[RE-wrenches] Remote battery disconnects

[Starlight]

Have you (or any) installed surge suppression on the PV input side to clamp the open circuit voltage surge? 

Larry Crutcher
Starlight Solar Power Systems



On Feb 1, 2023, at 1:23 PM, William Bryce via RE-wrenches <re-wrenches at lists.re-wrenches.org> wrote:

I can also backup what Jim has said, and have seen the same gear destroyed by removing the battery abruptly when the controller is under heavy load. They can die, and sometimes die spectacularly.

Have seen SolArk inverters integrated MPPT controllers blow up when lithium battery BMS disconnects. Not a field fixable issue.

Just flip off the breaker when the solar is working hard and and the SolArk will give up the smoke. 

Like I originally said, it’s the non talked about issue that is a big issue depending on what gear your using.

On Wed, Feb 1, 2023 at 2:49 PM James Jefferson Jarvis via RE-wrenches <re-wrenches at lists.re-wrenches.org <mailto:re-wrenches at lists.re-wrenches.org>> wrote:


On 2/1/2023 12:25 PM, Alex MeVay via RE-wrenches wrote:
> Although our controllers are probably smaller than what you would be
> considering in this discussion, surviving a load dump (suddenly
> disconnected battery) is an engineering requirement for us, and likely
> would be for other responsible MFG's on this list (boB?).  The charge
> controller can either handle full input voltage on the output, or
> there is a comparator that will shut the controller down instantly
> when the output voltage gets too high.

So that's part of the picture.

The other part of the picture is the rest of the system. The discussion 
and what NEC is mandating is that the battery be disconnected. As far as 
I can tell, there isn't an explicit requirement in all cases that all of 
the power inputs into the system are all going to go away at exactly the 
same time .... or ever. This leaves things like charge controllers 
getting input power potentially from PV or wind or grid or generator or 
something else. And nothing in the NEC, as far as I can tell, is 
mandating that all of the DC loads be disconnected. So the loads are 
online. Now the regulation circuit (switching power supply in the case 
of a MPPT controller) is regulating variable loads and nothing is 
providing substantial resistance to change. Normally the battery is 
acting like a very very very big capacitor. But without the battery, 
there isn't enough damping in the control loops and voltage stability 
will suffer. This is where you get 250 volts on your normally 48 volt 
battery bus. Or 3 volts. Or -80 volts. Or all of those in a fraction of 
second. This sort of thing is hard on electronics and will cause failures.


Alex's comment about Genasun's controllers handling full input voltage 
on the output is probably unique to their niche product. Looking at 
their biggest controller, it appears to support VOC of 34 volts. At 34 
volts, he can use 50V or 100V rated components on his output. For a 150V 
input controller, you probably can. But you wouldn't because it would be 
too expensive. But on a 600V or 1000V controller, there's just no way 
that you do that because 1000V rated components are big and expensive 
and their spacing requirements are huge compared 100V level sort of 
stuff. So the way bigger MPPT controllers deal with load dump is with 
transient voltage supression or other diodes to handle the voltage spike 
caused by the inductor when the load goes away. When operated in 
parameters, these parts don't wear out. But it can be interesting to 
size them adequately to account for inductance elsewhere in the system 
adding to voltage and energy that has to be absorbed.


There is a very simple experiment that anybody can perform to see how 
equipment handles a load dump: Simply wait for a sunny day and turn off 
the battery breaker. If no magic smoke was released, turn back on 
battery breaker. If still no magic smoke was released, then great, your 
system survived a load dump.

If you are at all uncomfortable doing this and/or your system gets 
destroyed in the process, think back to my earlier security comments 
about having a self destruct switch on the outside of your building 
allowing anyone walking by to do this experiment for you.

I have personally destroyed Outback, Midnite, and Morningstar 
controllers inadvertently or intentionally doing load dumps by shutting 
off their output breaker. Typically the TVS diodes short out and 
secondary over current protection (circuit breaker) trips before things 
catch on fire. Usuaully the UL94V0 rating on the circuit board and the 
box the circuit board is in prevents fire from spreading when things do 
get wild.

But I've also seen all of those brands survive a load dump.

I have multiple customer who have fielded lithium battery systems to 
cold locations and have had battery BMS disconnect the battery from the 
rest of the system. Ten's of thousands of dollars of equipment has been 
destroyed in these islanding events.


Some thoughts, for what they are worth.

-James Jarvis
APRS World, LLC

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