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thanar avatar image
thanar asked

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 ControllersBMS
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wkirby answered ·

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 avatar image thanar commented ·

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 avatar image wkirby ♦♦ thanar commented ·

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 avatar image thanar wkirby ♦♦ commented ·

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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wkirby answered ·

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 avatar image thanar commented ·

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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thanar avatar image
thanar answered ·

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

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wkirby avatar image wkirby ♦♦ commented ·

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

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