SmartSolar: What if arbitrary, proprietary or random BMS cuts off battery?

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thanar asked Jan 16, '19

SmartSolar: What if arbitrary, proprietary or random BMS cuts off battery?

I have been using a SmartSolar 100/15 for some time now, charging a 7s lithium (li-ion 3.7V nominal) battery, by setting bulk and float values to 28.7V. Disabled equalization of course. I have also set load cut-off values so as not to over-discharge the battery. All loads pass through the SmartSolar unit.

I will be installing an arbitrary BMS that will do some cell balancing, as well cut-off the battery if a cell group drops below 2.5V or goes above 4.25V.

What would happen if the BMS would cut-off the battery from the SmartSolar unit? Would the unit be damaged in any way? I am asking because you are supposed to disconnect solar first, battery next.

edit:
It wouldn't matter if battery gets disconnected during the night, but what would happen if it disconnects while charging, because a cell group went over-voltage?

MPPT Controllers BMS

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wkirby answered · Jan 20, 2019 at 09:37 AM

It is just not fussed at all.
When disconnecting the battery and PV is sufficient then the Voltage on the battery terminals stays at the level it should be at. Even when MPPT changed from Absorb to Float, the Voltage at the battery terminals dropped down to Float Voltage, without a battery connected.
I tried with load and without load. If PV power was not enough to keep a load going without a battery then the Voltage at the battery terminals and load output would drop until the MPPT just turned off.

No smoke came out, no flames to see.

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thanar commented · Jan 21, 2019 at 07:55 AM

No smoke, no flames seems like a good thing. That's what I was hoping for, that the charger would not get damaged in the event the battery would get disconnected by the BMS.

You start your reply with "no joy I'm afraid"; what were you expecting?

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wkirby ♦♦ thanar commented · Jan 21, 2019 at 08:04 AM

Hehe, by "no joy" I meant that nothing unexpected happened and it was rather a boring experiment.
They are good controllers. I think overvoltage events are their only threat.

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thanar wkirby ♦♦ commented · Jan 21, 2019 at 08:21 AM

I was afraid that there could be a capacitor popping or something, because MPPT charge controllers are actually buck converters which means that they switch a couple of inductors/capacitors quickly to reach the required charging voltage. Such devices, when there is current flowing through and you disconnect the load (battery in this case), then there could be a too high spike on the inductor/capacitors before the IC takes over and lowers the current, hence they can be damaged.

I know Victron products are amongst the best in this field, that's why I went with Victron in the first place.

Thank you for the experiments.

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Modifications Space

Node Red Space

Answer 1 · 2019-01-16T09:25:56Z

wkirby answered · Jan 16, 2019 at 09:25 AM

It's not ideal, but I don't think any damage would occur. It would only be in an emergency anyway and more likely to happen in the night when the PV is off by default.

Alternatively, if all of your loads are powered by the LOAD OUTPUT terminals on the MPPT it's self, then the load output can be switched off using the Rx pin on the MPPT. The BMS could signal this pin and disable the LOAD OUTPUT.

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thanar commented · Jan 16, 2019 at 10:15 AM

If it disconnects during discharge, I am sure there will be no damage to the charger. My question is more focused on what happens if the BMS disconnects the battery from the charger during CHARGING.

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Answer 2 · 2019-01-18T08:30:53Z

thanar answered · Jan 18, 2019 at 08:30 AM

No anyone has any insight on the matter? Or should I try it myself and find out?

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wkirby ♦♦ commented · Jan 18, 2019 at 08:33 AM

I've been wanting to try it, but I have been at work during the week.

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thanar wkirby ♦♦ commented · Jan 18, 2019 at 09:14 AM

Cool; I guess you have a bunch of extra chargers laying around that could take a hit! ;)

Please report back on any results.

btw... any connections to Andy Kirby?

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wkirby ♦♦ thanar commented · Jan 18, 2019 at 09:33 AM

I have one I can test with, but no sun today. I'll see what I can come up with.

Haha no, no relation to Andy at all! I wondered when someone was going to ask me that.

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thanar wkirby ♦♦ commented · Jan 18, 2019 at 10:21 AM

did it really take that long? Andy Kirby's videos actually is where I first learned about victron, lol!

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