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

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In a Camper Van or Motorhome, you’ve already got a generator built-in. It’s called your alternator. The engine spins the alternator which charges your starting battery which 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, computers, and such.

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. This blog post is going to talk about the various ways to charge your house batteries from the Alternator:

Why You SHOULDN’T Use the Alternator to charge your house battery bank:

Something to keep in mind: an alternator is a mechanical device with lots of moving parts. These ‘parts’ will wear out as you use them. The harder you work your alternator, the sooner it will fail. If your alternator stops working; so does your engine until you replace your alternator. This is why, over time, I’ve migrated to a mindset of opting for slower alternator charging (fewer amps) and a way to turn off alternator charging so that the solar panels can do their job, even when driving down the road.

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.

How fast will a Battery Isolator Charge a House Battery Bank from the Alternator?

In order to find out how fast our house batteries can charge from the Alternator, we need to know how big of an alternator we have. From there, we can estimate that ABSOLUTE BEST CASE SCENARIO we can anticipate a charging amperage of 70% of the nominal alternator output.

So, if we have a 220 amp alternator… that would mean the most amps we could see coming into our house battery bank is 154 amps. Now… based on real world info gathered from readers of this site, the actual charging rate of a 220 amp alternator is closer to 100 amps.

This means that if we were pushing 100 amps into our house battery bank over the course of an hours drive, we will have refilled 100 amp hours into our house battery bank.

This is also a very rough figure because as the batteries get closer to full, the slower they will charge.

So, to answer the question: “How fast will my house battery bank recharge while I’m driving?” You could VERY roughly calculate 50% of your alternator size in amps times the number of hours you are driving, and this will tell you how many amp hours will be pushed back into your battery bank.

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:

  • Higher charging rates can lead to premature alternator wear and potentially alternator overheating.
  • It’s likely that your alternator does not put out high enough voltage to charge Lithium batteries to 100%
    • Lithium Batteries generally need to be charged at 14.4-14.6 volts once they get near 100%
    • Vehicle Alternators generally put out 13.5V to 14.5V depending on their quality, design, and age.
  • Large wire size, potentially up to 2/0 wire, should be used due to minimizing voltage drop (Less than 3%) while accommodating high charging rates.
  • Varying alternator sizes and vehicle setups make predicting charging rates with accuracy, difficult.
  • If the Isolator is not specifically designed for Lithium, it’s connect/disconnect parameters will be too low for use with Lithium batteries and may not disconnect the house and starting battery bank properly when the engine is off.

Charging DIY Camper Batteries with an Li-BIM

The Li-BIM (Lithium Ion Battery Isolation Manager) is a popular isolator designed specifically for use with Lithium Batteries. It has higher voltage open/close parameters that allow the isolator to open and close at more appropriate times depending on if the alternator is charging the house battery bank or shore/solar is able to charge the starting battery.

The most unique thing about this isolator is it attempts to combat alternator overheating by providing time based ‘cycling’. This means that when the engine is running and the isolator is allowing the house battery bank to charge, the Li-BIM will allow charge for 15 minutes, disconnect charge for 20 minutes to allow the alternator to cool, reconnect for 15 minutes, disconnect for 20 minutes, and this cycle will continue the entire time the engine is running. This takes some stress off of the Alternator.

How fast will the Li-BIM charge my house battery bank from the Alternator?

Since the Li-Bim allows charging for 15 minues and then disconnects for 20 minutes, this means that it is only charging for 42% of the time.

Now, Using the same specs from earlier. If we have a 220 amp alternator that is charging our house battery bank at, say, 100 amps; this means that it would be pushing 100 amps into our battery bank, 42% of every hour we are driving. This would give us 42 amp hours into our battery bank for every hour we are driving.

Pros of Charging Camper House Batteries with the Li-BIM Isolator

  • It can charge at a rapid rate, potentially over 100 amps depending on the vehicle.
  • It is an isolator with voltage connection/disconnection set-points specific for Lithium Batteries.
  • The Li-BIM has the capability to jump-start a weak starting battery in emergency scenarios.
  • The time-based charging ‘breaks’ allow the alternator to cool between high amperage charging rounds.

Cons of Charging Camper Batteries with an Li-BIM Isolator:

  • Higher charging rates can lead to premature alternator wear.
  • It’s likely that your alternator does not put out high enough voltage to charge Lithium batteries to 100%
    • Lithium Batteries generally need to be charged at 14.4-14.6 volts once they get near 100%
    • Vehicle Alternators generally put out 13.5V to 14.5V depending on their quality, design, and age.
    • Although the Li-BIM is designed specifically for lithium, it cannot and does not boost voltage in any way.
  • Large wire size, potentially up to 2/0 wire, should be used due to minimizing voltage drop (Less than 3%) while accommodating high charging rates.
  • Varying alternator sizes and vehicle setups make predicting charging rates with accuracy, difficult.

