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Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: glyndwr1998 on November 05, 2017, 11:33:21 AM



Title: tesla powerwall. does it actually require a compatible inverter?
Post by: glyndwr1998 on November 05, 2017, 11:33:21 AM
Hi,
please dont slap me around the head too much.......

Why does the powerwll V1 which is DC coupled need a comptiible inverter to operate?

Ive tried searching on net and youtube for install videos looking for the cabling and connection, but info is sketchy.

Is it possible to just connect the powerwall to any inverter (in my case a sma 4000tl - 20 ) without any rs232 or bms info cable, and run my own independant BMS with its own cut off set points

I seen a powerwall for sale used recently which made me think about such an install.

Thanks.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: RIT on November 05, 2017, 06:45:34 PM
The issue is that you don't just need a standard BMS/charging solution to handle the charging of the powerwall. You also need a solution that manages the powerwall's output to the inverter if you are using a standard inverter.

A standard inverter will invert all the power available on its input. In a normal configuration this is not a problem as the PV panels set the maximum input that the inverter will receive. Connecting a battery to the inverter without some form of limiter will just overload the inverter.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: glyndwr1998 on November 05, 2017, 07:16:17 PM
ah rite, understood.

That makes sense now that youve pointed it out. With the sb4000tl connected to the grid, the inverter will just dump the energy from the battery into the grid. ok.

Ive got 7.5kw of solar, 3.5kw on east elevation with a ABB aurora 3.6tl inverter and 4kw array on west elevation with sma sb4000tl inverter. Ive noticed a few gen 1 tesla powerwalls on ebay lately for around 2k, i thought at that price maybe worth a look, but not if its not suitable with my existing system.
Looks like i`ll need to wait for a cheaper AC coupled system then.

I have got a full 24kwh leaf battery here to be utilised. I used 16kwh of it in the plug in prius i converted. No use of that now i`ve got an e-nv200 wheelchair adapted van instead.
Its finding equipment out there where i can utilise the battery.
If the powerwall could be installed quite easily i would have sold the leaf battery on, to help pay for the pv storage project.

Thanks for that info, it does make sense what youve said. Cheers.
Anthony.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: RIT on November 05, 2017, 07:35:46 PM
If the prices are right you should consider watching out for an old Powerwall, plus an SMA Sunny Boy Storage device and SMA Energy meter. This configuration results in the battery being connected on the AC side, so beyond your current inverter. This way the current system does not need to be touched and if you receive FITs payments there is not a reduction in your income.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: pj on November 05, 2017, 10:26:54 PM
Connecting a battery to the inverter without some form of limiter will just overload the inverter.
I'm not sure that's exactly correct. Many of us have more kWp of panels than the inverter could take - I have 3.6kWp for my 3kW inverter. Inverters will take as much or as little from the input as needed to give up to it's designed maximum output. Too much available power at the input will not overload an inverter. It takes what it wants.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: glyndwr1998 on November 05, 2017, 11:00:29 PM
Thanks RIT.

You`ve given great help. If i knew what commands the sma sunny storage needs to control the battery i`m sure thered be a way of interfacing something like a leaf battery to it.
I already have the leaf battery, I can extract battery info from it like battery temp, pack voltage, state of charge, current, cell voltage etc.... im sure an arduino type program cold be written using can commands to tell the inverter what state the battery is in.

Thanks.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: glyndwr1998 on November 05, 2017, 11:06:44 PM
Hi pj,

i suppose if a battery was to be connected directly to a non compatible inverter then it would probably work to its max output until the battery is depleted, which may overload it if worked at max for an extended period of time.

Its just a thought i had thats all as i did see a powerwall go for quite reasonable money the other day. As i say I have a 24kwh nissan leaf battery here, it would be ideal if i could get that interfaced with an approriate inverter, ideally on the ac coupled side.
Im ok with working with the battery, I did install 16kwh of it in a plug in prius project i completed back in 2015 which worked really well. Its the programming skills i am lacking. Folk do say to me that arduino is quite a simple platform to learn but im better at ladder plc, i did a short course on plc ladder programming when i was a tech at Sony manufacturing which helped out in the workplace, but arduino is alien to me, wish i knew someone locally that could walk me through it for the basics, id gladly pay too for their time maybe a for couple of days  maybe 2 day crash course.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: RIT on November 05, 2017, 11:33:34 PM
Connecting a battery to the inverter without some form of limiter will just overload the inverter.
I'm not sure that's exactly correct. Many of us have more kWp of panels than the inverter could take - I have 3.6kWp for my 3kW inverter. Inverters will take as much or as little from the input as needed to give up to it's designed maximum output. Too much available power at the input will not overload an inverter. It takes what it wants.


