Controller disconnect sequence and timing

[Bob D]

I am sorting through a conversion from FLA to LFP batteries, and have installed shunt trip breakers to shut things down if voltage extremes are reached, beyond the normal charging/discharging range.
I'll disconnect the PV panels first, but how long should one wait before disconnecting hte battery?
is a couple milliseconds enough?
Trying to decide whether a time delay is needed.

[boB]

No waiting necessary for the battery side.   As soon as its relay clicks off, everything is disconnected except for its little auxiliary power supply and even it discharges almost immediately.

boB

[Bob D]

What actually happens if the battery is disconnected first?
Does it immediately damage the controller?

[boB]

What actually happens if the battery is disconnected first?
Does it immediately damage the controller?

Nothing bad should happen.   Depending on what state the controller is in at the time, it might stay on by using the PV to power the Classic for a few seconds before the Classic realizes it is not charging anything.

Don't you have a battery side circuit breaker or disconnect and also a solar disconnect breaker or switch ?

Usually the sequence does not matter unless the user has some other kind of requirement in the system.

[Bob D]

Thanks for the info.
Yes,, I have shunt trip breakers on both sides.
They should never trip, only if other things have failed and the BMS needs to protect the battery.
The simplest for me is to first trip the pv breaker, then trip the battery breaker from the PC breaker auxiliary contact. That would provide a couple milliseconds delay.

[boB]

Turning off the battery first is what I like to do and then, when (or IF) the classic stays on because of the PV being connected, turning off the PV input will discharge the PV input capacitors and the Classic and display will go out as those input capacitors are discharging.   But that method is more of a kind of satisfaction to watch and also knowing that the input caps are already discharged and won't take several seconds at least.

You certainly do not need to do it that way though.

When I think of shunt trip breakers, I think of our breakers with the main breaker physically linked to a second half that is just a solenoid that trips it when voltage is applied.  But we can use the breaker like a regular switch as well.  I am guessing that your BMS breaker is very similar but maybe not ?   

boB

[Bob D]

The breakers are as you describe - a breaker with a gadget attached that trips the breaker when 24 volts is applied.
The 24 volts will come from the fault output on the BMS, which triggers if the normal charger and inverter shutdowns fail.
I am using AUX 2 as an input from the BMS to stop charging, and a similar approach on my Magnum inverter.
Those controls will normally keep the battery in good voltage territory.
However, if one of those controls fail, and the battery continues to charge or discharge beyond safe voltages, then the BMS will fault and trip the breakers.

The shunt trips I am using have an auxiliary contact, so I can daisy chain them to trip one after the other, although the time between them will be minimal.
If it is critical, I can install a time delay between the trips.

It sound to me that since this is a seldom (if ever) occurrnece, then I wouldn't be taking a huge risk if I tripped the PV in then the battery.

In your rapid shutdown system, do you sequence the shutdown or just dump everything at once.

[boB]

This is really neat that your BMS is one that tells the Classic (with AUX2) to stop !  That was the very first BMS interface feature for the Classic !  I could come in handy for other uses as well as for BMS's.

No sequence for rapid shutdown.  Just turn off the PV high voltage.  Then, if they have a battery disconnect module (BDM ?) it can also turn off the inverter if needed.  I think that BDM is for use with the Birdhouse unit with the red lever.

What company is your BMS ?   I have long lost track of who uses that charging trigger stop feature.

[Bob D]

The BMS is from Electrodacus.
http://www.electrodacus.com/

I will use the MPPT capability of the Classic, but will set the absorb voltage a bit higher than the charging stop voltage set in the BMS.
In my 24 volts system, charging will stop at 28.4 volts, so I will set the classic absorb to, say, 29 volts.
There are arguments about spending some time in absorb but it doesn't seem necessary if one isn't trying to completely fill the battery.

Basically, the Classic will be in bulk charging mode all the time until the BMS says stop, then then Classic will rest.
At that point the BMS sets the SOC to 100%.
Once the SOC drops to 87, it restarts charging until SOC = 90%, and cycles in that range until there is no sun.

Of course, the BMS also monitors cell voltage and stops charging or inverting if individual cells hit the individual limits.
It also does a bit of balancing.


The nice thing about the BMS is that the current capability is more or less unlimited - power doesn't run through the BMS.
It has outputs to cycle the charging, to shut down the inverter on low voltage, a fault output in case the voltages go hi-hi or lo-lo, where the battery would be damaged.  I am using that one to trip all the breakers - PV in, Controller out, Inverter, generator.

You will see on the website they advocate not using a charge controller but rather in a 24 volt system use 60 cell panels, and just turn them on and off using solid state relays.

[boB]

Sounds like you are doing all the right things.

A lot of people will try to use the extra energy of the sun during the day to do useful things after the battery is charged.

Thanks for the company name.  I know I have heard of them but can't remember if they were the ones that requested the Aux2 turn off feature or not ?