Input AC-DC charger and Solar microinverter parallel connect

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Rosy_Posy
Rosy_Posy Registered Users Posts: 5
Hi dear solar lovers,

I have a boat with on board AC-DC charger 3.3kw (AC: 100-250V) DC 96V nominal (72-132v range), battery 42kwh 96V with smart BMS.

Can I just parallel connect input AC charger and 2-3 solar microinverters 300W , (say, 130V AC output nominal) ?
1.
Will such system work off grid while boat on the water, charging from 1kw solar PV through AC-DC charger instead of traditional solar charge controller?
2.
Can it work correctly  if just connect charger to 110AC regular grid, but keep solar microinverters connected parallel too?

The reason I want to use microinverters is that they have IP67 protection, but solar charge controller doesn't.

Thank you so much for any suggestions/comments.



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  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Welcome to the forum Rosy_Posy,

    It is difficult to understand what you want to do here... With electrical/solar/engineering questions, the details matter--A Lot!

    Can you post brand/model numbers/links to the micro-inverters / charge controller / AC inverter you currently have installed and/or thinking of using? I am not quite sure what you are talking about here (Grid Tied/Grid Interactive solar panel=>AC power? Or what)?

    Also--Can you tell us more about your battery bank? A 96 VDC lithium battery bank + BMS is not common for non-commercial installations. Typically, for residential solar systems, they tend to be 12/24/48 VDC based battery buses. 96 to 380 VDC power systems tend to be in large data centers or similar locations that need backup power systems.

    Are you using shore power / Genset / Solar? And/or/all?

    What is your "need"? More charging power? Where is that power coming from (shore or what)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Rosy_Posy
    Rosy_Posy Registered Users Posts: 5
    edited May 2021 #3
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    Hi Bill,

    Thank you so much for such detailed response. Please see more information below.

    I am building RIB e-boat with Chinese suppliers, mostly for fun, but keeping in mind producing more advanced copies in near future depends on how things go on.

    The 42kwh 96V battery I mentioned previously is assembling now for
    26kw outboard electric propulsion engine that has 96V nominal voltage.

    The battery will have 4 blocks 48.1V each, combined 2 parallel 2 in series, with low level BMS in each block and master BMS to balance whole package.
    It is Li-NMC battery.

    So basic charging way is shore grid.

    120V AC grid => AC-DC charger => battery.

    3.3kw 96v nominal AC charger will be this one:
    https://m.alibaba.com/product/1600146571862/LNLEE-EV-Charger-3-3KW-Waterproof.html?__detailProductImg=https://s.alicdn.com/@sc01/kf/H30bcce112be649d69f55545059ece3ecU.jpg_200x200.jpg

    So while on water I want to use 400w solar panel on T-Top roof, plus additional 400w fold panel.
    For six days a week when boat is not used these panels can generate about 20kwh energy, so I don't need to plug in boat to the grid, which is always problematic.

    So the task is to suggest/discuss off grid microinverter and/or structure that would work for this application.

    Once I have regular connection to the grid, I cannot use off grid microinverter simultaneously.

    Therefore, final question is, how to  combine solar power and grid charging together, having switched between them depending on need.

    All the best
    Kuz

    aka Rosy Posy


  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Kuz,

    In general, you can parallel multiple/different battery chargers to one battery bank. You need to program each charger "correctly" for your battery bank... And the total charging cannot exceed the maximum charging current for the bank (you need to check the battery and BMS specifications for those specs).

    Looking at your solar panels... Several issues here. Most solar charge controllers are rated for a maximum battery bank voltage of around 48 to 60 VDC nominal. So "standard" solar charge controllers will not work as is--You can take two 48 VDC chargers, one for the "low group" of cells, and the second for the "high group" of cells. You can run into issues if you try to network them (the "high charger" will have its "ground" at +48 VDC--And you cannot connect the two controllers in parallel with "ground referenced" communications links (RS 232, some RS 422, etc. unless you use optical isolators or other comm. channel).

    Another issue is almost all solar charge controllers are "buck type"--I.e., they take higher voltage (and lower current) and "buck"/down convert to lower voltage (and higher current) to charge the battery bank.

    You need to look at the Vmp of your solar panel(s)... Most run in the range of 30-40 VDC or so (at 25C/75F). So, they would not charge a 96 VDC bank, unless you put (for example) 3x 36 VDC Vmp panels in series for a 112 Vmp array.

    It sounds like you want to take the solar panel(s) and drive AC inverter(s) to power your 120 VAC input for your AC battery charger. While it is possible--You run into a bunch of issues...

    One is the panel Wattage should be higher than the load Wattage... Or the load will "collapse" the AC voltage. 3.3 kWatt battery charger an 400-800 Watts of AC load--I would not expect to work/be stable.

    If you want to parallel off grid AC inverters--Most do not parallel AC output and will not function correctly or possibly damage each other.

    Most off grid AC inverters are expecting stable DC Voltage bus... Connecting solar panels to "typical" AC inverter DC input causes issues (many inverters "shut down" if battery bus voltage falls below cutoff voltage and you need to turn off/on the DC voltage again to reset (inverters are different--And there are some larger AC inverters that can run from a solar array only--Most seem to be >>400 Watts).

    There are Grid Tied (Utility interactive) AC micro inverters... But they do need a separate AC off grid inverter to set grid voltage and frequency (may be a couple companies that can run GT micro inverters "off grid"--Still the issue of 400 Watt array and 3.3 kWatt AC battery charger).

