An issue with "big" Inverters and LFP batteries. Pre-charge!

[rickst29]

A 4-position, high current battery switch (rotary type) contains 3 larger terminals inside. They are intended for "battery #1", 'battery #2", and outgoing "load".

But, in this application, you wire the switch with the big +12v battery bus wire(s) on the "load" terminal. On "B1" (the marking for the "battery 1" connection), you have equally an equally big cable (or 2 smaller cables) going to the Inverter's 12v input connection lug. On "B2", you can use a much smaller wire (AWG-14, allowing up to 15A, would be more than adequate). This wire also ends at the Inverter's 12v input lug. But along that wire, you add a high-wattage resistor of size 10 or 25 ohms.

25 ohms on the "12v" circuit would allow current of about 0.5 Amps, dissipating about 8 watts of waste heat. That resistor could probably be used without being attached to a separate heat sink, although I would not mont it directly to a wood or plastic surface. A 10 ohm resistor would generate 16 watts of heat, but allows the Inverter to become "charged up" in half the time. It will become quite warm during the process, and should be mounted on a heat sink.

A 5 ohm resistor would cut the pre-charge time even more, but would allow 2.5 amps, and generate about 33 watts of waste heat (and would definitely need a BIG heat sink.) I do not recommend using a 5-ohm resistor for this job.
- - -

After you have turned on the 'Converter' and 'Air Conditioner' circuit breakers, you would activate the Inverter by rotating the battery selector from "OFF", to instead connect "#2 ONLY". Then, after around 20 seconds, you rotate to "1&2", connecting the main circuit. Immediately rotate the switch by another step, to leave the switch connected on "#1 ONLY".

Switching to "#1 ONLY", from "BOTH", stops the resistor from wasting power while you run the Inverter. When you are finished with the Inverter, having first turned off all 120-VAC loads and then turned off the Inverter itself, you can rotate the switch another 90 degrees, ending at "OFF".
- - -
If you will be using the Inverter for more than 1200 watts (e.g., a household 120-VAC coffee maker @ 1400 watts, or a "1000 watt" microwave) your main wire size from the "12V" battery bus should be pretty large, as follows:

The microwave oven is worse than the coffee maker, because it is a very reactive load. "1000 watts" / .65 typical "power factor" of microwave ovens = 1540 watts consumed. In contrast, the "power factor" of the coffee maker, being a purely resistive load, is 1.0. Its power consumption at the Inverter is simply the labeled rating, e.g. 1400 watts, divided by 1.0 = 1400 watts.

1540 watts consumed, / 85% Inverter efficiency, is going to be about 1800 watts of DC input power at the Inverter. At 12.6-12.8v delivered Voltage from the LFP battery pack, that's around 143 Amps. I'd want to have AWG 4/0 connecting that Inverter, or dual AWG 2/0 cables (for each direction). Even if the wires are short, the maximum ampacity is a serious issue - and you want to keep the generated heat (within the cables) well below their rated limits.

My own Inverter is rated higher, and perhaps used a bit higher. I have dual AWG 2/0 leads on each Inverter connection bus. If the BMS on your upcoming battery pack doesn't allow for up to 200A continuous, don't buy it.

[rickst29]

Originally Posted by Wavery

I have a Newpowa, Powall 2000W inverter. It has a remote read out showing all of the input and output loads. When I first turn it on, I can see the voltage ramping up on the 110V volt read-out. I left mine on for 5-days on our recent camp-out (because I was running a 110V fridge) and it seemed to draw so little at idle that the current usage didn't even read out.

However, on previous campouts, I did turn off the inverter when not in use and I noticed that it came on instantly with no "ramp-up" sequence (after the 1st time turning it on)...
I do intend to buy a 200AH LFP battery soon.

First: Lead-Acid Batteries have no need of precharge, because the internal resistance of the batteries themselves is so much higher than the internal resistance of LFP battery packs. The requirement (which may or may not be present with a particular Inverter) only becomes apparent with LFP batteries.

Second: There are very, very few vendors of "good" pre-built battery packs, and even fewer vendors of "good" pre-built packs in sizes larger than 100A. To assure quality and capacity, I recommend that you build your own battery pack. Under normal circumstances, that costs considerably less money. It would take about 3 hours (uncompressed) or maybe 4-1/2 hours (State of the Art - Compressed).

However, I have a special deal for someone "building their own" 12v LFP pack during the next month or two. I have a 200A/400A "Daly/Delgreen" Smart BMS, for 12V 4S only, (4 battery cells in Series = 12.8 Volts), which I will be removing from my new 230aH battery pack. It runs perfectly, and I've tested all of its functions except "charging current limiter". It's going to be for sale, either here, or on the leading LFP battery builders' forum.

I am the designated tester for a brand-new BMS design from 'E&J Technology Group', AKA "JK BMS". Within a few hours, I will be removing the DALY from my battery pack, and replacing it with the JK (for testing of the JK). The DALY should be sitting on my test table, waiting for sale to someone else (unless I succeed in "blowing up" the new JK design while staying within its 200/350 design limits).

I can sell that DALY BMS to you at a good price. You would only need the 4 battery cells, 3 GOOD bus bars, and a home-built case (maybe with compression springs and rods) and a "better" power converter, to build the battery pack and make your conversion. Building the case takes a relatively short time, because it's mostly "open" within the TM rear compartment.

photo is my current 230aH battery pack, show with a few accessories, but lacking the second pair of cables into the Inverter. The DALY/DELGREEN Smart BMS (and it's Bluetooth Interface) are still attached. (I'll be headed to Lowes Hardware today, to buy more AWG 2/0 for those additional Inverter cables.)

Amazon wants $173 for this BMS - sold to you without correct programming yet done, and balancing wire lugs not yet attached. I'll sell my programmed and ready-to-go unit for $120, and I will pay the shipping.