Here's a general capacitor charging and current curve so you have an idea:
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MASTER THREAD: Powering house or other things with Model 3
- Thread starter Vines
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Nice. Thanks! Those curves are beautiful! But my question also is: what is T roughly for an 500 to 1200 VA inverter (typical)? If the capacitor is a microFarad then it would be just a few microseconds... right? But I don't really have a clear idea even to within an order of magnitude what the capacitance might be. Do you have a sense of that?
Additionally, I have little knowledge of inverter circuitry and don't feel completely confident that a single time-constant picture captures the complexity of actual inverters? I guess probably it does, I just feel a bit a bit uncertain about that...
Additionally, I have little knowledge of inverter circuitry and don't feel completely confident that a single time-constant picture captures the complexity of actual inverters? I guess probably it does, I just feel a bit a bit uncertain about that...
stopcrazypp
Well-Known Member
There may be multiple capacitors in the inverter design, but for the purposes of the precharge circuit you only care about the lower voltage DC link capacitors, meaning the ones that are directly parallel to the input and are directly charged when you connect your low voltage power source (even with the inverter off).
You can tell which ones they are easily by their proximity to the input wires and their rating (like 25V or 16V). There are also large ones used on the output, but those are rated for much higher voltages.
https://www.digikey.com/en/articles...acitors-for-dc-link-and-inverter-applications
There is no "general" case because there is no set standard for how many are used. For example just looking at 1000W inverters:
Here's one that uses two 4700uF capacitors:
https://www.amazon.com/Inverter-Camping-Outdoor-Suitable-Production/dp/B0B6PTD855
Here's a design that uses four 4700 uF capacitors.
https://www.yumoelectric.com/YUMO-P...rd-with-independent-radiator-pd522137698.html
This one uses 6:
https://www.aliexpress.us/item/3256804456073412.html
There may be others that use other values.
I don't know if this is the same model, but here's a 700W Renology that appears to use 4:
The time constant (as per chart ~64% charge in terms of voltage, full charge would take about 5T) with a 2 ohm resistor would calculate to:
2x 4700uF: 0.0168 seconds
4x 4700uF: 0.0376 seconds
6x 4700uF: 0.0564 seconds
https://www.digikey.com/en/resources/conversion-calculators/conversion-calculator-time-constant
The only way you can know for your specific inverter is to take a multimeter at the input to monitor the voltage, a stopwatch (can probably use your phone), connect your precharge circuit and power source and record how long it takes for it to fully charge your input capacitor (when it reaches steady state which is approximately 5T as noted above). That's what I did for mine (unfortunately, I didn't write it down so I don't remember what it was exactly).
Generally the charge is so fast for these smaller inverters that it doesn't really make much sense to have to use such a low resistance.
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n2mb_racing
Active Member
There was an automatic precharge circuit earlier in this thread that might be easier. Nothing to manually switch. And it would work if the car turns itself off and back on again.
n2mb_racing
Active Member
I found the link again. This looks like a good way to do it:
stopcrazypp
Well-Known Member
Certainly it is much easier to use, but a bit more complex to wire and depending on how you configure and power it, the relay has a parasitic draw to keep the coil energized (which may or may not matter to you depending on your load).
n2mb_racing
Active Member
Although, I would probably put the relay on the positive side, just because the chassis is negative ground. But, same idea. The resistor charges the input capacitors on the inverter. When they are charged enough, the voltage on the relay coil will reach 12V, turning on the relay and connecting the inverter to the 12V supply.
I'm getting ready for winter in Ukraine, and want to extend the autonomous time of my existing setup consisting of UPS ZASILACZ AWARYJNY SINUS PRO 800 E 12/230V (500/800W) | VOLT POLSKA with external 100Ah lead battery.
By connecting under-seat 12V DC outlets on my 2018 model 3 to the buffer battery, like in MASTER THREAD: Powering house or other things with Model 3
Invertor's spec says it provides a charging Voltage of 13.8V.
If I don't have anyth in place of (???):
Would Tesla trip when the power comes back? What if Tesla is powered off?
Would such a setup be over-charging the buffer lead-acid battery?
By connecting under-seat 12V DC outlets on my 2018 model 3 to the buffer battery, like in MASTER THREAD: Powering house or other things with Model 3
Invertor's spec says it provides a charging Voltage of 13.8V.
If I don't have anyth in place of (???):
Would Tesla trip when the power comes back? What if Tesla is powered off?
Would such a setup be over-charging the buffer lead-acid battery?
Wouldn't a simple voltage relay like
Not a great idea. Reasons: 1) the Wall Connector only allows 48 Amp max. The wires for the L1 L2 going into the panel are really big, much bigger than the 60A wires. 2) the car battery system must have an inverter to convert DC voltage to AC, that costs more and make the car more heavy. Bottom line: the EV car is not meant to be a PowerWall.
An relatively inexpensive power backup for home use is a gas generator. A PowerWall certainly can do but at very high cost.
Also, the HPWC has no neutral which is required to serve 120V loads. This limitation could be significant, depending on the rest of the system design. An autotransformer could perhaps create the 120V that is needed if that was part of the design.
n2mb_racing
Active Member
That part is fine. You can supply power through a lower amperage generator. The problem is that the wall connector would also need a transfer switch.
Maybe for Tesla but not all EV's. F150 Lightning shows 10 PW's on wheels is more than doable.
As I have already written, Tesla could easily make a charger/battery-less powerwall that would have the electronics but no extra batteries.
I'm not a hardware engineer, but I'm guessing such a device could be made in the range of $1000, even with a manual transfer switch and a 50A amperage limit which would be a healthy 10kW. My house uses 1kW on average.
No one is trying to turn a tesla into a full powerwall, but simply to use one for emergency use a few times a year at most.
In the meantime, we have my solution which is better than nothing, but not nearly as good and limited to 2kW
Mentioning this with 20/20 hindsight of course, but the battery-less Powerwall turns out to already exist as you describe, it is exactly the same Tesla Gateway from Powerwall installations - grid isolation, automatic transfer switch, at least 11.5kw support (max that the Cybertruck can output) - as announced as Powershare feature. Interesting to note the Gateway is itemized at exactly $1000 in Powerwall quotes, though there's already some suggestion that battery-less installation costs may run around $4K, which sounds expected given it's almost the same labor as doing a full Powerwall install. Already some discussion in the Energy subforum too about how Powershare deals with the lack of neutral in the Wall Connector portion as well....
MontyFloyd
Member
Incorrect, fuel powered generator will cost more fairly quickly, and have several more downsides.
Being a machine it will require regular operation to keep parts moving, annual oil changes, new filters, and cleaning.
Plus is noisy, exposed to elements (yes, generators can freeze), release CO2, CO, and possibly NO2 (others if using liquid fuel)
Mmm, sort of. A propane sine wave generator is pretty low maintenance. Run it 3 or 4 times a year. Oil rarely needs changing under those conditions. Usually under a thousand dollars for a portable unit with electric start. It’s obviously not going to power a whole house but will keep a refrigerator going, run the furnace motor etc. A good solution for many.
PianoAl
Active Member
A neighbor had a whole-house propane generator and followed the manufacturer's CYA advice to run it once per month. That puppy was loud.
Could be. Ours is super quiet. But it’s also only a 3400 watt genny. We use the car to exercise it every few months. It charges at 24 amp 120 volts. We typically run it for about 20 minutes.
n2mb_racing
Active Member
That works, but I'd rather use the 100 kwhr (Model S) of capacity I already have sitting in my garage in the event of a power outage.... And not have to store another large item in the garage.
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