kenkoh
Solar Enthusiast
Want to make sure I get the right resistor.
Do I need to pre charge a 24v 1500w inverter before connecting to LiFePO4? Can anyone recommend specific resistor to buy?
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sunshine_eggo
Happy Breffast!
ANY resistor helps massively. You just have to find a balance between resistance and rating.
Memory tool: About same volts in Ohms and 50% or more in Watts, e.g.,
24V system = 24Ω, 12W
You could round up to 50Ω and drop to 10W. More power rating on the resistor is never bad.
These would be great:
Memory tool: About same volts in Ohms and 50% or more in Watts, e.g.,
24V system = 24Ω, 12W
You could round up to 50Ω and drop to 10W. More power rating on the resistor is never bad.
These would be great:
uxcell 2 Pcs Aluminum Clad Wire Wound Resistors 25W 30 Ohm 5%: Single Resistors: Amazon.com: Industrial & Scientific
uxcell 2 Pcs Aluminum Clad Wire Wound Resistors 25W 30 Ohm 5%: Single Resistors: Amazon.com: Industrial & Scientific
www.amazon.com
kenkoh
Solar Enthusiast
Do I need tp precharge everytime before I turn the invertor on?
sunshine_eggo
Happy Breffast!
You need to precharge any time you connect the inverter to the battery well BEFORE turning the inverter on. The battery connection is where the surge happens.
If you turn the inverter off, disconnect battery and, move a connection and then reconnect within a minute or so, you probably don't need to pre-charge again, but it doesn't hurt.
kenkoh
Solar Enthusiast
If I never disconnect inverter from battery but turn it off, I can turn it back on without issue correct?
sunshine_eggo
Happy Breffast!
Yes. In the off state, the inverter caps will stay charged.
kenkoh
Solar Enthusiast
Thanks. Solar is deep.
sunshine_eggo
Happy Breffast!
and W I D E!
fromport
Solar Addict
I ordered some of these: https://www.ebay.com/itm/284470104226
50 Ohm 100 watt for my 48 volt setup.
Even if the input of the inverter would be a short, 50 volt -> 50 ohms = 1 amp (relative easy to find a push button that can do 1 amp)
P = UxI = 50 x 1 = 50 watt of heat disapation. so the 100 watt is a little bit of overkill ;-)
50 Ohm 100 watt for my 48 volt setup.
Even if the input of the inverter would be a short, 50 volt -> 50 ohms = 1 amp (relative easy to find a push button that can do 1 amp)
P = UxI = 50 x 1 = 50 watt of heat disapation. so the 100 watt is a little bit of overkill ;-)
Hedges
I See Electromagnetic Fields!
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If you have the ability to turn the inverter off and on, by something other than a circuit breaker (so caps remain connected even when off), then any resistor or light bulb will do, so long as it doesn't burn out. Bulb rated for battery voltage, or resistor of value & wattage able to take battery voltage continuously, does that.
If an inverter auto-starts when battery applied, you need to figure out how fast precharge needs to be done.
In the case of my Sunny Islands, they have an internal breaker which disconnects battery, plus a membrane button to start operation. If I used lithium rather than the AGM I have, I would turn on breakers first, then precharge, then close external switch (or otherwise connect battery), then start. They draw 4W at idle, so resistor value would need to be low enough to supply that current (75mA per inverter) without much voltage drop. Around 3 ohms, 16A surge to precharge 4x inverters.
If an inverter auto-starts when battery applied, you need to figure out how fast precharge needs to be done.
In the case of my Sunny Islands, they have an internal breaker which disconnects battery, plus a membrane button to start operation. If I used lithium rather than the AGM I have, I would turn on breakers first, then precharge, then close external switch (or otherwise connect battery), then start. They draw 4W at idle, so resistor value would need to be low enough to supply that current (75mA per inverter) without much voltage drop. Around 3 ohms, 16A surge to precharge 4x inverters.
73powerstroke
Solar Enthusiast
Anything. I've tried many from a few ohms to 200k
100w? Why? A 2w resistor works. you seriously don't need anything near that big. I usually use 5 or 10w sandbars a few ohms to 1k. Whatever i grab first in my parts bin...Works every time.
sunshine_eggo
Happy Breffast!
Have you actually used a 200kΩ resistor to precharge inverter caps? I tried a 600Ω one, and it was agonizingly slow.
73powerstroke
Solar Enthusiast
what did it do? Keep sparking? I've never had a problem. Hold it on a few seconds with unit off or it will spark some. How are you watching it charge. Shouldn't need but a few seconds then hook the wire.
Hedges
I See Electromagnetic Fields!
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600 ohms into 100,000 uF would be 60 seconds time constant, charged about 63% of the way or 34 volts out of 54V.
After 3 to 5 minutes it should be mostly precharged, reducing the surge current to something in the 135A to 1000A range.
If a smaller inverter with smaller capacitor bank, would be much more fully precharged.
After 3 to 5 minutes it should be mostly precharged, reducing the surge current to something in the 135A to 1000A range.
If a smaller inverter with smaller capacitor bank, would be much more fully precharged.
Bud Martin
Solar Wizard
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It has to do with TC (Time constant), using 200K to charge about 10,000uF capacitor bank will take a long time (5TC) for the Voltage on the cap to reach the supply Voltage.
sunshine_eggo
Happy Breffast!
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.
73powerstroke
Solar Enthusiast
I don't get into all that I just grab a resistor that has a couple of W rating and put it on there and hold it for a couple of seconds and take it off and put my terminals on there and I've not had a spark problem on any of these inverters
Bud Martin
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That is why we wonder how you come up with 'Anything. I've tried many from a few ohms to 200k'.
Re-Volt-Oz
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73powerstroke
Solar Enthusiast
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
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