inverter wattage to charge an x2?

[rotorblades]

for an electronic device i wonder why seg is so secretive on how many charge watts. i know it varies ive read everything i can find i used to be an electronics student soo iwould thinkthat infot would be a little more available.especially in the manuals i glide on solar power= nuclear power direct to me.if i wasnt so laid up i would hook up my killa-watt meter up to see.

[Bob.Kerns]

Originally Posted by rotorblades

for an electronic device i wonder why seg is so secretive on how many charge watts. i know it varies ive read everything i can find i used to be an electronics student soo iwould thinkthat infot would be a little more available.especially in the manuals i glide on solar power= nuclear power direct to me.if i wasnt so laid up i would hook up my killa-watt meter up to see.

Well, if you do a forum search, you will find many, many discussions of this.

It's a little tricky to give a hard number, because the best answer depends on both the inverter and what your question is (inverter output vs input).

My observations with a Kill-a-Watt is that it doesn't go much over 120 W. But that doesn't give you much headroom for peak load, duty-cycle limitations of the inverter, inadequate wiring on the input leading to voltage variations, etc.

People report using 400W inverters successfully. You could probably get by with less -- 200W would likely work, if the unit is conservatively rated and properly wired. In between? We're in guesswork land.

[KSagal]

Originally Posted by Bob.Kerns

Well, if you do a forum search, you will find many, many discussions of this.

It's a little tricky to give a hard number, because the best answer depends on both the inverter and what your question is (inverter output vs input).

My observations with a Kill-a-Watt is that it doesn't go much over 120 W. But that doesn't give you much headroom for peak load, duty-cycle limitations of the inverter, inadequate wiring on the input leading to voltage variations, etc.

People report using 400W inverters successfully. You could probably get by with less -- 200W would likely work, if the unit is conservatively rated and properly wired. In between? We're in guesswork land.

Bob,

I have used inverters in the past, and what one company calls a modified sine wave may not look like what another calls one.

Typically when scoped, the wave from my cheap inverters have only a few large steps, making the AC wave look like a staircase up and back down. On the better inverters, the steps were much smaller, and there were more of them, making the AC wave look more like a pile of gravel.

I believe that RMS average wave values might list both of the above waves as the same voltage and frequency, but I wonder if the larger steps as opposed to the smaller ones provide less available current.

What think you?

[rotorblades]

[QUOTE=Bob.Kerns;234593]Well, if you do a forum search, you will find many, many discussions of this.
you are soo right but the maxasked forbulk charge rate should be in the manual im thinking.
im so picky because i run a big solar system to run my whole house.and soon a chevy volt.before actually owning an x2 it might have made planning easier for someone else.i have juice to waste most might not.

[Bob.Kerns]

Originally Posted by KSagal

Bob,

I have used inverters in the past, and what one company calls a modified sine wave may not look like what another calls one.

Typically when scoped, the wave from my cheap inverters have only a few large steps, making the AC wave look like a staircase up and back down. On the better inverters, the steps were much smaller, and there were more of them, making the AC wave look more like a pile of gravel.

I believe that RMS average wave values might list both of the above waves as the same voltage and frequency, but I wonder if the larger steps as opposed to the smaller ones provide less available current.

What think you?

Well, the point of RMS (Root Mean Square) is to make the waveform irrelevant. Since higher voltages mean higher currents (and vice versa), the higher voltages end up being weighted more in the calculation. That's why the "square" in RMS. It's the square root of the average of the square of the voltage (or current).

That doesn't really answer your question, though. You're really asking about what it does *under load*. If the RMS voltages remained the same, they'd still be producing the same power output -- but will they remain the same as the load approaches the rated limit?

For that, we'd have to consider how the waveform is being produced. I haven't really looked at cheap inverters since the days when they used vibrating reed switches, so I don't know what they do to make them cheap, but having designed a few power supplies (LONG ago), I can tell you some of the tradeoffs.

The easiest, most efficient way to approximate a sine wave is to periodically turn on a switch to charge a capacitor up to the voltage it should be at at that point in time for a sine wave. This is efficient, because an ideal switch dissipates no power when it is on or off -- it only dissipates power when it's in the in-between state when resistances is neither zero nor infinite.

The switches (FETs, SCRs, Triacs, etc.) are not ideal switches, however. They have some resistance even when fully on, and they take some non-zero time to turn on and off.

A "chunkier" waveform will require more of the total charge to be passed in each chunk, and a bigger capacitor to store it. That means higher peak currents in charging, unless steps are taken to limit it (for example, putting a resister in series).

So bigger capacitor, higher peak currents, a (bigger) resistor in series wasting power -- that's not a design direction you want to go. I'd be surprised to find an inverter like that today!

Historically, I think they were designed like that because of the slow switching times of available high-powered solid-state switches. That's when most of the power is lost. Power lost = heat. So I think they had to limit the number of times they switched the power.