Just about all inverters support a higher input than their rating(DV overloading), but within limits. I have seen comments in the past that Fronius' products are warranted for up to double the PV DC input than their AC output. This issue is that for general systems they are not designed for what is an 'unlimited' supply from a directly connected battery.

Just found Solaredge's statement on the issue - depending on the product they allow 135% or 155% overloading

    https://www.solaredge.com/sites/default/files/inverter_dc_oversizing_guide.pdf


The major issue is how an inverter controls the input - it does this by causing the PV voltage to rise by limiting the current, as the voltage rises the panels become less efficient and so the panels produce less energy. A battery on the other hand will not produce less as this current limiting control kicks in. The end result I guess is that the inverter's control circuit will fail.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: RIT on November 05, 2017, 11:54:59 PM
Thanks RIT.

You`ve given great help. If i knew what commands the sma sunny storage needs to control the battery i`m sure thered be a way of interfacing something like a leaf battery to it.
I already have the leaf battery, I can extract battery info from it like battery temp, pack voltage, state of charge, current, cell voltage etc.... im sure an arduino type program cold be written using can commands to tell the inverter what state the battery is in.

Thanks.

You would need the development docs for a battery solution that the SMA storage device supports, rather than anything published by SMA. The reason being is that your home-brew solution would need to emulate a supported battery. So you would need to make your leaf battery look like a Powerwall 1 to the SMA inverter.

Considering the possible risks involved in such a project (to life, home and the national grid) you may want to start on something smaller or wait until Nissan ships their inverter solution.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: pj on November 06, 2017, 01:01:08 PM
Well you live and learn - everyday is a school day.
I can quite understand that the input voltage limit must not be exceeded, but am really struggling with the concept of too much power.
In an electrical/electronics context, power is usually sucked in by a device from a source, not forced by a source into a device, unless the source keeps increasing the voltage level.
e.g take a 12v lamp across a car battery. If you put multiple car batteries in parallel, the lamp will be no brighter, despite the higher source power potential. If you put those same batteries in series, the lamp will blow straight away.

I just found this link on SMA
https://en.sma-sunny.com/en/7-reasons-why-you-should-oversize-your-pv-array-2/ (https://en.sma-sunny.com/en/7-reasons-why-you-should-oversize-your-pv-array-2/)
This includes as an example a 15kWp array driving a 9kW inverter (166% overpowered). Also stated is that it's the Voltage that must not be exceeded, no mention of a limit on how overpowered the system is.

However, I do agree that a controlled inverter is needed to control the power output to match the required load for the purposes of the OP, and regular inverters will just drain the batteries at the maximum power output rate of the inverter.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: Tinbum on November 06, 2017, 02:53:33 PM
I think it is technically possible to add as much PV to an inverter as long as you don't exceed the maximum voltage. The reasons its not recommended beyond a certain amount are;

1) you waste a lot of the potential from the PV as it will nearly always throttled to the max power of the inverter.

2) I'm not sure on this but are inverters rated for 100% duty cycle.

3) Its not recomened to parallel up lots of strings of PV.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: RIT on November 06, 2017, 03:25:04 PM
Well you live and learn - everyday is a school day.
I can quite understand that the input voltage limit must not be exceeded, but am really struggling with the concept of too much power.
In an electrical/electronics context, power is usually sucked in by a device from a source, not forced by a source into a device, unless the source keeps increasing the voltage level.
e.g take a 12v lamp across a car battery. If you put multiple car batteries in parallel, the lamp will be no brighter, despite the higher source power potential. If you put those same batteries in series, the lamp will blow straight away.

I just found this link on SMA
https://en.sma-sunny.com/en/7-reasons-why-you-should-oversize-your-pv-array-2/ (https://en.sma-sunny.com/en/7-reasons-why-you-should-oversize-your-pv-array-2/)
This includes as an example a 15kWp array driving a 9kW inverter (166% overpowered). Also stated is that it's the Voltage that must not be exceeded, no mention of a limit on how overpowered the system is.

