96v inverter, two banks of 48v batteries

jhines97jhines97 Registered Users Posts: 2
I have inherited a 96V inverter.  I can buy (for reasonable cost) two 48V MPPT charge controllers that are UL listed.  Would I be able to hook the MPPT charge controllers up to their respective battery banks, and then use a connection with diodes to put both battery banks in series so I can get the voltage the inverter wants?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    You can connect two 48 volt controllers to the lower and upper 1/2 96 volt battery banks. The big issue you have is that the "upper bank" controller has a +48 VDC offset--Both for "system ground" and the "+48 volt" terminals.

    You do have the option of series/paralleling the battery bank (96 volts for inverter, 48 volts for charging). There would no diodes needed.

    Or you can get a 96 volt capable charge controller... Here is one from our host:

    https://www.solar-electric.com/aerl-coolmax-srx-45-290-mppt-charge-controller.html
    https://www.solar-electric.com/lib/wind-sun/AERL_COOLMAX-SRX-Datasheet.pdf

    Not cheap....

    The other thing--Look at the AC inverter... At what point  doe a "free" 96 VDC input AC inverter become more expensive than a 12/24/48 volt AC inverter+battery bank+charge controller+etc...

    While higher DC input voltage AC inverters are common for industrial installations (computer UPS, etc.), you are getting into non-standard things for "residential/DIY" systems.

    More or less, over 60 VDC is considered to be hazardous voltage in most cases and should only be worked on by a qualified/trained technician and needs to be behind a locked door/enclosure... Anyway, that is a common commercial rule.

    Also, high(er) Voltage DC can be hazardous in its own right... DC voltage/current sustains arcs very nicely (DC Arc Welders >~25 VDC are used in many places). So, any switches/breakers/fuses need to be rated for >96 VDC (there are 150 VDC breakers/fuses out there--You just need to look at the specifications for the details). Typically, 12/24 VDC hardware is very common (cars, trucks, boats, etc.).

    My suggestion, is that define/measure your loads, always look at conservation (use the least amount of power needed for the job). Next, do a paper system design (solar array, battery bank) based on your loads. Then lastly, look for equipment (96 VDC inverter and battery bus, vs 12/24/48 volt battery system) that will meet your needs (probably several iterations, because there are different solutions with different costs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jhines97jhines97 Registered Users Posts: 2
    BB. said:
    You can connect two 48 volt controllers to the lower and upper 1/2 96 volt battery banks. The big issue you have is that the "upper bank" controller has a +48 VDC offset--Both for "system ground" and the "+48 volt" terminals.

    You do have the option of series/paralleling the battery bank (96 volts for inverter, 48 volts for charging). There would no diodes needed.

    Or you can get a 96 volt capable charge controller... Here is one from our host:

    https://www.solar-electric.com/aerl-coolmax-srx-45-290-mppt-charge-controller.html
    https://www.solar-electric.com/lib/wind-sun/AERL_COOLMAX-SRX-Datasheet.pdf

    Not cheap....

    The other thing--Look at the AC inverter... At what point  doe a "free" 96 VDC input AC inverter become more expensive than a 12/24/48 volt AC inverter+battery bank+charge controller+etc...

    While higher DC input voltage AC inverters are common for industrial installations (computer UPS, etc.), you are getting into non-standard things for "residential/DIY" systems.

    More or less, over 60 VDC is considered to be hazardous voltage in most cases and should only be worked on by a qualified/trained technician and needs to be behind a locked door/enclosure... Anyway, that is a common commercial rule.

    Also, high(er) Voltage DC can be hazardous in its own right... DC voltage/current sustains arcs very nicely (DC Arc Welders >~25 VDC are used in many places). So, any switches/breakers/fuses need to be rated for >96 VDC (there are 150 VDC breakers/fuses out there--You just need to look at the specifications for the details). Typically, 12/24 VDC hardware is very common (cars, trucks, boats, etc.).

