Unusual? Lifepo Battery Charging Behaviour

Hi Everybody,
My solar system went live 2 weeks ago and with the exception of a few minor teething issues (due to my own learning curve) has been working very well (HURRAH!)

A recap of my system overview:
-25 pcs Trina 450W solar panels

  • 2 units MPP Solar PIP5048MKX hybrid solar charger-inverters (standalone, NOT in parallel)
  • 3 pcs 48V 200AH Lifepo batteries (BCT- Blue Carbon Tech) connected in parallel

I have programmed the MKX MPPT chargers to ‘bulk charge’ up to a cut-off of 56.8V (3.55v per cell) and then to ‘float charge’ at 53.5v. Incidentally the MPP charger inverters do NOT support communication with the BCT BMS so the charging must be managed via the MKX bulk and float charge settings.

My question relates to what I view as possibly unusual battery voltage ‘behavior’. In monitoring the voltage of the battery bank during the sunlight hours, the voltage does usually reach 56.8v during the bulk charge phase, but then ‘falls’ to 53.5v or even below, for the rest of the day.

I thought that maybe I was seeing the float charge voltage being applied by the MKX charger, so I disconnected one battery from the bank and measured its voltage again (using a multimeter), expecting it to rise back up to its fully charged ‘resting voltage’ which I read should be around 54.4v . However the voltage stayed around 53.5v. The MKX displays show that there is sufficient PV power being generated to run my household loads without drawing down the batteries and in fact sending the excess into the battery bank so its voltage should not be depressed by loading, right?

Solar newbie that I am, my assumption was that the battery voltage should reflect its state of charge, which after completing bulk charging should be 57.8 (during charging) or at least 54.4v?

Would appreciate if more experienced/expert solar-cum-lifepo users could help me to understand if what I am observing is normal or else something I should look into further.


Hi William, I finally got mine up and running as well. Mine acts the same except my voltage never gets above 54v, we’ve been having lots of clouds and rain here in the afternoon so that might be a contributor. I seem to be adding about 10.5 amps per battery most of the time when it’s cloudy. Richard

Generally, under no load and no charge current, a LiFePo4 battery will revert to 53.5V. This is normal. The charge and float voltage is always higher (during charging) when current is being fed into the cells.

Are you using a shunt or battery monitor to measure battery SOC?