you.

Final Thoughts & Opinions:

The B2B charger is the safest and easiest to plan for option all-around. That said, both options are perfectly good options. MANY people with 100 and 200 amp hour battery banks are using an isolator with great success (especially the 200 amp hour battery bank + isolator combo). If that’s what you want to do, go for it!

For the purpose of this website, I sometimes have to opt to recommend the safer, more reliable option. But just because I recommend one over the other, doesn’t mean I don’t think both are still fine choices.

Now that you know most of the basics of how to charge your camper van electrical system with your alternator, lets dive a little deeper into the specifics of choosing a battery isolator.  Check that out here: https://www.explorist.life/how-to-install-a-battery-isolator-in-a-camper-van/

Everything that you are learning here is put to use in our FREE Interactive Solar Wiring Diagrams.  If you haven’t yet, check them out as they are a complete solution for a camper van electrical system.  Check them out here: https://www.explorist.life/solarwiringdiagrams/

Remember, this is just one part of a full camper van electrical educational series.  To see all of the individual guides, click here: https://www.explorist.life/diy-campervan-solar

Finally, If you found this guide helpful, It’d truly mean the world to us if you’d share it with somebody who can use it, pin it to pinterest for later reference, or share it to a facebook group when somebody has a question about this subject.  Click the bubble in the lower right corner to subscribe to be notified of future updates and as always, leave any questions you’ve got in the comments below.

25 thoughts on “How-to Charge a Camper Van Electrical System with the Alternator”

    1. I think that if you have a secondary alternator, you will be able to drive more power for longer to your house batteries than you could with a single alternator.

  1. I am wondering if you can go into more detail on the li-bim 226 isolator on where yo wore the ign to and the sign.

  2. I am also wiring my BIM isolator from Battle Born. I can find nothing to show me where and how to wire the ignition wire and where is the +(red) wire with the manual switch going to to get the 12v( to the +bus bar or house battery.. and does the 2/0 red get hooked right on to the + pole of the van battery. I have a Ford Transit 250 with the battery under the drivers seat. Also want some info on how to get you a couple hundred$. I can do PayPal.

  3. Thanks very much for the great advice, I’m a marine electrical engineer with 35 year experience, 30 of that in the Navy, and am very impressed with your knowledge and pragmatic approach. I have found the product advice very useful. My question is, “most inbuilt BMS have a relatively low max charge rate. Is there a risk of a direct alternator connection through an isolator overloading these. Thanks

    1. The Battle Born batteries have a recommended max charge rate of 50 Amps per 100 Amp Hour battery; so It’s recommended to take your Alternator output, multiply that by 70% (since alternators don’t put out what they are rated for (basically)), and make sure your battery bank size is within that parameter.

  4. Hi Nate,

    1. If my energy usage/day ~40Ah, and I’m driving every other day, could I get away with only B2B charger without solar and only a single 100A Battle Born Battery? (I have a used 2007 Sprinter with after-market alternator of unknown Amps).

    2. How many hours of driving does it take to fully charge a 100A BBB(Li) with…. a) 30A B2B ..vs.. b) 60A B2B?

    3. What would happen if I use the 60A Sterling B2B with only one 100A Battle Born Lithium battery? (basically can I get away with this even if it isn’t ideal for the battery?)

    4. Do I have to wire the Sterling B2B charger to an ignition wire or will it turn on automatically? (I’m intimidated by wiring to ignition).

    Thanks! These articles are pure gold!!

    1. Hey Alex! So, as much as I love Battle Born batteries; I don’t know that going that route in such a small system is what I’d recommend. Personally, for a system that small; I’d be going with a goal zero Lithium unit.

  5. 1. I am replacing a 685 cca Deep cycle marine battery, (house battery), with a 100Ah AGM battery. I used to charge the Dcmb through a Battery Isolator. Can I do the same with the AGM? I have a 2013 Ford F-150.
    2. I also bought a 20A MPPT solar controller so that if I am parked somewhere I can use one of my 100W PV panels to charge the AGM battery as needed.
    I would guess that the PV panels go to the Charge Controller and the Charge controller to the battery. And the Line from the Battery Isolator would go to the Battery also. Am I thinking wrong to do both at the same time? Do I need other equipment?

  6. Thanks for all this info! The datasheet for the Victron LFP-Smart 12,8/100 battery states a recommended charge current of <= 50A, and a max charge current of 200A. I won't go into all the details, but to summarize an email from Victron, charging at rates higher than 50A may effect the life of the cells and above 200A may physically damage the battery.

    Since a battery isolator does nothing to limit the charging current, whereas a b-to-b charger regulates the charging current to a known safe amount, I don't understand the popularity of the isolator. Unless you know for sure that your alternator output will never exceed the recommended charge current for your battery, it seems risky.

    My 2019 gas sprinter has a 250A alternator, and I plan to have two 100Ah batteries and therefore would prefer to limit the current to 100A. How do I know my alternator would never put out more than that?

  7. Nate, thank you for all you do. Your knowledge is impressive and I appreciate you sharing it with us.

    Do I need a battery to battery charger such as a sterling if I am using a Goal Zero Yeti with the Yeti Link? If my Yeti is at the back of the van (30 foot run to under the front seat…2019 sprinter 144 4×4), I see people recommending 8 gauge wire…does that seem small?

    Thanks,
    Matt

  8. Hi Nate, I’m looking to put a 12v 800ah lithium Iron “house” battery bank in my not yet purchased RAM Promaster charging both it and my starter battery off of a single, beefed up, (Nations maybe) alternator. Later I’ll add solar panels as well. Is the single alternator doable while protecting both the house bank and starter battery from issues? What other “magic” devices will I need in a system like this?

  9. Isn’t the real danger of a depleted lithium battery bank that it might ask for too many amps from your alternator and shorten its lifespan or leave you stranded? I plan on 3 Battle Born batteries and if they’re run down to 20%, when I start my van, that battery bank will be able to accept the maximum amperage from my alternator, won’t it? What are the chances that overheats my alternator? 97 E450 Super Duty BoxVan with a 7.3 liter Power Stroke diesel for reference. I would like to be able to send the battery bank 150 amps for maximum recharge during a short drive, that’s a major selling point for lithium batteries. Thanks for all the great videos.

    1. Yes. That’s definitely the downside of charging with an Isolator. A battery to battery charger would decrease wear and tear on the alternator. I’ve got a diagram out now that features a battery to battery charger: https://www.explorist.life/3000w-inverter-400-600ah-400-to-1200w-solar-camper-solar-kit/

      But if you are wanting to send 150A, yes you will risk alternator damage depending on how long that load is applied. That is the tradeoff when trying to charge quickly. Remember, an alternator is a mechanical device. The harder you work it, the more wear it will endure.

  10. I have the Battle Born / Precision Smart Isolator and it’s all hooked up, my Battle Born 100AH is below 13.4 which allows for a charge, but it says it does not charge when voltage above 14.4. I can clear say with a volt meter that 14.7-15 amps are coming in to the isolator, and therefor not going out of the isolator as the other side of the isolator registers my current battery voltage. I have a 2019. Anyone have this problem with the vehicle putting out too high of voltage??

  11. I’ve been following for a while now and your videos and tutorials are so good and easy to follow. Just one question. I have an Orion dc/dc smart charger, does that type of charger simply wire directly into the starting batteries and then into the house battery’s or do you have to run a wire to the alternator?

  12. Hi, and thanks so much for your tutorials. This is the part of van building where I’m most out of my depth! Your calculator recommends the Blue Sea add-a-battery kit. I am sure it works based on alternator capacity and hose battery type, and I would like the ability to jump my start battery from the house bank if necessary! However, I was under the impression that I absolutely needed a B2B charger if my vehicle has a “smart” alternator. 1) Is this true, and 2) how can I determine if my 2009 GMC van has a “smart” alternator anyway?!? Please pardon my ignorance here…

    1. I’m going to guess that your 2009 vehicle does NOT have a smart alternator. There should be a sensor of some kind on top of your starting battery if you have a smart alternator. In that case, you should be able to simply use an isolator like suggested.

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