    Probably the simplest setup... Just as a starting point--Setup your Vmp-array voltage to be at least 1.25x your battery charging voltage set-point (Vmp falls as solar panels get hot). And use a simple relay to cut off the solar power when Vbus>charging set point. You can get more complicated--But I am not sure I would try... Simple 120 AC power to charge at dock. Solar panels in series with a DC relay charge cutoff at camp.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Rosy_Posy
    Rosy_Posy Registered Users Posts: 5
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    Hi Bill,
    First of all, thank you so much again for all this information, that's incredible! And it illustrates that possible scope of troubles with AC charger "on the go" does not worth efforts. I will read your post couple of times before I understand everything and make the decision. But last question might be: do you think using solar charge controller may be 'OK' with connecting it to batteries? (DC-DC structure now)
    I found IP67 MPPT 96v DC output controller. (Input PV range is about 120-300v)
    It is about 25 pounds, but if it works, I can ignore size and weight, and consider this.

    All my best
    Kuz
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Good morning Kuz,

    That sounds like a "reasonable" charge controller... But look at the size of your solar array vs the capacity of the charge controller.

    Some large charge controllers are pretty inefficient at lower loads (i.e., "wasting" 50 Watts on a 400 Watt array is "a lot", "wasting" 50 watts on a 4,000 Watt array is not much. (no idea of the specifications on the controller you are looking at--Just an exaggerated example).

    Another issue to watch for on "portable solar charging systems". Most solar charge controllers boot and configure from battery bank power. If you connect the solar panels first, then the battery bank second, in some cases that can cause the charge controller to boot incorrectly/in a confused state, or even damage the charge controller if hit with solar panel energy first.

    The typical solution is to always connect battery first and solar panels second... And when shutting down, disconnect panels first, then the battery bank.

    Have fun!
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Rosy_Posy
    Rosy_Posy Registered Users Posts: 5
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    Hi Bill,
    I likely will try a solar controller, eventough it can consume something at low PV input. I do have one small question left. Do you think it is too dangerous to connect PVs directly to the battery without an extra relay, since the battery has BMS and high/low voltage cut off?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Lots of ways of looking at the issues there...

    Simple, many times, means more reliable. Complex, means more ways to fail (and many times, less obvious ways to fail).

    A basic BMS is there, in my humble opinion, as the last line of defense to protect the battery bank against anything else that may fail. I would suggest that the BMS operates (disconnects the battery/cells) only in exceptional situations. I would not suggest that the BMS over voltage cutoff be used like a "battery charger/controller".

    There are also some BMS systems that "balance" cell voltages to keep them nearly identical... That way, the charge controller setting to XX.X volts for the whole bank will keep the bank between 20% and 90% (or your choices) state of charge. You do have to look at the "balancing" BMS design to ensure your charging set-points are correct.

    There is/was at least one major BMS design that used a large fuse on the output bus of the battery bank. If anything went wrong (say a solar or inverter-charger was overcharging the battery bank), there was a "crowbar" relay that literately shorted out the battery bus and would pop the (large) battery bank fuse (as I understand from reading here).

    For lead acid batteries--The controller needs to plus width modulate to vary the average charging current to the battery bank (hold the set-point voltage for 2-6 hours). A more "complex" charging system vs the "typical" lithium charging system (no temperature compensation, charge 100% until "set-point", then stop charging.

    If I was going to inexpensive and "simple"--Let the BMS do its job (monitor per cell voltage, bank temperature, max current, etc.) and warn/shutdown if anything is exceeded. And a simple solar charger/controller that keeps the solar panel connected until the battery is full. That also lets you run DC loads (radio, GPS, lights, etc.) while charging with solar... Instead having the BMS shutdown the entire battery bus as you are charging/running DC gear on the boat.

    You also need to look at the cost of failure... With LiFePO4 and similar chemistries--The cost of failure (over/under charging) is possibly a new battery bank (no fire/explosion). With other common lithium rechargeable chemistries--The cost of failure (over/under charging, charging near/below freezing, etc.) can be a major explosion and/or fire--With possible toxic waste cleanup.

    I am certainly not an Li Ion expert... Just suggestions on how to look at the problem.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Rosy_Posy
    Rosy_Posy Registered Users Posts: 5
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    Hi Bill,

    Thanks for your comments on BMS. It is very helpful to me. Indeed, I didn't think that battery shut down will also cut off all electric equipment on the boat. And of course more layers of proper protection, as I see, is the right idea.
    So I will go with simple "boost" MPPT (say, 36V PV, 96v output, I found some on Chinese market), like you similarly suggested and double check numbers and this "simple" design with local engineers, once my battery and engine arrive.
    Thank you once again for your support, it helped me a lot to avoid problems.

    All the best
    Kuz

  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
    edited June 2021 #10
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    You are very welcome Kuz.
    By the way, don't pull (for exsmple) 12 VDC from the low cells on the lithium bank. That will unbalance the Li cells.
    Draw from 96 vdc and down convert to 12 VDC or have a backup 12 VDC battery for radio/lights/trolling motor/etc. as needed.
    Let us know what you did and how it is working.
    You may be the first person here doing this, but probably not the last.
    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,533 admin
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    Another thing to look at...
    Boost DC to DC converters should be designed for connection to solar panels...
    DC converters are usually designed for battery input power, not solar panels which are (mostly) behaving like current sources. Many times,  the standard DC converters will lockup or behave unpredictably when powered from solar panels throughout the day (dark light dark cycke).
    Some converters designed for solar panel power will say the have MPPT maximum power point tracking input.
    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset