Hello! I have a Classic 250 installed on a DC solar trailer that originally had 48V lead batteries, and I replaced them with LiFePo batteries from LithiumUpgrade. Actually, I have a fleet of these for some years, recently converting to LiFePo. The new battery charge voltage is 58.4V, and peak charge current is 200A. I set the Absorb, float, and rebulk voltages according to this guide: https://www.midniteftp.com/support/kb/faq.php?id=127
So, Absorb is 58.4v, float is 56.4v, re-bulk is 54.4v. charge time is 5 minutes.
Here's the problem: The charge controller will not display the correct voltage of the battery consistently. It will charge from PV to battery for a while, then the classic's indicated battery voltage will jump up suddenly, and it will read a battery voltage of pretty close to its absorb voltage of 58.4v, and start to float. Meanwhile, the battery is actually 52V or less, and the Sunny Island SMA inverter also shows 52V, and the battery state-of-charge is very low and needs charging. But the classic is not delivering power from the PV array, in full sun.
At some point, this will change, and the battery voltage will indicate correctly, and the classic 250 will start to deliver charge again. But it's not enough. It should be sending power from the PV array all while the sun is shining.
Clearly this incorrect voltage indicator is the culprit. How to fix?
Other things that are relevant: I have a fleet of these. All of them do this. I have checked resistors, taken back ones that needed replacement, pressed the control boards, and all of them do this, so this means that likely it's a classic programming issue.
These systems worked fine with the lead-acid batteries.
Also, there is a backup Kubota LoBoy 11KW diesel generator connected to this. It is controlled by the SMA inverter and will kick on when state-of-charge is below 20%. When it's on, the LiFePo battery will happily receive 150A of current and charge up, and the SMA shows proper battery voltage the whole time. I can also charge the battery manually with a golf cart charger with no issue. I mention these points as a context for what the problem *may not* be.
I'm at a loss, and my customers are grumpy about it. Any help is appreciated! 73