Battery Inverter Design with multiple small inverters (in Series) - Page 1
Forum section: electronics / projects

author: jmibk
author_rank: Regular Contributor
timestamp: November 06, 2022, 10:52:47 am
content: I fell over a - for me - new idea of battery ac inverters (in that case a power station) that they don't boost the battery voltage up to 350 - 400V DC and then doing PWM switching to get the AC voltage of 230V/50Hz.They use multiple small inverters, that are driven by a 6S liIon battery pack and are synchronized to get a high AC voltage when they are in series.Attached you can see a simple schematic what is meant.The company is called instagridhttps://instagrid.co/product. I'm not a member of this company, I'm only interested in that new approach and like to build my own (ac inverter).There are a few videos out there, where they tell how this is working (in general) and there are also some pictures of the PCBs.Parts they are using:Mosfets:https://assets.nexperia.com/documents/data-sheet/PSMN1R0-40YLD.pdfISO Drivers:https://www.ti.com/lit/ds/symlink/iso7041.pdf?ts=1667731449521They seem to use a 6S system for the individual inverters and pack 24 of these in series. They claim that they have also the full output voltage, if two mini inverters fail.Another thing is that they can control the output voltage of each module, so you can do a kind of battery balancing if you lower the output voltage on modules with a lower cell voltage than the others.The output voltage seem to be generated via SPWM technolgy - thats not a new kid on the block.My questioning about charging the modules.- they can use the H-Bridge body diodes for charging (they build a full wave recifier)- they can use the H-Bridge to realize a full active rectification- is it possible to reduce the voltage if you generate a little sinusodial wave via the H bridge to "lower" the voltage on the module when charging?Whats yout ideas about that concept and what do you think is that working in detail?

author: moffy
author_rank: Super Contributor
timestamp: November 06, 2022, 11:39:40 am
content: While an interesting idea it seems to be a very complicated way to go about a battery to 240V inverter. It would make some sense if the output power devices were not available, but they are. It just seems, complex, unecessary and expensive. You have to have more controllers, as you control not only each unit but also have to monitor and control them as a group. Sorry I can't see a compelling advantage.

author: jbb
author_rank: Super Contributor
timestamp: November 06, 2022, 11:35:53 pm
content: I think this would be called a “stacked H-bridge” converter in the scientific literature. From the photo it looks like 20 modules are stacked in series.MOSFET H bridge can indeed be used ‘backwards’ to charge the batteries. Turning on a MOSFET gate will ‘short out’ the body diode and reduce voltage drop / conduction losses.Producing lower voltage is (prepare for hand waving!) basically accomplished by scaling down the duty cycle commands to each switching block.IMHO the upsides are:- no big DC-DC converter (which would be required with 48V pack)- those cells could put out heaps of surge current to start up a load (eg power tool)- with 20 modules, you could get 2x20+1=41 levels out of the inverter. This would allow a huge reduction in the 50Hz output filtering compared to a classic ‘big H bridge’ design (2x2+1=3 levels) saving weight and condition losses.- switching actions are a shared around many modules, so the aggregate switching frequency (which also determines filter size) could be several times the at-the-silicon switching frequency. This reduces switching losses.- losses are spread over many devices, which might help with heat sinking.Downsides:- Clearly there are more switches, gate drivers etc.- how to synchronise switches?- all modules would be ‘danger high voltage’- does spreading all that switching stuff around cause ElectroMagnetic Compatibility issues?- no opportunity to tap off some DC for eg a USB C output

author: David Hess
author_rank: Super Contributor
timestamp: November 07, 2022, 03:01:35 am
content: Reliability will be lower with so many extra parts. The series stacking creates a much larger high voltage hazard.What problem are they solving? High power density inverters are a solved problem at this point.

author: Phoenix
author_rank: Frequent Contributor
timestamp: November 07, 2022, 05:25:07 am
content: Relectrify are doing something similar, but even more discrete. I think they switch/bypass individual cells to create the sinewave.https://www.relectrify.com/technologyQuote from: jbb on November 06, 2022, 11:35:53 pmYou'll also find it called "cascaded H-bridge".