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MPPT, BMS and Inverter wiring question

Bugzee

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Joined
Apr 15, 2022
Messages
12
Location
Sri Lanka
Hi all...

I'm a newbie here and I'm planing to build an off-grid system for my home.
So far I'm planning on using a DIY LiFePO4 battery 280ah 24v (From Amy).

Referring to the sketch attached (ignore any wire sizes, switches or fuses) I have a question.
Assuming it's day time (charge controller is up and running) and for some reason the battery is very low (assume the battery was used the previous night).
If the inverter is turned on, won't the power from the charge controller sent to the inverter as well?

The ideal situation would be that the inverter will draw power only from the battery and not the charge controller.
Is this the way it would work or is there some sort of solution to this?

I'm planning on using a 300A or larger Daly Smart BMS.

Thanks.

1650091750731.png
 
Hmm... The solar charge controller can output only 40amps. Can the inverter run with that power?

Is the design above commonly used by everybody?

I'm not comfortable powering the inverter from the solar charge controller...
Hi Bugzee,

If you use this inveter @dyatime, then you could get power from the solar panels. This means the power from solar through the MPPT to battery. And it seems your inverter is little bit bigger for the battery. 300A BMS is enough.
 
@indalecio I think I maybe able to use the load of the solar charge controller to activate/deactivate a relay to control the inverter.
This way, when the bat level is low, inverter will be turned off...
 
Some inverters have a programmable parameter called low voltage disconnect that does just that.
The inverter draws power from the battery. If it's on, some of the battery charge will go to it. But
a) You can turn it off yourself.
b) If no loads are drawing power from it, the idle draw will likely be small enough to not really matter.

Your BMS should really have a low voltage disconnect setting, anyway.
-
 
Most folks want their solar charge controller to run off the panels.

If you want low voltage disconnect that is a different issue but it can be arranged by choosing the correct inverter and if necessary using a Victron battery protect.

I made a little textual drawing of the core of your system.
I added a shunt because I suspect you will want one.

Code:
positive
|<->fuse<->battery<->shunt<->|
|->fuse->inverter------------|
|<-SCC<----------------------|
                      negative
The systems we make are like a ladder.
The risers are the busbars and the rungs are the circuits.
In this topology either or both the battery and solar charge controller can supply power to the load.
Say the charger is providing 40 amps and the inverter is drawing 50 amps.
The 10 amp balance will come from the battery assuming the battery can provide it.
Say the charger is providing 40 amps and the inverter is drawing 30 amps.
The 10 amp balance will go to the battery assuming the battery can accept it.
 
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Some inverters have a programmable parameter called low voltage disconnect that does just that.
The inverter draws power from the battery. If it's on, some of the battery charge will go to it. But
a) You can turn it off yourself.
b) If no loads are drawing power from it, the idle draw will likely be small enough to not really matter.

Your BMS should really have a low voltage disconnect setting, anyway.
-
yeah, there is low voltage cut off in Daly Smart BMS.
 
Most folks want their solar charge controller to run off the panels.

If you want low voltage disconnect that is a different issue but it can be arranged by choosing the correct inverter and if necessary using a Victron battery protect.

I made a little textual drawing of the core of your system.
I added a shunt because I suspect you will want one.

Code:
positive
|<->fuse<->battery<->shunt<->|
|->fuse->inverter------------|
|<-SCC<----------------------|
                      negative
The systems we make are like a ladder.
The risers are the busbars and the rungs are the circuits.
In this topology either or both the battery and solar charge controller can supply power to the load.
Say the charger is providing 40 amps and the inverter is drawing 50 amps.
The 10 amp balance will come from the battery assuming the battery can provide it.
Say the charger is providing 40 amps and the inverter is drawing 30 amps.
The 10 amp balance will go to the battery assuming the battery can accept it.
A newbie question here, what's a shunt? :)
 
This is the solar system circuit drawing.View attachment 91620
OK you have the solar charge controller and the inverter connected directly to the battery.
That is an alternate way to do things.
Your way will work but so will mine.
In your system you have the dc loads running off the load ports of the solar charge controller.
That works but only the low end, low power solar charge controller's have load ports.
I urge you to reexamine my diagram and I assure you that it works.
I also assure you that it is very common topology.
 
hmm... lots more information to take in...

Anyway, I'm currently drawing a detailed diagram of the system I wish to make (with fuses, wire sizes and etc).
I'll post it in a new thread in the next week or so (I'm currently reading the Resources for beginners stuff), so that you guys can take a look, review it and see..

Thank guys for the help so far... (y)
 
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