But how does this relate to your question? Well, the relationship is not simple. It depends on the design choices that were made. How big is the output capacitor, that stores the current while it's in the "off" state. Does the design turn it back on sooner when the voltage drops? (Probably not, since we're talking cheap&simple). How much is the current limited by the limits of the device? How big a heat sink is there, to dissipate heat? The thermal engineering is a critical part of designing power circuitry.

The device with the smoother waveform will have the same issues, though. It will switch more frequently, but will likely have a smaller output capacitor that drains more quickly under load. It will be using a modern device that switches more quickly, but it'll still be producing heat with each switch.

If the capacitor drained by the same percentage on each cycle for the two inverters, the RMS voltage would be affected the same. If each can bring the voltage back up to the same % of intended, likewise the RMS voltage would be affected the same.

But that's math & physics, not engineering. The engineering question is: can they?

So my *inexpert* expectation would be, that it would depend not so much on the waveform, but on how close the design pushes the limits for the devices in question.

And that's where price comes into the equation. I'd expect the cheaper device to push the limits more closely, be rated less conservatively, and perform more poorly.

But that's not necessarily true in individual cases. A clever designer and efficient manufacturer can produce a good product cheaply, while a sloppy design and inefficient manufacture can produce a poor product expensively -- and then spend a bunch on marketing, have a high markup, and drain your wallet.

I know, it's a non-answer. But I hope I at least answered your core question of how waveform interacts.

[KSagal]

Just to add a real life example that may more clearly demonstrate the question I was trying to ask...

I had an inverter that I thought was working well, and seemed to provide the power I required, including the charging of my segway, on the road.

I did not pay particular attention, but one day I noticed that using that inverter to provide 120v AC from my car, it would not work with my new company computer. I did not have a dc to dc transformer that would work, so I was using the regular wall plug into my inverter...

This inverter worked well enough that it was invisible for the laptop computer that my work mate was using, but mine seemed to reject it. (two different computer mfgs).

I scoped it, found the large steps in the wave.

Bought a new inverter, scoped it, found the smaller steps. Works fine with my company computer.

Digital multimeter claiming RMS readings gave same or similar readings for both inverters. Computer energy draw is way below either inverter's ratings.

Riddle me that, Bob...

[Lily Kerns]

OK, you geeks! Let's try a simpler question... For the non-engineers among us, if I wanted something to recharge my seg when I'm traveling between power sources, what should I buy, and where would I find one?

[Bob.Kerns]

Originally Posted by KSagal

Just to add a real life example that may more clearly demonstrate the question I was trying to ask...

I had an inverter that I thought was working well, and seemed to provide the power I required, including the charging of my segway, on the road.

I did not pay particular attention, but one day I noticed that using that inverter to provide 120v AC from my car, it would not work with my new company computer. I did not have a dc to dc transformer that would work, so I was using the regular wall plug into my inverter...

This inverter worked well enough that it was invisible for the laptop computer that my work mate was using, but mine seemed to reject it. (two different computer mfgs).

I scoped it, found the large steps in the wave.

Bought a new inverter, scoped it, found the smaller steps. Works fine with my company computer.

Digital multimeter claiming RMS readings gave same or similar readings for both inverters. Computer energy draw is way below either inverter's ratings.

Riddle me that, Bob...

Did you scope it under load? Measure the RMS voltage under load?

I have had a couple inverters fail to charge some high-load laptops that they should have been able to handle according to their load. The culprit was easy to identify -- voltage drop on the input lines. In fact, one inverter would shut down and alarm, then power right back up again as the input voltage rose, over and over again.

Were you feeding the inverter from a lighter outlet? That's the context in which I've seen this happen. Was the engine running? In most cars that will raise the input voltage a little.

Bottom line is, my experience is that inverters tend to be rated rather optimistically.

The difference in laptops is also a variable. Different chargers will respond differently to a low-voltage situation. If the first stage is an isolation transformer, the output voltage will dictate how much headroom they have for low input voltage. When the input voltage drops enough that the transfomer's output is less than the laptop's input voltage plus voltage drops across the switches, etc, then it will no longer be able to deliver the intended voltage. Most laptops will then fail to use any power from the power input at that point.

Waveform does come into play there. At the same RMS, a broader waveform with lower peak will have less of a cycle that reaches the minimum voltage threshold for the power supply. If you graph the current draw, I bet you would see it's more of positive/negative pulses around the times of the peaks, than a sine curve. Inductance would minimize but not eliminate that effect.

But when I worked with these things, switching power supplies were still rather exotic. Trying to extrapolate to modern ones is rather iffy. The switching principles remain the same, but the control circuitry is far more advanced, and I'm sure they go to more lengths and complexity to ensure either a stable clean output, or none.

[jgbackes]

Www.walmart.com


Cobra 400-watt Power Inverter. $25.00

[Amimoto]

I use a no brand 300 watts pure sine wave inverter on my H2, it charge 2 Segways just fine.
I would recommend a pure sine wave inverter rather than a cheaper inverter (simulated or modified sine wave)


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