BMS common port vs seperate port

[GXMnow]

It is possible that a poorly designed unit might have a problem dealing with a disconnect, but any well made unit should not have a problem. The BMS opening the charge side is really no different from a fuse or breaker opening. If a fuse blowing will cause a device to self destruct, it is a bad design. When the output becomes unloaded, for any reason, the voltage will start to climb up. The regulation circuit should quickly reduce it's output current all the way down to near zero to keep the output voltage at the maximum absorb voltage setting. It should then pull less current from the solar panel, so the panel voltage will rise, but if the system was properly design with the open circuit voltage below the charge controller limits, this should also not be a problem. I did see a few super cheap PWM charge controllers, that limited the battery voltage by actually shorting out the panel with a transistor. This will cause the voltage limiting transistor to try and dissipate the entire power that the solar panels are producing. That may work for a 50 watt panel, but much more than that is a bad situation. Stick with a quality MPPT charge controller, and it should not be an issue.

 

[Dzl]

Just a second thought, if the BMS disconnects the battery from the charger, wouldn't the charger burn out from the panel? I remember Will's video said I should always connect the battery to the charger before connecting the panel.

This will not be an issue for a Victron controller, others I am not sure about. To paraphrase the general feeling around here, with the exception of charge controllers that explicitly recommend against it, and the possible exception of cheap off brands, its a non-issue. But as a default, always follow the guidance the manufacturer gives. I suggest reading the thread @BiduleOhm linked to above.

 

[Cal]

This will not be an issue for a Victron controller, others I am not sure about. To paraphrase the general feeling around here, with the exception of charge controllers that explicitly recommend against it, and the possible exception of cheap off brands, its a non-issue. But as a default, always follow the guidance the manufacturer gives. I suggest reading the thread @BiduleOhm linked to above.

It's not the charge controller that can get damaged when battery is disconnected prior to disconnecting PV, but other devices that could be connected to the same bus. It's possible for the CC to output panel open circuit voltage (Voc) during this condition.

Edit, did Will have an oscilloscope on the CC output to check for spikes?

 

[Dzl]

It's not the charge controller that can get damaged when battery is disconnected prior to disconnecting PV, but other devices that could be connected to the same bus. It's possible for the CC to output panel open circuit voltage (Voc) during this condition.

Edit, did Will have an oscilloscope on the CC output to check for spikes?

I'm not sure that Will has an oscilloscope at least I've never seen him use one in his videos, I'm not sure what sorta testing he did, as I recall this was just a conversation not a published video. Justin, a tech from Bay Marine Supply, also did his own testing, and saw no issues, he may or may not have had one. Not sure. But he struck me as a fairly conservative guy. I suggest reading through that thread if you haven't already, your input would benefit the conversation.

 

[Cal]

My Blue Sky CC is 15 years old. Here's their warning.

WARNING: The 2512 operates on battery power, not PV power. A battery must be connected with a minimum voltage of 9V

for the unit to operate. Battery voltage on the battery terminals of the 2512 must be the same as actual battery voltage within a
few 10ths of a volt for proper operation. DO NOT disconnect the battery with PV power applied as appliances still attached to
the output of the 2512 may be damaged by voltage spikes resulting from sudden removal of the battery, particularly when used
with higher voltage 60 cell PV modules. ALWAYS remove PV power first before disconnecting the battery.

 

[grizzzman]

I'm not sure that Will has an oscilloscope at least I've never seen him use one in his videos, I'm not sure what sorta testing he did, as I recall this was just a conversation not a published video. Justin, a tech from Bay Marine Supply, also did his own testing, and saw no issues, he may or may not have had one. Not sure. But he struck me as a fairly conservative guy. I suggest reading through that thread if you haven't already, your input would benefit the conversation.

He showed a simple DSO when he destroyed a LFP in his last utube

 

[Dzl]

My Blue Sky CC is 15 years old. Here's their warning.

WARNING: The 2512 operates on battery power, not PV power. A battery must be connected with a minimum voltage of 9V

for the unit to operate. Battery voltage on the battery terminals of the 2512 must be the same as actual battery voltage within a
few 10ths of a volt for proper operation. DO NOT disconnect the battery with PV power applied as appliances still attached to
the output of the 2512 may be damaged by voltage spikes resulting from sudden removal of the battery, particularly when used
with higher voltage 60 cell PV modules. ALWAYS remove PV power first before disconnecting the battery.

