question

Hi @epretot

I can give an answer in the context of LiFePO4 battery packs but other chemistry types I'm not as familiar with so this may not be accurate if you're using another type of battery.

If individual cells within the battery pack have different internal resistances or different overall capacities or have never been top (or bottom, usually top for solar applications) balanced or weren't of the same State of charge when built then they can have differences in their balance/level of State of charge in relation to each other.

For LiFePO4 the voltage throughout the charging of the battery remains relatively constant. Therefore unbalanced cells are difficult to spot during the main charging phase of battery. However LiFePO4 battery voltages peak when nearly full (starts around 3.45v) and also drop off at almost empty, this is when the imbalance will become apparent.

For this battery chemistry symptoms of unbalanced cells tend to only present themselves when one or more of the cells within the pack is almost full or empty as this is when the voltage peaks/differs from the other cells.

The main symptom I can think of is the BMS (battery manage system aka the safety device) of the battery disconnecting the battery from the system to prevent the peaking cell from being overcharged and damaging it. Most BMS will then have a built in balancer (usually slow) which will discharge the peaking cell(s) into the others eventually balancing them. Most BMS have a voltage at which the balacner will turn on, if you leave it on all the time for LiFePO4 funky things can happen due to the flat voltage curve. ie for LiFePO4 cell 3.2v could mean 20% could also mean 80%, so balancing is only worth while/useful when you're either almost full or almost empty as thats the only time you can correlate State of charge percentage to voltage.

Unbalanced battery packs can therefore result in you receiving less power out of the battery than one that is properly balanced.

Best way to spot if a pack is unbalanced is to check the BMS. Most BMS will have an app or screen that lets you monitor the voltage of each cell which will make it easy to see how out of balance your pack in.

The solution to balancing is usually leaving the battery in absorption phase (full) for a longer period of time to allow the BMS to balance the battery. The length of time can vary per BMS and per balancer built into the BMS.

Thanks for the reply. I have been experiencing some symptoms that led to that question.

When viewing the graph in the trends section of my BMV, I saw a voltage jump every day at the same window of time every morning. However, I never had any visible symptoms.

Then I had a glitch on the AC side which led me to look at the graph. I saw the voltage jump for two weeks when looking atbthe graph.

Here is what I'm trying to determine.

1. What causes it? Possibly unbalanced batteries.

2. Why does it occur even on shore power?

Might help if you describe your system and the symptoms. At the moment we have nothing to go on.

Those spikes are odd. Sounds as if you have done a lot of trouble shooting already. Guess you've tried most of what I say below.

Maybe try elimination by disconnecting or turning off parts of the system. Also loads. Are these lithium or AGM batteries? If lithium, they would cause the BMS to cut the batteries for protection. Or could be from the chargers overshooting when a heavy load suddenly disconnects.

Is it happening at specific times?

Do the spikes show on the PV voltage readout?

The battery balancer will equalise between the 12V batteries, but not internal imbalance in lithiums, the BMS should handle that. Only cuts in at 27V. Not clear from the graph what voltage represents fully charged.