one alternator, two batteries, and an inverter

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May 13, 2023
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buying a mercedes sprinter van. want to add a second battery and a 1500 watt pure sine wave inverter to run frig and microwave. is it possible to isolate the secondary battery that is connected to the inverter so that I don't run down the main battery by running the frig and the microwave, and yet still charge both batteries with a single alternator?
If you set up a constant duty solenoid that charges the house battery (and starter battery at the same time) with the ignition is on and the engine running this set up will do it. When you turn the engine off the power to the constant duty solenoid goes off and this then isolates the two batteries. This will accomplish what you are wanting to do.

Watch Bob Wells video on Youtube here:

Installing a constant duty solenoid
Have you considered the house battery set up you'll need to power the fridge and microwave ? A couple of 6 volt deep cycle batteries (think golf cart batteries) wired together in series would be a dependable solution to that need.
There are several ways to do an electrical system.

The simplest way is to buy a power station and plug everything into it. These units combine all the components (inverter, battery, charge controller, outlets, fuses, wiring etc.) of an electrical system in a compact, portable package. Many power stations come with factory or third party warranties. Power stations are sufficient for most simple to medium complexity van builds. Mid sized power stations are capable of running a fridge and microwave.

Power stations by unknown Chinese brands are starting to hit the market and prices are dropping precipitously. With 4 year warranties available from Asurion, I wouldn't hesitate to try something like these:,aps,363&sr=1-3
At the other end of the spectrum is a completely self built system assembled with individually selected components. With this complexity comes the flexibility to cater to specific/specialized needs, like a high amperage appliance. This approach takes a fair bit of knowledge and a willingness to accept responsibility and risk. Should you wish to go this route, YT vlogger Explorist Life has a couple of dozen videos dedicated to electrical systems in vans. There used to be a big difference in price between DIY electrical systems vs power stations, but nowadays the delta is minimal.

Somewhere in the middle is a hybrid system that tries to utilize the advantages and minimize the disadvantages of the above approaches. Permanently installed accessories (eg. rooftop AC, fan, heater, lights, USB ports) are hard wired to a distribution box, which is connected to a power station. This gives flexibility in the placement of accessories while letting the power station do the heavy lifting.

There are multiple ways of indirectly charging via alternator.

Almost all power stations can be charged via a vehicle's cigarette lighter. Typically this maxes out at 8-10A (96-120W). The VS30 cigarette ports are regulated to 10A.

Some vehicles (like the Transit) have built in OEM inverters capable of delivering 400W. This is sufficient for 110V charging of some power stations (like the Bluettti AC200P).

It is possible to charge a power station using 110V by connecting an inverter to the starter battery. It is crucial to select an inverter that doesn't draw too much from the alternator. It is also good practice to tie it to a busbar in the vehicle that is only powered when the engine is running. A warning based on personal experience: AC charging on EcoFlow power stations is pass through, this makes them ill-suited for this charging method. It is very easy to trigger an error code, and could lead to damaging the power station and/or prematurely age the starter battery.

Both power stations and standalone battery banks can be charged using a DC DC charger. Here too it is critical to stay below the limits of the alternator as specified by the vehicle manufacturer. Here too, tie it to a busbar in the vehicle that is only powered when the engine is running.

One last option: a cheap but basic power station. Dr Prepare makes a 100 Ah LiFEO4 battery with a 12V module that makes it capable of powering devices directly. It doesn't have an inverter, so it won't power a microwave, but for $400, it could be used as a starting point for a simplified DIY electrical system.
Well written and good ideas. I’d add the information that I have recently posted in other threads on inexpensive AGM and lithium batteries, as well as the lower priced lithium battery boxes. Prices are indeed coming down.

YouTubers like Hobotech, Will Prowse, City Prepping and GoTechGeek come to mind as some who have made comparison review videos. Be advised that some YTbers are “very positive” about sponsored products. The saying ”Don’t bite the hand that feeds you” comes to mind.