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

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.


Jay

Friday 20th of May 2022

Great article and I bought an Orion on your recommendation and because it's what I saw in the wiring diagram I purchased from you.

Anyways, one thing I find funny is I once needed a jump-start because I left my van headlights when I had 600Ah of batteries in the back. The BIM would solve that problem, but I opted to not get the BIM. Are there other options you can recommend to be able to top off the starter battery from my house batteries in case I do something stupid like that again?

Kyle

Tuesday 3rd of May 2022

Hey Nate, thank you for all of your knowledge you share with us.

Quick question, I noticed that on your youtube sprinter builds you use the ground under the drivers seat for the Orion Isolated dc-dc charger. Have you had any issues with that ground? I have been told that it needs to be connect directly to my starter batteries negative terminal but my sprinter doesn't have any extra mounting points. Thoughts? Thanks!!

Nate Yarbrough

Friday 6th of May 2022

We have not had any issues with it; no.

Kaileigh

Sunday 1st of May 2022

Hey Nate! We are looking to wire our Orion-TR Smart 12/12V-30A Isolated Charger to the alternator but our 2021 Ford Transit has a 175 A CCP on the seat, but there is no ground. Based on the wiring diagram we purchased, we are unsure where to run the black wire from the Orion to.

Does that need to be grounded to the chassis? If so, should that wire be the same length as the red wire going to the alternator?

Nate Yarbrough

Sunday 15th of May 2022

The negative should indeed go to a ground location as close to the starting battery or CCP as possible, but the wire lengths do not need to be identical; just 'as close as feasible'.

Rich

Wednesday 13th of April 2022

"If your alternator stops working; so does your engine until you replace your alternator."

This is not true; many vehicles can run for hours just on a battery.

Bruce

Thursday 7th of April 2022

Hi Nate, Our next build will be essentially the same Transit AWD van you are building out now. We will be adding a 600AH (+) battery bank to run a 12V A.C. unit and a factory installed 2nd alternator. Any wiring plans that you have available that would cover that sort of configuration? We're needing to figure out if we can go direct from the alternator to the batteries for high recharge rate, or do we need something in between? Thanks!