    My suggestion, is that define/measure your loads, always look at conservation (use the least amount of power needed for the job). Next, do a paper system design (solar array, battery bank) based on your loads. Then lastly, look for equipment (96 VDC inverter and battery bus, vs 12/24/48 volt battery system) that will meet your needs (probably several iterations, because there are different solutions with different costs).

    -Bill
    Thanks!  I am looking at the TS-MPPT-60-600V-48-DB-TR (my solar panels are wired into strings of ten at 32.8V max each, so a string has a max voltage of 328V DC with only 9A).  I am guessing they ran that way to use much smaller wire since the Fronius string inverter that I have is capable of up to 4 input strings with each up to 600V.  The TS-MPPT-60-600V-48-DB-TR includes a DC transfer switch so i can transfer to those in the event of a power outage.  I will look at a 48V inverter instead of re-using the 96V inverter that I have (yep it is a pure sine wave output UPS that was used for 3 years in our data center here at work so it is capable of 6000VA or 4200W) I know I would lose some efficiency using a modified sine wave inverter to the power input and then using the UPS to charge it's internal battery bank and run the inverter.  I was just trying to figure something out that was less expensive.  However, it sounds like buying a 5 kW or 6 kW pure sine 240/120 inverter is the way I should be heading to keep from using something dangerous.  
  • BB.BB. Super Moderators, Administrators Posts: 32,212 admin
    Yes... The "high voltage" of (typically) 600 VDC max input solar controllers (Grid Tied inverters or MPPT solar charge controllers)--The high voltage and low current is much easier to send 100's of feet... Vs trying 12 volts at tens to hundreds of amps more than 10-20 feet without lots of very expensive copper cabling. The DC to DC buck type switching (down) converters do the energy conversion very nicely (at 95% or so at rated power). The problem is that 600 VDC input silicon transistors are expensive, and makes for expensive controllers.

    If you can, avoid using an MSW (modified square/sine wave) AC inverter... There are lots of "harmonics" in MSW wave forms (vs the single 50 or 60 Hz in a "pure sine wave"). For filament lamps, electric heaters--The extra harmonics are not an issue. But when connected to induction motors (refrigeration compressor, etc.) the extra harmonics don't "help turn the motor shaft", they instead just cause circulating currents inside the motor--Which is just waste heat. Can also cause "noise" in wiring and devices. And for transformers, some "power bricks/adapters", and (some) electronic power supplies, they do not like MSW either (again overheat). The problem is that 80% of the devices probably work OK on MSW, and 10-20% do not... It is very hard to find the 10-20% that do not (and tend to "die young").

    And I highly suggest that you look at your real power needs... Ideally, you want to first conserve energy (cheaper to save energy than to generate energy). Second, measure/estimate your daily energy usage... A 6 kWatt AC inverter (or inverter-charger) is not that expensive--But when you tie it with a really large (aka expensive) battery bank (that may need replacing every 5-10+ years), and a large solar array and backup genset (if you want a genset)--This stuff adds up to big $$$, and a good amount of $$$ down the road for maintenance (battery replacement every 5-10+ years, or more often if something "goes wrong", new inverter and charge controllers every ~10+ years, etc.).

    For an "off grid solar power system", it is not unusual to run the costs to $1-$2.50+ per kWH... And you may be paying $0.20 per kWH or so for utility power.

    Yes, this is a "solar/off grid power forum"--But I/we really want you to be fully informed and prepared for what the costs and complexity are.

    Off grid/hybrid solar rarely saves money. Grid Tied systems can save lots of money (solar array+GT inverter--Cheap, efficient, not many expensive things to go bad in just a few years). If this was for a utility connected home that loses utility power for less than 1-2 weeks every year (or less often)--A backup genset is usually way more cost effective.

    If you need power where there is no utilty, or need backup power for weeks or months out of the year, then Off Grid/Hybrid solar can make sense....

    But for a full off grid system--Conservation is usually the first step--What would your power usage look like if your utility bill went up by 10x tomorrow... All of a sudden, conservation looks a lot better (or only run the minimum loads needed while on backup power).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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