This is good info, It would be interesting to see if this advice is current. From what I know, Victron also used to recommend against it, but has since revised that and determined/decided that while the reasons for their initial caution were hypothetical problems their engineers/technical writers were concerned about, they did not prove to be problems in the real world and they have since removed the warning. It should be stated that what applies to Victron may not apply to other manufacturers who may design things differently or be more or less cautious. I also don't know if Victron was considering the effect on other devices or just their own controllers (I would hope both).

On a practical note, if the Blue Sky CC cautions that harm can/will come from disconnecting the battery, how is OCP and BMS disconnection handled?

 

[Bob B]

This is one of the most commonly ignored warnings you will find in this forum. Many will find a tidbit or source that justifies ignoring the manufacturers warning about disconnecting the battery .... and others like me will use it as a reason for implementing a more complex solution than just disconnecting the battery power.

For my RV pack, I am attempting to disconnect charge and load independently using remote shutdown or by switching off input power from chargers if no other way is available ... and only disconnect the battery via a manual disconnect or fuse.

For my boat electronics, I am going with the more simple ... just disconnect the battery if there is a problem. I have a smaller 100ah battery with the JBD style BMS for that application. I don't expect any higher than 10A draw on that battery, and am also a lot less likely to have the BMS disconnect because one of the protection thresholds was exceeded.

There's good info supporting both schools of thought already one this thread ... only you can decide which way is better for you.

 

[Cal]

On a practical note, if the Blue Sky CC cautions that harm can/will come from disconnecting the battery, how is OCP and BMS disconnection handled?

OPC = over current protection (?) should be no different than over voltage protection where a relay opens panel supply. I also have a toggle switch to manually disconnect solar panels. I've gotten surprised with sparks several times when working on power cables and only disconnecting the battery. The 12V power bus is still HOT even after disconnecting the battery (at least when the sun is out).

Disconnecting solar at the battery side of the CC is just not smart. It can easily lead to surprises.

 

[GXMnow]

I wonder if just installing a large capacitor at the charge controller output would be enough to keep it stable in the event of a fault disconnect? It might slow down the voltage rise rate enough for it to regulate, lower the current, and keep the voltage from going too high. I would suggest a cap rated for at least double the battery bank voltage, and several thousand micro farad. I added a large cap on my battery bank to reduce the ripple current at the battery a bit. Since my Schneider XW-Pro is a true sine wave inverter and a power factor corrected charger, it does produce a lot of ripple current at the batteries. Virtually any proper install should have a fuse on the battery bank. It would sure be dumb to have a charge controller pop because a load blew the fuse on the battery.

 

[BiduleOhm]

I wonder if just installing a large capacitor at the charge controller output would be enough to keep it stable in the event of a fault disconnect? It might slow down the voltage rise rate enough for it to regulate, lower the current, and keep the voltage from going too high. I would suggest a cap rated for at least double the battery bank voltage, and several thousand micro farad. I added a large cap on my battery bank to reduce the ripple current at the battery a bit. Since my Schneider XW-Pro is a true sine wave inverter and a power factor corrected charger, it does produce a lot of ripple current at the batteries. Virtually any proper install should have a fuse on the battery bank. It would sure be dumb to have a charge controller pop because a load blew the fuse on the battery.

I bet it would if it's big enough (probably at least tens of thousands of µF)

A MOV or a TVS are other possible options (far cheaper), but no guaranties it'll work.

 

[Dzl]

OPC = over current protection (?) should be no different than over voltage protection where a relay opens panel supply.

Correct OCP = Overcurrent Protection. I play in / pay attention to the kiddie end of the pool when it comes to system size and complexity, but I am not familiar with the type of relay based OCP you are referring to. Essentially you are saying that if your main battery fuse or breaker blows, this will trigger a relay on the input side of the SCC to disconnect? Same deal with the BMS disconnecting?

 

[Cal]

We're talking about a catastrophic event should the main fuse blow. The likes of a vehicle accident or possibly operator error where a wrench is dropped in the wrong place. The probability is so small that it doesn't warrant further circuit protection.

I consider it sub-par engineering if OVP is switched at the battery and not at the charging sources. Major voltage spikes can be generated during turn-off. The alternator dc/dc is still operational and loading the alt. The converter is still operational and loading the generator or mains. Why look for problems?

