Sure the resistor will limit the current to (100V/R) but it may not be necessary or even desirable.
One needs to know how the inverter in question works and what it would do with a very low impedance source (essentially a battery can deliver several hundred amps at 12 V; lets say on the order of 50 milli-ohms). A string of PV modules may present 400V with a short circuit current of 8A (a source impedance of 50 ohms a factor of 1000X higher impedance). This much I know: grid tied inverters start at very high input impedance say 50,000 ohms (drawing very little current) and iterate their way up the I-V curve presenting lower impedances as they go (and checking the power input at each step). When the most recent step actually decreases the input power, the MPPT algorithm reverses the step and goes back and forth, constantly maximizing the input power. Faced with a source on the order of 50 milli-ohms, the inverter will never be able to find a maximum power point by lowering its input impedance. So my question is the following Will another specification (like maximum out put power) limit this process and cause the inverter input impedance to sit at that point which causes a maximum output power? I doubt that things are that simple. But the question could be asked of the inverter manufacturer. One other thing to remember is that inverters have a very large capacitor on their input which gets charged by the PV string. The capacitor is very much like a battery (very low impedance) so at least in the beginning of the MMPT algorithm, an inverter should not have a problem with a battery vs a capacitor. Finally, there are relatively simple circuits that can perform current limiting functions and could be used with a battery. Unlike a resistor, current limiters offer no impedance (to speak of) until the current limit has been reached and then they do a good job insuring the current set point is not exceeded. However, they are an active circuit and may not be compatible with a MPPT algorithm. Sound like someone should spend some time with a super-tech at one of the grid-tied inverter manufacturers and suss this out. - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 ________________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of boB Gudgel Sent: Tuesday, November 16, 2010 12:00 PM To: RE-wrenches Subject: Re: [RE-wrenches] 120 volt grid tie inverter On 11/16/2010 11:20 AM, Richard Scott wrote: The KACO blueplanet 1502xi inverter has a minimum MPPT voltage of 125Vdc but cannot be connected to a battery. I would not recommend hooking a battery up to the input of just any grid tie inverter unless it was designed for it. The current from a battery is significantly higher than that of a PV array and can damage the inverter or cause it to fail in a way which could be unsafe. Sincere regards, Richard Scott KACO New Energy One thing that could help to make these kinds of connections work would be to add a power resistor in series, in-between the inverter input and battery. This will limit the current and make the battery look "more like" a PV array. Of course you will lose some energy in that resistor, but the MPPT algorithm would have a much better chance of getting it right and current limiting safely. If it's better than going without, then that's better than nothing. And, of course, one would have to pick a proper resistance (a few Ohms, typically), and a minimum power rating of the resistor. boB On 11/16/2010 10:34 AM, Peter Parrish wrote: Bob, So he is grid-tied. But his household loads are all 100 Vdc? I wonder how he charges his battery bank If it is PV with a charge controller (or wind), then I think he can export to the grid and get both a rebate from his utility and a Federal Tax credit. I assume that he is not interested in powering the 20 HP motor for his sawmill! If he is charging his batteries some other way (small hydro) I dont know about the incentives. But it looks like he is staring at a fully charged battery bank with generation to spare and is thinking Why not sell this juice to my utility company? Fair question. I have no direct experience with the SB700, but it has been around for eons, and presumably is rock solid and can MPPT at 100 V all day long. The Fronius inverters only work down to 150 V. The Exeltech inverters that I am familiar with work down to 200 V. There must be lower voltage, grid tied inverters out there but the battery backup versions assume 24 Vdc or 48 Vdc battery banks. So, with the exception of the SB700, there seems to be a gap between 48 V (maybe 60 Vdc) and 150 Vdc. - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 ________________________________________ From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of bob ellison Sent: Tuesday, November 16, 2010 5:23 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] 120 volt grid tie inverter Peter, The fellow is living with a 120 volt DC battery bank, using it to power his place and a 20 HP motor on his sawmill. He wants to reduce the electric bill further than he already has. So we are looking for an inverter that will take 120 volt DC nominal from the panels / batteries and send it to the grid. I am supposed to meet him tomorrow and hope to have more info afterwards. Thanks, Bob Ellison From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Peter Parrish Sent: Monday, November 15, 2010 8:38 PM To: glenn.b...@glbcc.com; 'RE-wrenches' Subject: Re: [RE-wrenches] 120 volt grid tie inverter Bob, Do you mean they have a nominal 120 Vdc input? Or do you mean that they want a 120 Vac (RMS) output? If the former, what output voltage do you want? If the latter, what is their nominal dc input voltage (better yet the range of DC input voltages over a range of typical temperatures and irradiance values)? - Peter Peter T. Parrish, Ph.D., President California Solar Engineering, Inc. 820 Cynthia Ave., Los Angeles, CA 90065 CA Lic. 854779, NABCEP Cert. 031806-26 peter.parr...@calsolareng.com Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885 -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Glenn Burt Sent: Monday, November 15, 2010 5:29 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] 120 volt grid tie inverter I think there is still an SMA 700W inverter available that uses 120VAC. -Glenn -----Original Message----- From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Bob Ellison Sent: Monday, November 15, 2010 11:14 AM To: RE-wrenches Subject: [RE-wrenches] 120 volt grid tie inverter I have a customer who wants to grid tie a 120 volt dc system Does anyone know of an inverter that can do this? 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