Builiding of a complete 6kW PV inverter with MPPT chargers

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Joined: 13/10/2014
Location: Australia
Posts: 1522
Posted: 01:27am 03 May 2024      

  -dex- said  KeepIS - you said earlier that you have your own lifepo4 packages and a lot of your own experience with them. Can you despair of the scenario of events when, for a battery with a nominal voltage of ~51V, we supply more than twice as much - the question is whether the voltage will immediately be at ~100VDC or it firstly will drop to the battery voltage and slowly increase?

Apology for my overly long reply.

All of this depends on the maximum current @ ?? voltage available from the Solar array, the size of the LifePO4 battery bank and the configuration of the bank and array.  

With my 12kW of solar for the LifePO4, I have more on a separate Mains feed in, there is some serious power available. If a Solar regulators shorts the Solar array voltage to the Battery, then a big LifePO4 bank with a good BMS will switch off, but as has happened before, the BMS is not designed for a sudden high voltage across the FETS at the moment of OV protection and isolation, some use a relay for isolation or have an external drive for one, much better.

As you noted and found:

Reports of FETS shorting and destroying the LifePO4 bank have been reported - so now you basically have NO BMS, and you have a big problem. A big battery bank capable of charging at around 500A in bulk voltage mode will, as you said, will pull the solar array down to a much lower voltage, but if you are not there, disaster is a short time away.

The way mine is configured, there is never going to be enough voltage or current unless all 5 solar regulators short.    

I have 4 parallel banks of LiFepo4, each has a BMS, each has an isolation switch, and each bank has a low current 80A Fuse. But an 80A fuse will hold high current for X time, but you can see where most of the banks would isolate, and the rare possibility of one bank taking out a FET.  

However my Solar arrays have a VOC of around 90v, under any load the mppt sits at around 70V, so it's likely the 4 BMS devices will handle that. I also have each Solar regulator de-rated, there are 5 solar regulators, and all solar array outputs are fused just above the max current of each array, each Solar regulator has a current tripped isolation switch on the output, so it's pretty hard for the system to fail in that manner.

There is a lot to be said for dividing everything up having the ability to isolate any part of the system that failed. Not the least for redundancy in an emergency failure situation, but also to lower the power each section has to handle, and much lower fuse ratings can be used.

IMHO the Kilovac would be a really good backup fail safe.  

They are rated at high voltage and current on release. I bought four of the 48v to 72v coil with economizer for $38.0 each in AU about 4 month ago, I had have three 24v units from a few years back, not a single failure - and man have I abused them !!!
Edited 2024-05-03 11:48 by KeepIS