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Mixing lead acid and Pylontech battery


KobusK
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I have two concerns with this.

The first is charge voltages. The Pylontech battery is 15s, so it really doesn't want to be above 53V and definitely not above 54V. Your 24-cell lead acid battery is just barely being floated at that voltage, in other words, you can never properly put the lead acid bank through a proper absorption phase. Yes, you can charge a lead acid battery full at 2.25V per cell, but it takes days to complete the process. You'd have to use a 16-cell battery to make it work.

The second is what happens in a fault condition. If you have two batteries in parallel, and one of them develops a bad cell and wants to live at a lower voltage, current from the other string will flow into the bad string naturally trying to pull the voltage up. If the bad cell is on the lead-acid side, with a high-energy lithium battery in parallel, that just does not strike me as a good idea. You would have to (at the very least) fuse the individual strings to protect against this.

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I have seen an installer do this with a very complicated system of of voltage regulators and a BMS hub with an ICC if it works properly remains to be seen but I am dubious..batteries used solar gel and I think Revov second life with a custom Built BMS hub or that is what he tells me.. personally I would not mix the two far too risky..

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6 hours ago, SteveFury said:

I have seen an installer do this with a very complicated system of of voltage regulators and a BMS hub with an ICC if it works properly remains to be seen but I am dubious..batteries used solar gel and I think Revov second life with a custom Built BMS hub or that is what he tells me.. personally I would not mix the two far too risky..

Of course one could use a buck/boost converter between the banks so they can live at different voltages, but you still get the benefit of using both. But I don't think something commercially available exists for this application.

If you had separate PV arrays charging the banks individually (the efficiency would be too dreadful otherwise), and you had a way to move charge from the old lead-acid bank into the LFP bank (using buck/boost, maybe 90% efficient), I think you can make it work. It would be like the buck/boost used in vehicles to charge the second battery from the main one, moving charge from one battery to the other.

Edit: Another example would be electric cars that charge their 12V battery from the main HV battery using such  converter. Also, this.

Edited by plonkster
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