[rich2468]

Originally Posted by rickst29

An interesting new hassle occurs between an LFP battery string and a "big Inverter" (1500 continuous watts are higher).



When 120v Inverters are connected to 12v batteries and turned on, they want to "charge up" some big internal capacitors. With a lead-acid battery pack, internal resistance of the batteries themselves will prevent this "pre-charge" current from becoming too high.



But LFP batteries have almost no internal resistance. High current will occur. The connecting DC power leads, if sized for the maximum continuous load, will not be harmed by momentary current mayn times higher than their ampacity ratings - but the high current may either blow a fuse or cause the battery "BMS" current protection circuit to shut down discharge completely. If the BMS shuts down the discharge circuit, the Inverter can't even complete startup.

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This should be prevented by using a 2-way battery selector, the "dial" type which selects from 2 battery power sources. (The 4 dial positions are "off", "battery-1-only" "both", and "battery-2-only".) They have 3 connectors, one for each of the 2 batteries, and the third for the "load" port.



But, in this application, your "battery-2" wire goes to the common bus (12v) for all of your batteries together. That's the huge load-carrying wire. The "battery-1 wire", which need only be AWG-10, goes from the common bus and through a 25-ohm, 100 watt resistor before reaching the selector switch.



Before turning on the Inverter, you switch the "battery selector" from "OFF" to "Battery-1 Only" for several seconds. This allows the Inverter to charge up those capacitors through a high-resistance circuit, slowly and smoothly. Then switch the selector switch through "BOTH", wait another second, and then switch it to "Battery-2-only". This second switch position prevents the Inverter from pulling 12v power through the resistor circuit and wasting energy.



If you have an automatic transfer switch and the Trailer is NOT plugged in on the main power cord, you may then Plug in your 120-VAC appliance(s), and use them. Or you may use your appliances directly on the inverter's own power sockets. Either way, when you are done and want to turn off the Inverter, turn off the Inverter first, and then turn the "battery selector switch" to the OFF position.

Rick- thanks for sharing.

I have a 3000 Watt pure sine wave Renogy Inverter powered by at Lithium bank and an auto switch setup to automatically cut my shore power connection when the Inverter kicks in to power my 120 volt needs in the camper.

I've always ensured that I have all 120 volt loads off during the switch, but i`ll need to investigate the need for pre-charge (as well).