So your LiFePO4 battery is acting up:

• won't charge fully
• seems to have reduced capacity
• voltages in the system are spiking, perhaps tripping your inverter

It's fairly common for excessive charging voltage to indirectly cause these symptoms. Unfortunately it's very common for preconfigured Lithium profiles to charge at voltages high enough to start the problematic chain of events.

If this is the problem then reducing the charging voltage will cause the symptoms to improve or disappear.

about these summaries

1. reduce charging voltage (aka “absorption voltage”, “boost voltage”) to something like 13.6v. Float if used, can be set to something like 13.3v-13.4v. You may have to use a USER profile instead of a canned LITHIUM profile to make these changes.¹⁾ If you choose to modify the Li profile itself take a pic or notes of the original settings so you know what they were and can return to them later if desired.²⁾
2. observe for a few days. Charging will be slower and may not finish by sundown³⁾; we want to see if it can keep on charging rather than trigger BMS cutoff.
3. if symptoms do clear up you can start to raise the charging voltage back up gradually (13.65v, 13.7v, 13.75v, etc) or you can leave it low if you are getting enough charge. {note from secessus: there is little benefit to charging LiFePO4 >13.8v.] Float can remain low at 13.3v-13.4v.
4. if it starts acting up again drop charging voltage back down a notch or two

LiFePO4 will fully charge at voltages as low as the mid-13s, but it will take more time. That extra time is called “Absorption duration”⁴⁾. At the kinds of current levels we usually see in offgrid charging⁵⁾ the duration looks like this:

• ≥14.0v will charge to 100% SoC with zero minutes of absorption durations: hit the voltage setpoint and stop. Cell voltages tend to diverge as charging voltage increases above 14.0v because they are further up the knee (see below)
• 13.8v will charge to 100% SoC with a small amount of Absorption (10-30 minutes?) and cells tend to stay in balance.
• 13.6v will charge to 100% SoC with several hours of Absorption ←- where we are starting
• 13.4v will eventually fully charge the bank but it would take days. It takes so long we can use 13.4v as a Float voltage.

1. lithium cell voltages are very “flat” for most of their usable capacity. When deeply discharged the voltage drops off dramatically, and when they are overcharged their voltage spikes dramatically. Because of the shape of this voltage curve we sometimes say there is a “lower knee”, broad flat middle, and “upper knee”.
2. when one cell hits the knee first it can “run away” voltage-wise, and do so quite suddenly. This unevenness in cell voltage is called “imbalance” and is more likely to happen with higher charging voltages.⁶⁾
3. the BMS sees the runaway and shuts down charging to avoid damaging the cell⁷⁾
4. which stops all charging of the battery⁸⁾
5. if the cell imbalance is bad enough the battery can't charge anywhere near full because the misbehaving cell is causing charging to shut down early.

So we ease up on charging which encourages the cell to stay in line with the others, which allows for a full battery charge.

The bigger question is this: why do battery and charger manufacturers specify such high charging voltages in the first place? It's not a plot; there really are some benefits to charging at higher voltages, especially when selling to the general public.

• an approach to greater LiFePO4 longevity