Do I need to pre charge a 24v 1500w inverter before connecting to LiFePO4? Can anyone recommend specific resistor to buy?

[73powerstroke]

24V 25W Stop light from an automotive shop works perfectly. Starts bright, goes dim, when off, you're done

That makes more sense. And then you know it's not going to spark. Excellent

 

[squowse]

10W resistors are cheap and easy.
10W / 24V = 0.42A
24V / 0.42A = 57 Ohms

Probably have to choose between 47 ohm and 100 ohm. I would go 47 ohm on the basis that it could handle more than 10W for a few seconds.

 

[Re-Volt-Oz]

That makes more sense. And then you know it's not going to spark. Excellent

I really don't know why this method is not discussed more. It works really well with readily available parts. I used this method yesterday while testing an MPP-2424. As well as giving me an idea when the capacitors were charged, I could also test for how long it took for the caps to discharge.

 

[73powerstroke]

I really don't know why this method is not discussed more. It works really well with readily available parts. I used this method yesterday while testing an MPP-2424. As well as giving me an idea when the capacitors were charged, I could also test for how long it took for the caps to discharge.

Definitely easier than ordering some odd resistor off Amazon if you don't have a parts bin. Got to auto store or junkyard and buy what you need. Visually confirmed charge too is a real bonus.

 

[kenkoh]

Is there a such a thing as a disconnect switch that also has a precharge ability and can anyone recommend one for my setup? If not, using the resistor is fine.

 

[squowse]

Is there a such a thing as a disconnect switch that also has a precharge ability and can anyone recommend one for my setup? If not, using the resistor is fine.

you can wire one fairly easily with a dual battery isolator rotary switch

 

[Hedges]

3-5 minutes feels like FOREVAR!!!! when waiting for the circuit to pre-charge, and that sounds about right.
The voltage was within about 10V of battery when I cut it over. No biggie.

Assuming short-circuit current of LiFePO4 battery is 20,000A (my calculation from cell internal resistance, best guess until someone measures and reports an actual figure which may be limited by ion transport),

Connecting a 50V battery to capacitor bank would deliver 20,000A surge.
Waiting until capacitor bank reached 40V (within 10V of battery) would deliver "only" a 4000A surge.
If switch is momentarily a resistive contact, power = I^2R would be 1/25th as much at 1/5th the voltage.
Close to the voltage of a 12V battery, but still 1/5th the current.

That still sounds like a lot, but is about what a 100 Ah AGM battery was reportedly measured delivering. So your surge may be what my system experiences.
My Sunny Island documentation doesn't have a precharge recommendation, just switch on the internal breaker. This inverter was developed in the days of FLA, VRLA, NiCD.

The new European model 8.0H also doesn't have any such recommendations. That one has no internal breaker, just a membrane keypad. I guess when an (approved, e.g. BYD and many others) lithium battery is connected it relies on whatever the battery does. BYD instructions have external switch turned on first, battery second, inverter switch third. Doesn't see like the way to enable a precharge function of the battery.

https://www.zerohomebills.com/wp-content/uploads/Installationsanleitung-BYD-B-BOX-Premium-LVS-4-8-12-16-20-24kW-Solar-Battery-DE.pdf

That is why we wonder how you come up with 'Anything. I've tried many from a few ohms to 200k'.

Some people have suggested leaving resistor permanently connected.
If inverter draws zero current, that would keep capacitor bank charged (once charged the first time, which would take about a day for the 100,000 uF I assumed.)
You would want to disconnect it and drain capacitor before doing any wrenching that might short it.

Works for me. I don't have any near that big. I have use these here. I mean you don't have to precharge you'll just get a nasty spark if you don't. And could have some bite marks. Anything is subjective. I wouldn't try a mega ohm rated Anything or over 1k. Look at my first reply. I said a few ohms to 1k. It doesn't need to be fully charged, you just don't want the spark. Maybe I'm doing it wrong , but I'm not getting a spark and I'm not using big values. Seems like common sense to me. Ohms law. Sometime when I find a 200k I'll try it and see what happens. Lol

Small values like 3 ohms will draw about 16A, briefly, dissipating 85W briefly. So brief that 5W or 10W resistor won't overheat unless left carrying the current when inverter turns on.