Benefits of interfacing nverter/charger and Lithium BMS

MattWallin

Hi all, I'm designing my system for my off-grid home.  I've been trying to make a decision on an inverter/charger and have settled on a Victron Multiplus. My hangup is deciding between the 2000VA and 3000VA model. The low standby power and capacity of the 2000VA model, with future intentions to buy another for split phase capability, makes that my first choice. However, the ability of the 3000VA model to interface with a Lithium BMS through a serial port, a capability the 2000VA model lacks, makes me wonder if that is worth going with the larger model?  I have done a bit of reading, and many sources seem to indicate that a reasonably flexible inverter with programmable operation parameters would do fine without a direct BMS interface.  I'm looking for real world experiences of those using lithium battery banks and would love to hear for/against arguments regarding foregoing the direct BMS interface. Thanks for any advice.

-Matt

mike95490

The goal is to not ruin your solar gear, to preserve your battery.
 The battery BMS  ONLY cares about the battery.  It can provide a signal to the Chargers and Inverters to "shut down" before the BMS disconnects the battery to save it.

When the BMS disconnects the battery, you have no reference voltage in the system, and the charge controllers can go berserk with 98 volts on the PV array and nowhere to send it to, so the system voltage can start to rise, since there's no battery to stabilize it.
What will your household inverter do when it's battery input goes to 75V ?  Will your charge controllers fry themselves ?

So having communication between the BMS, Charging sources and Loads, helps preserve the entire system.    You can carefully engineer and match components, and still have 2 sunny days with light loads, overvoltage the system.

MattWallin

Mike, thanks for bringing up the scenarios of potential issues. If I'm understanding your point, the the greatest danger is when generator charging brings the battery to a state of charge where the BMS cuts it off from said inverter/charger. The energy having nowhere to go, and the loss of reference voltage then damages other parts of the system. It makes sense that communications between components would be most desirable.  My question centers around whether the inverter/charger settings, in concert with proper BMS settings, could be configured such that, in the absence of a fault or malfunction, it would properly curtail charging before triggering the BMS to disconnect the battery? I guess I'm looking at it same way as the interaction of the solar charge controller and the BMS, with a similar coordination of the configurations to prevent the BMS from disconnecting when charging has reached full charge by solar means. It appears to me in my research, that many lithium systems run in this manner, with no MPPT to BMS direct communication.  I'm stretching the budget to get high quality equipment and I don't want to let the magic smoke out of any of it, so I appreciate advice and guidance from those who have lived with this stuff long term.

BB.

I like to treat the Lithium BMS as a last ditch emergency shutdown situation.

In normal system desgin/operation, the BMS will never cut out for over/under voltage, over current, over/under temperature, etc...

The BMS should keep everything humming along nicely, and ideally give you some warning that things are not working correctly.

And if something does go "south", the BMS will help prevent smoke and fire from your lithium bank (and protect against one cell/battery failure from killing the rest of the bank).

If the BMS does trip and cut off the battery bank--Then there is something horribly wrong and while it may not be great for your DC devices (charge controller, AC inverter)--It hopefully prevented a worst situation (destroying your battery bank and/or setting your home on fire).

Having some sort of communications/control between BMS and "other solar equipment" (shut down charging, shut down AC inverter "safely") can at least give you warning/protection before pulling the final system protection.

-Bill