what happens if bms disconnects and solar charger

[big jerr]

Ok, something I just thought about as I was preparing to purchase lithium batterys, I have a victron 100-30 solar charger fed by 4 100 watt panels and a 22OOwatt sine wave inverter proposed to connect to 200 amp lithium battery but a thought just occurred to me on the BMS ??
1. what will happen if the BMS should disconnect for whatever reason and cut power to the victron solar charger ??
every thing Ive read says never cut battery power to victron before disconnecting solar input first, bad things can happen to the victron
thanks Jerr

 

[Bob B]

This warning is ALMOST universally ignored and I haven't seen any reports of people blowing up their equipment by doing that.
The standard explanation is that it is only important for the batteries to be connected first at the initial power up so the SCC can determine the battery voltage..... and if the SCC is properly configured this won't happen very often.

I personally am in the relatively small group of people who don't like the idea of switching the battery power on and off for the SCC or the inverter ..... even if there was no risk of damage .... I just prefer switching at the lowest power point I can find .... so I use a relay / SSR based BMS and get creative about remote controlling and powering things off.

 

[big jerr]

AH, I think I get -it, thanks Jerr

 

[HRTKD]

Some of this comes down to you properly configuring your equipment. Buying quality, programmable equipment is another factor.

The solar charge controller should be set to charge to a certain voltage, let's use 14.4v. The BMS should be set to cut off charging at a slightly higher voltage, say 14.6v. That way, the BMS never really has cut off charging.

But lets say something does wonky and the solar charge controller charges past the cutoff used by the BMS. For a good quality BMS that means that the charge circuit is cut off (open), but the discharge circuit is still active (closed). Your solar charge controller would still have power.

If your battery goes dead overnight and the BMS cuts off the discharge circuit, who cares? The solar charge controller isn't getting any input from the PV so no harm, no foul. Besides, if that happens, you have bigger problems to deal with.

 

[sunshine]

The solar charge controller should be set to charge to a certain voltage, let's use 14.4v. The BMS should be set to cut off charging at a slightly higher voltage, say 14.6v. That way, the BMS never really has cut off charging.

Yes!
A less technical approach-
If your battery disconnects while there are other loads on in the system @ less than the PV input there wont be much of a voltage difference anyway? The CC wouldn't know the difference.

 

[HRTKD]

If your battery disconnects while there are other loads on in the system @ less than the PV input there wont be much of a voltage difference anyway? The CC wouldn't know the difference.

I'm not following you. Can you explain this in more detail?

 

[sunshine]

Can you explain this in more detail?

Using your figures- the CC is putting out 14.4v and current is going to the battery and a load. While there is power coming from the PV the CC only sees a single load out - doesn't differentiate between load or the battery - so will only react to the changed resistance if one or the other is disconnected and at the 14.4v.,,,battery or no battery.
It is also possible that the load within the CC itself, the display etc, that comes on when the battery is first connected may be do the same as the external load.

 

[HRTKD]

Using your figures- the CC is putting out 14.4v and current is going to the battery and a load. While there is power coming from the PV the CC only sees a single load out - doesn't differentiate between load or the battery - so will only react to the changed resistance if one or the other is disconnected and at the 14.4v.,,,battery or no battery.
It is also possible that the load within the CC itself, the display etc, that comes on when the battery is first connected may be do the same as the external load.

I think I see what you're saying. In that case, the BMS could disconnect the charge circuit. But the discharge would still be active. This scenario would happen only if the solar change controller (or any charger/converter) was providing more watts than the non-battery load was consuming.

 

[sunshine]

This scenario would happen only if the solar change controller (or any charger/converter) was providing more watts than the non-battery load was consuming.

Yes. This would be the case in most situations. Probably why I haven't burnt out one of my pwm's yet,

Most likely the destroyer of CC's is the voltage spike at panel connection (obviously says everyone!!). This may even exceed the Voc that we base the maximum V of the CC's input on. We don't get this spike when the CC is permanently connected - look at all those pwm's that are wired into foldup camping panels that only come with alligator clips to hook up the battery.

Probably be justified to put a dummy load across the pv's terminals before connecting to the CC at any time, specially under load, to take out any voltage spikes and save those mosfets or at least connect the common terminal first.

 

[mikefitz]

every thing Ive read says never cut battery power to victron before disconnecting solar input first, bad things can happen to the victron

Many controllers will operate at either 12v or 24v or higher battery voltages and their charge profiles are adjusted to match, thus the requirement to connect the battery first before the solar panels so the controller knows to implement the correct charging program. Some controllers can become confused and charge at the wrong voltage if the connection/re connection suggested sequence is not followed. Thankfully with Victron controllers, once first connected, 'remember' the battery voltage, so any subsequent incorrect disconnect sequence does not cause issues.
Its possible in theory that a battery disconnect could cause damage but evidence suggests this rarely happens on quality products. However I suspect that there could be issues if the disconnect were to occur frequently with the controller operating at the limits of solar voltage and current at the input to the controller.

With a correctly deigned systen the battery BMS should not operate, as the various chargers and loads should have setups such that they operate within the BMS protection limits. The BMS should be regarded as a last line of defense to protect the battery and systen from unsuspected faults, a better term for the BMS would be battery protection system.

Mike

 

[big jerr]

great info ,thx

 

[Substrate]

Part of the reason SCC's typically don't like to see panels connected first, is being designed for user-safety.

Ie, when the panel is connected first, the SCC can't mind-read, and what it thinks is on the other end is a ZERO VOLT BATTERY.

Whether it's lead, LFP or something else, trying to charge a zero-volt-battery is an invitation to disaster, and for some SCC's won't work initially, or if they do when they see a magical return to real voltage when you do attach a real battery, go into a fail-safe mode that may differ from what your intended internal settings are, like only doing float charging, when you have specifically set up for higher CV's.

It all depends on how your SCC manufacturer views things.