Repeated charging and discharging when battery is full - multi voltage reading inaccurate?

I’ve been watching this for a while now:


I can’t remember seeing the MPPT sitting at idle, while there are critical loads consuming from the battery?

Now the battery has dropped to 99 % and the MPPT has started charging again:

It could just be the battery not allowing charging?

@plonkster explained this the other day.

Yes, that looks likely.

Also, don’t know what battery you have there, but a FreedomWon Lite (for example) will lower the charge voltage when it is full and it has to accommodate a high cell. When it does that, the MPPT will stop charging, and the inverter will run from the battery until the voltage has reduced to the point the BMS demands.

I am not sure… it is a bit weird to me. Earlier today it was “fine” at 100 % and then later on this flapping started. I.e. the MPPT was powering the critical loads even though the battery was at 100 % - now it seems like the battery is powering the critical loads until it goes to 99 % and then the MPPT starts charging again.

It is a BYD LV Flex Lite 5kWh.

It should be something like this

Battery at 100% but since there is load, the MPPT is supplying the battery that is serving the load.

Maybe explain a bit more which configuration are you running?
The screenshot below is system running ESS

That larst graph (Charge and Discharge Limits) show what is happening.
The battery is setting the charge current to 0A (orange) when it is full. Then the MPPT will stop charging and use some of the battery when a load starts and before the MPPT can ramp up.
It also lowers the charge voltage (blue) at the same time, so the MPPT will be aiming for a lower voltage.
This seem to match when the max cell goed above around 3.55V by the looks of things (min/max cell graph)

I agree with all of the above about the battery charge limits etc. The bit that is confusing is that it was fine between 10:00 and 12:00 today with similar limits, and the the MPPT was supplying the critical loads?

That is a battery that lowers the charge voltage. You can also see it on the chart you copied. It lowers the charge voltage from about 58.4V to 57.3V (ish, I’m estimating). Once it has done that, the MPPT is way over voltage and will not continue to charge until the battery voltage has dropped down to the lower voltage. During that time… your loads will be powered entirely from the battery.

Once it reaches a kind of equilibrium, you’ll again see the MPPT making about as much power as the loads need.

There is something a bit odd going on here. This morning, from around 8:50 to 9:30 - happy equilibrium…

Later in the morning, 11:45 until 12:50. Up and down we go - is it because the battery is at a slightly higher voltage than earlier?

Mine does that too. 99, 100, 99, 100… I ignore it.

Just adding ‘my understanding’ from what I can see:

08:50 - 09:30:
Battery voltage ~54.2V
BMS charge voltage 57.2V, but 0A
As the battery is ‘requesting’ a higher voltage than what the battery is at the MPPTs will supply what’s needed on the DC bus (the loads)

11:45 - 12:50:
Seems like some balancing or something taking place.
BMS is now asking for ~58.4V and ~15A
MPPTs ramps up to supply it.
Once the battery voltage hits ~57.4V the BMS asks for 57.2V again
As the battery is requesting a lower voltage than what the battery is at the MPPTs waits for that voltage to be hit before supplying the DC bus again, so the DC bus is using the battery.
Rinse and repeat.

This seems to be normal based on what plonkster is saying.

I viewed the thread, absorbed it, and thought, what a interesting development to keep lithium batteries well top balanced.

Charge to 100% SOC.
Then drain some of the batteries.
Recharge a bit so that the balancer can work again.
Then drain a bit …


Similar issue, but my BMS maxes out at 92%. Then this happens, and I draw quite a bit from the grid, even though my PV charger is capable of giving more power than the PV Inverter:
Screenshot from 2023-11-02 12-07-33

Here you can see the battery shows 92% when it is full:

  1. How can I fix the battery level?
  2. How can I prevent drawing power from the grid when I have more than enough solar and inverter capacity, but the batteries are full?

A few minutes later:

Screenshot from 2023-11-02 12-33-25

Why would the battery do that?

Maybe something to do with what TTT suggests about cel balancing?

It would keep the individual cell voltages going above their max values. They spike up fast when full (lithium cells)


Two things pop to mind:

  1. Is that the BMS SOC or a BMV? If BMV, then it needs calibration. If BMS, speak to the battery supplier.
  2. Panels not doing anything: Set the system to Keep Charge when the batts are full. Do the panels then “kick in” again?