However, I do agree that a controlled inverter is needed to control the power output to match the required load for the purposes of the OP, and regular inverters will just drain the batteries at the maximum power output rate of the inverter.

I have to say that I'm no expert on the design of the DC side of a grid connected inverter, but your example of a light bulb connected to a battery is a good comparison. A light bulb has a certain resistance which limits the current flow to a known amount at a specified voltage. A PV connected inverter does not have a fixed operational input voltage and to be as efficient as possible the design will try and keep the resistance of the input circuit as low as possible. It can do this as it expects the 'maximum' input current to be within known limits and it also knows that if it does start to limit the current 2 things may happen, firstly the voltage from the PV will start to rise, so making the PV panels less effective and so lowering their overall output and secondly if the voltage rises to much it can just shut down. These features/controls are just extensions of what the Maximum Power Point Tracking (MPPT) controller does anyway so incur no additional design costs.

I think the key thing is the very low resistance of the DC side circuit. Very few electrical devices we use are designed in this way, where the circuit uses the characteristics of the power source to limit the energy that the circuit receives.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: biff on November 06, 2017, 03:26:53 PM
H,mmmmm.
        Right. I am not into On grid applications but I do Off Grid stuff.
  The on grid systems use the grid as a battery,,,,right,?
  The offgrid has to have a controller between the Inverter and the Battery (grid). In my own case. The controller can cover a range of voltage from 110vdc to 161bvdc. It cuts out below and above these voltages.. My bank is some 600ah x 120vdc,,it can be less and it can be more. It does not make a lot of difference to the performance of the inverter as long as the controller can satisfy the inverter within the parameters of the set voltage  110-----161vdc.
  So It would be easy to suspect that the said GTIs must have an internal  transformer/controller that regulates the DC supply withing it,s set parameters...simple,??
                                                                        Biff


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: pj on November 06, 2017, 04:29:37 PM
Hi RIT, I think we've got to the point of violent agreement here.  :)

In terms of the input side of an MPPT inverter, it modifies its input reactance rather than resistance to find the Maximum Power Point, and it's not necessarily the lowest reactance. If you keep dropping the reactance (resistance) the panels actually drop the voltage since they act as a constant current source at a given light level over most of the voltage curve. This leads to lower power transfer (power=volts x amps)


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: eabadger on November 06, 2017, 05:47:35 PM
again no idea about on grid.
but i went over the top with pv and morningstar said as long as oc voltage is below limit, no problem at all, we see the controllers sitting at 60a each (3 of them) even now, which i am well pleased with.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: RIT on November 06, 2017, 07:23:05 PM
Hi RIT, I think we've got to the point of violent agreement here.  :)

In terms of the input side of an MPPT inverter, it modifies its input reactance rather than resistance to find the Maximum Power Point, and it's not necessarily the lowest reactance. If you keep dropping the reactance (resistance) the panels actually drop the voltage since they act as a constant current source at a given light level over most of the voltage curve. This leads to lower power transfer (power=volts x amps)

That was something I had to read up on last night as I assumed the same thing, but what I found indicates that the MTTP does the exact opposite. The MTTP is able to make a PV panel operate at all points along its published IV curve, an example diagram of which can be found here

      http://www.daviddarling.info/encyclopedia/I/AE_I-V_curve.html

The few comments I was able to find indicate that to control the input power the MTTP changes its characteristics so that the current flow is reduced, which then pushes the voltage up. I guess it is safer/easier to implement a solution that takes the panels to an 'open circuit' configuration, rather than an 'short circuit' configuration.
While I had a look around I could not find a good document covering this process.


Title: Re: tesla powerwall. does it actually require a compatible inverter?
Post by: glyndwr1998 on November 06, 2017, 08:34:32 PM
Thanks gents,

Its a shame the pv and storage stuff wasnt modular. I know they are now becoming so, i suppose its the price early adopters pay for the latest tech. They are getting cheaper to be fair.
One part of my brain tells me to go get one, the other tells me wait a while, which is probably the best thing to do.
Id love to be able to use the leaf battery i already have.

Anthony.