Electrical Shopping list
AGM Battery system
AGM batteries have a few advantages: they're simple, inexpensive and do well in freezing temperature. Lithium Ion batteries are good for other reasons, I will discuss later.
Link as many as you think you'll need. I like the Weize battery because it comes with useful covers for the terminals that help protect from shorting.
Voltage Sensitive Relay is a type of Smart Battery isolator. When the voltage is up on you starter battery, it will let current pass to your house battery.
Safety First! This particular fuse also doubles as a switch. If you leave the van parked for a long time, it's a good idea to disconnect.
Wiring
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For a Sprinter with a battery under the passenger seat (or within 13 feet of the starter battery, I use 4 gauge.
These will need to be soldered to the wire, and heat shrink finishes it off professionally.
In case you need help soldering, I have a demo here [coming soon]
You'll also need a propane torch.
I have one of these where I can see it when I drive.
This is mostly necessary if you are not setting up solar, as most solar charge controllers will show your battery life.
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Know your gauges!
Gauge depends on max amperage, and also depends on length of wire. When in doubt, go fatter.
Power (Watts) = Potential (Volts) x Current (Amps)
So if you're powering a 3000W inverter, calculate your current back from 3000W.
On a 12V system, 6000/12=250 Amps.
So, if your inverter is within 7 ft of your battery. use a 4 gauge copper wire.
(Note: this chart is for Copper wire.)