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