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airtime avatar image
airtime asked

Can SmartSolar 100/20 output overshoot exceed 35V after a 24V BatteryProtect disconnect?

I have a SmartSolar 100/20 set up in a 24V system. I want to use both a remote on/off under BMS control, and connect VE.Direct to a Cerbo. So I can't use the RX pin remote disconnect option. Thus, I'm considering using a BatteryProtect to disconnect the output of the MPPT when instructed by the BMS.

I did read another very good thread that confirmed that the MPPT will not be damaged by an output disconnect, but that its output voltage may overshoot before settling back to the absorption voltage setpoint. However, the potential magnitude of the overshoot was not quantified.

For reference, I have four 160W panels, two parallel strings of two in series. (Yes I know my 100/20 SmartSolar is slightly undersized). So PV voltages can be in the 40V range, and solar charging current of 20A. The MPPT output will go to the BatteryProtect input, and the BatteryProtect output to the 24V LiFePO4 battery positive bus. No other loads on the MPPT.

My question: Is there any condition where the MPPT output voltage overshoot could exceed the maximum BatteryProtect input voltage of 35V, when the BatteryConnect disconnects the MPPT output?

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2 Answers
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wkirby answered ·

The amount of overshoot is not quantified because the MPPT is not supposed to be used in that way. For an example: It similar to asking how long a cellphone will work under water? The manufactures did not test that because the did not intend for the phone to be used under water.
If the MPPT is producing a lot of power into a load like a battery, then the battery is suddenly removed, the time it takes for the feedback loop to control the new situation is unknown, but probably a few milliseconds. It is not intended for the battery to be disconnected from the MPPT output.
If you need to stop the charge for any reason, then the best way is to disable the MPPT rather than use an external disconnection device. This is a much more elegant way.
The Battery Protect is primarily intended for disconnecting loads from the battery to prevent over discharge.

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airtime avatar image airtime commented ·

@WKirby where does Victron say that the MPPT is not supposed to be used that way? On the contrary, in another thread on a similar topic, a reply by Guy Stewart after consulting with the Victron HW engineering team explicitly allows this:

"Using the MPPT with a BMS like this that disconnects the battery is allowed by Victron, from our equipments perspective. In fact our own SuperPack lithium batteries operate on this same principle, where the battery disconnects itself internally to protect itself."

As for the more elegant ways of disconnecting the MPPT, Victron did not see fit to provide a seperate Remote On/Off switch on the smaller MPPTs. You have to sacrifice the VE.Direct port, and I'd rather not do that. Thus the question on using a Battery Protect for disconnect.

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Andrew Jefferies answered ·

I do not accept @WKirby's answer as several LiFePO4 battery packs have internal, un-managed by Victron, HV& LV disconnects. Upon un-managed disconnect the "Over Voltage" he describes would then be presented directly to attached loads.. I , personally, would not state or guarantee that it will not overshoot, but sincerely would hope that it would be minimal, a transient. It could be verified by using an oscilloscope. Overshoot, if present, could then be managed by transient suppressors.. I have the feeling that some of the Expert answers are not 100% correct.

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kevgermany avatar image kevgermany ♦♦ commented ·
There are no guarantees here, we're mostly volunteers doing our best. And in any line of work, experts can be wrong. I spent much of my career dealing with this. And there are plenty of world level examples, from range of radar, to man can't fly, or break the sound barrier.


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