 

[Will Prowse]

Will,

I am scared to go w/o a BMS. I posted previously about a Victron parasitic issue , (like you to look at in reviews going forward)

-Tim

Use a bms always

 

[Will Prowse]

He showed a simple DSO when he destroyed a LFP in his last utube

Yeah I didn't have an available 48V inverter charger and used a lead acid golf cart charger to test renogy pack. I was an idiot. It was transformer based charger and the voltage spikes shorted a fet. HVD didn't work, and I over charged the pack

I have a nice scope now, but my cheap scope can measure ripple. Doesn't matter how I measure it, I shouldn't use that cheap charger with fet based BMS. It worked fine on my other packs, so I didn't think much about it. I knew the output was dirty but continued using the charger. I'm the idiot.

 

[Will Prowse]

We're talking about a catastrophic event should the main fuse blow. The likes of a vehicle accident or possibly operator error where a wrench is dropped in the wrong place. The probability is so small that it doesn't warrant further circuit protection.

I consider it sub-par engineering if OVP is switched at the battery and not at the charging sources. Major voltage spikes can be generated during turn-off. The alternator dc/dc is still operational and loading the alt. The converter is still operational and loading the generator or mains. Why look for problems?

Yes good points.

 

[Will Prowse]

OPC = over current protection (?) should be no different than over voltage protection where a relay opens panel supply. I also have a toggle switch to manually disconnect solar panels. I've gotten surprised with sparks several times when working on power cables and only disconnecting the battery. The 12V power bus is still HOT even after disconnecting the battery (at least when the sun is out).

Disconnecting solar at the battery side of the CC is just not smart. It can easily lead to surprises.

True points, but try it. I haven't been able to destroy one yet. In theory, it should.

I would like someone to post pictures and proof of a destroyed SCC from doing this. I tried this with a box of controllers and they still work great.

And the capacitors in your inverter are probably keeping your SCC active. That's all. Put a volt meter on it and watch it fall slowly. Scc needs reference voltage to begin charging. If it doesn't sense battery, it cannot charge.

 

[Will Prowse]

My Blue Sky CC is 15 years old. Here's their warning.

WARNING: The 2512 operates on battery power, not PV power. A battery must be connected with a minimum voltage of 9V

for the unit to operate. Battery voltage on the battery terminals of the 2512 must be the same as actual battery voltage within a
few 10ths of a volt for proper operation. DO NOT disconnect the battery with PV power applied as appliances still attached to
the output of the 2512 may be damaged by voltage spikes resulting from sudden removal of the battery, particularly when used
with higher voltage 60 cell PV modules. ALWAYS remove PV power first before disconnecting the battery.

Yes, and nearly every SCC manual on the planet states a similar warning.

 

[Cal]

True points, but try it. I haven't been able to destroy one yet. In theory, it should.

I would like someone to post pictures and proof of a destroyed SCC from doing this. I tried this with a box of controllers and they still work great.

And the capacitors in your inverter are probably keeping your SCC active. That's all. Put a volt meter on it and watch it fall slowly. Scc needs reference voltage to begin charging. If it doesn't sense battery, it cannot charge.

The warning states that a voltage spike will be generated when disconnecting the SCC at the battery. This is a know fact, it will happen. Just like a relay may spark when the contacts are opened. The cable has inductance. One of the main properties of inductors is that they want to maintain the current it is conducting. If your SCC is outputting 20A and all of a sudden the cable is opened, the inductance in the cable will do all it can do keep the 20A flowing. It does that by drastically increasing the voltage. That's the reason for the spark when the circuit is opened via relay. The faster the circuit is opened the greater the voltage spike. In your testing you may not have opened the circuit quickly enough, or current was too small or the cable length too short. There will be a spike unless the SCC has voltage spike suppression. You really need an oscilloscope to view the spike. From the magnitude of the spike one can make a determination if it could damage electronics.

 

[walter-]

The split port has a charge port and a battery port. The BMS can cut off Charge separate of battery (Load). The common port the load and charge port are together. If the split port sees high voltage It can shut off charging. If it sees low voltage, It can disconnect load.

Jumping into the discussion:
The Daly BMSs; are they 'functionaly' separate port BMSs? I mean; they have indeed only 2 connections shared for charging and discharging; but if conditions need to block for example charging, they still allow discharging? And vise versa?
Is this the case?
In their descriptions of the BMS -no manuals except for the software? - it is not really clear to me. Does an under voltage block both discharge and charging -for overvoltage, don't think it is really relevant?