In short
- Most of what matters shows up at the top of charge, when the first cell reaches the knee of its curve.
- A spread that shrinks over the first cycles is normal; one that grows needs a look.
- A cell that always reads lower under load, but not at rest, often points to a connection, not to the cell.
New cells, even from the same batch, rarely start perfectly level. During the first cycles the Seplos BMS and its balancer have work to do, and the figures move more than they will later. Knowing what normal looks like saves a lot of worry.
Watch the top, not the middle
Between roughly 20 % and 90 %, LiFePO4 cells sit on a flat voltage plateau and all look alike. The differences appear when the first cell climbs the knee near full charge. That is the moment to read the spread: the gap between the highest and the lowest cell while the pack is still charging. A few tens of millivolts there is common on a new pack. More than 150 mV, again and again, means the cells are not level yet, or one of them has less capacity.
Is it getting better?
One reading tells you little. The trend over two or three weeks tells you a lot. Seplos Monitor keeps the spread at the top of each charge, day after day. If it shrinks, the balancer is doing its job. If it stays put, the balancer may hardly run, often because the inverter stops charging before the cells reach the balance start voltage. If it grows, find out which cell leads, and whether it is always the same one.
Connections before cells
A busbar that is not quite tight adds resistance. Under load, the cell behind it reads lower than its neighbours; at rest, it looks fine. That pattern, a cell that only sags when current flows, almost always points to a connection. Check the torque on that cell’s busbars before suspecting the cell itself, with the pack switched off and following the safety rules of your build.
Temperatures
Each temperature probe should read within a few degrees of the others. One probe much warmer under load can mean a poor contact nearby. In a cold garage, remember that LiFePO4 cells should not be charged below freezing; the BMS limits it, which is why charging can look slow on winter mornings.
After a month
Once the spread at the top is small and stable and the temperatures behave, the pack is settled. From then on, a monthly glance and an alert on the spread are enough.
Common questions
Does Seplos Monitor balance the cells?
No. Balancing is done by the Seplos BMS. The monitor shows each cell, the spread and the balancing activity so you can see whether it works.
What spread is normal at the top of charge?
On a settled pack, a few tens of millivolts. Higher figures are common during the first cycles of a new build and should come down.
Should I do a top balance before assembly?
Many DIY builders do, following the cell maker’s instructions. It shortens the settling period, but the BMS can also level the cells over time.
