DC to AC AND DC to DC? Power Loss & Configuration?

[Modular Hippo]

Happy New Year Fellow Solar Enthusiasts

Got a question that I hope someone might be able to give advise on.

I am putting together a traditional 48v system (2 x Seplos Mason as battery bank). The bank will be charged mainly by solar, but at times by generators and grid.

The battery bank will be traditionally connected to an inverter to give me AC for domestic use. The Inverter will allow me to run the load 100% of the grid when connected, or for example the generator if running (only use the battery bank when needed).

Here comes the question: The battery bank will not only be used for domestic AC use, but also to power an 48v DC Motor. What I am trying to understand is that if the DC Motors are connected directly to the batteries (and the solar panels generate power as well as my generator) then will the motors take electricity from solar and generator first before topping up any missing DC from the battery bank? Or will I potentially run into complications here? I guess that what I am trying to say is that I want the motors to always take what it can from the solar and generator before it uses the battery bank. If the motors draw less power then what the generator then would any surplus from the generator charge the bank? The behaviour I am after is what the DC to AC inverter gives but as this is DC to DC and not controlled by an inverter I wonder what the complications are?

I guess I could go via the inverter and go back to DC (in other words, from solar DC to AC via inverter, and after that us a rectifier to go back to 48v DC again). By doing so I could use the intelligence and configuration to set order of where the power should be taken from and what to do with surplus. BUT the reason for being reluctant in doing so is that I have read that between every conversion (AC/DC and DC/AC) you lose between 5-15% in energy loss. Converting twice could in theory (if correct) make me lose 30%. Any thoughts on how to best connect the DC motor highly appreciated

 

[TM48]

I'm not sure of the correct answer but definitely curious. What size motor and could it be replaced with an A/C motor? The battery charging with run at ~55v is that ok for the dc motor aka is it speed sensitive?

 

[timselectric]

All loads pull from the source with the highest voltage.
When charging, that would be from the charger first.

 

[Wibla]

I have to ask - what is the motor used for?

 

[mikefitz]

Any thoughts on how to best connect the DC motor highly appreciated

The motor draws power from the battery 'hub', if motor power if greater than combined active charger power, the battery SOC will fall. If the motor power is lower, the battery SOC will increase or hold at 100% (where all power is taken fron charging sources). If the charging sources cannot meet motor demand, the battery will make up the shortfall and the SOC will fall.
A suitable battery state of charge monitor with programmable output could control a DC contactor that only allowed motor activity above a preset SOC.

 

[RCinFLA]

The DC motor will act as a bleed on the battery and/or its charging current.

Whether the PV solar power or generator power immediately contributes to DC motor current depending on inverter/charger setup for charging bulk current, absorb voltage, and float voltage.

At the least, the battery will act as storage even if battery is not presently at SoC conditions requiring charging by inverter/charger.

You need to consider where in the DC path the motor is placed so any SoC Columb counting meter is capturing the DC motor load in the %SoC display. Assuming you are using the Seplos Mason SoC readout, the Seplos BMS provides the SoC and you are fine on the battery output terminals of battery pack which is after the BMS Columb counter.

If you have communications from Seplos batteries to inverter/charger, the SoC reported on inverter/charger should come from the numbers reported from Seplos batteries. If you don't have the comm link then the inverter/charger will not include the battery capacity sucked off by the DC motor. I would not trust inverter/charger self-generated internal SoC readout anyway.

 

[Modular Hippo]

Hi All (@TM48 | @timselectric | @Wibla | @mikefitz | @RCinFLA ),

Big thank you for the feedback, so glad I asked as seems that I have to thread carefully in relation to setup/configuration. Truth to be told, allot of the feedback is a bit over my head so feel I need to brush up on how it all hangs together. I now got a starting point for which I ma grateful.

The motors (actually 2), are meant to be used for leaving and entering harbour (in other words, the system is aimed to be used on a sailboat) Guess the off-grid concept is the same as for land, just on a boat The motor could be an AC motor but I am to deep in to the DC motors I already got in relation to outlay and other parts built around it so the aim is to find the best use for what I got.

Anyone interesting to see how it all ends, feel free to see more at: https://www.youtube.com/@modularhippo

Note: The 18650's and Daly BMS/Active Balancer/Parallel Units in background is the 'Secondary' battery bank and just a way for me to get my hands dirty. The 'Primary' bank is 2 x Seplos Mason



Thanks again and Happy New Year to you all!

 

[RCinFLA]

If the yet to be terminal spot-welded 18650 battery piles are LiPo type you are doing a relatively high-risk DIY battery pack for a boat.

When things turn badly, they turn into a fire blow torch you cannot put out until they exhaust themselves.

 

[Modular Hippo]

If the yet to be terminal spot-welded 18650 battery piles are LiPo type you are doing a relatively high-risk DIY battery pack for a boat.

When things turn badly, they turn into a fire blow torch you cannot put out until they exhaust themselves.

Hi RCinFLA - Thank you for heads up. I have seen the fires as plenty of them on YouTube. If I cannot safely contain the battery in a fire retardant box and if I am not 100% happy with them they will not come on-board Li-Ion and water is not a good match