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 the amperage down to a modest pre-set level and boosts the voltage as necessary to fully charge any batteries.

This decrease in amperage lowers the strain of your alternator greatly and also gives us a solid number from which to work with when being able to accurately determine how many amps the charger will operate at.

The boost in voltage is INCREDIBLY important for being able to charge higher voltage lithium batteries from alternator that can’t quite put out that much voltage. If the power from the alternator is hitting the DC-DC Charger at 13.8V and the Batteries need to be charged at 14.6V, the DC-DC Charger will boost the 13.8V from the alternator to the 14.6V the batteries require. On DC-DC Chargers, this voltage is also programmable so it will work for Lithium, AGM, Carbon Foam, Salt Water, or whatever other battery type you throw at it.

How fast will a DC-DC Charger charge my house batteries from the Alternator?

Each DC-DC Charger has it’s own charging rates. For example, the Victron Orion 12|12 – 30 will charge your house battery bank at 30 amps. This means, that if you drive for an hour, you will push 30 amp hours back into your battery bank.

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

  • Can regulate amperage making system planning easier and decreases wear and tear on the alternator.
  • Can deliver a specific programmable ‘charging profile’ to the batteries.
  • Can boost sub-optimal voltage to a level needed to charge batteries to 100%
  • Some models of DC-DC Chargers can be run in parallel, providing more amps if necessary.

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

Final Thoughts: Isolator vs DC-DC Charger

Since the introduction of the Victron Orion 12|12-30 DC-DC Charger, I have been all aboard the DC-DC charger train. The biggest selling point for me is the voltage boost. Having a device that will boost the voltage of an underperforming alternator to the 14.4-14.6V it takes to charge lithium batteries like our Battle Born batteries, is the main selling point for me. As much as I would like 100+ amps coming to my batteries, sometimes it’s risk vs reward and getting towed to the shop or performing a parking lot alternator change due to an overworked/overheated alternator, just plain sucks, IMO; but please use your own judgement when making this decision for your own camper.

39 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.

      1. Nate, thanks for another great article. My Sprinter conversion is all electric /w 620W solar and a 60W Sterling B2B and a Victron Multiplus 3000 all connected to solid copper bus bars per your directions.

        You provide solidly reasoned advice and it is much appreciated.

        Happy trails.

  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

      1. Goal Zero makes a product called Yeti Lithium 12v Car Charging Cable, which connects a 12v car outlet to one of their Yeti power stations. What is your take on this arrangement?

  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.

  13. Hi Nate,
    Good stuff. I recently decided to move away from the isolator that came with my truck camper. After paying $900 for a couple of 6-volt AGM batteries with a total of 300Ah I am motivated to protect my investment and get the maximum life from these batteries. So I added a 50 amp DC-DC Charger. This now provides a 3-phase charging profile that matches what my battery needs to charge it 100% and limit the current during the absorption and float phases of recharging. Otherwise there is no limit on the amperage going through an isolator and this could be detrimental to the battery life. In addition, some systems with lower Ah batteries would be getting overcharged even during the bulk phase. The DC-DC Chargers allow you to program the maximum amperage to eliminate this problem as well. The cost will be offset if I can get another couple of years out of my batteries. Keep up the good work Amigo.
    Dave

  14. What do you think about victron energy’s Cyrix-i battery combination because I don’t have much information about it yet

  15. Hi Nate, really appreciate your guidance and instructional videos to get a grip on motorhome/van electrical systems. Awesome work.
    I have an older motorhome and am putting solar into it. The coach currently has an isolator. Can this be removed if I use the sterling B-B charger, or should I install the charger between existing isolator and the Lithium battery bank? Thanks in advance.

    James

  16. Hi Nate,
    Thx for your support.
    I came across the opinion that using Victron Cyrix-LI-ct battery combiner for alternator charging would be the safe option that won’t harm the alternator. What is your opinion? Any pros and cons of such option? My setup would be : alternator from the 80′ and the LiFePO4 on the other end.
    Take care!
    Tom

  17. Nate, I tried using the Alternator Charging Calculator. But after the first result I kept coming up with the same Result, even with an input of only 40 amps. The calculator does not seem to be resetting properly.
    hanks
    COPY OF CALC INPUTS & RESULTS:
    Results
    Recommended Alternator Charging Method60A B2B Charger
    Estimated Max Charging Rate (Amps)60
    Inputs
    Alternator Size (Amps)
    40
    Battery Bank Size (Amp Hours)
    200
    Battery Type

    Lithium
    What’s Most Important?

    Alternator Heath
    Is Self Jump-Starting Important to You?

    No

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