In short
- SOH is the BMS’s estimate of the capacity left compared with new; cycles count the energy that has gone through the pack.
- SOH tends to move in steps, after the BMS has seen a full charge and a deep enough discharge to re-measure.
- Compare packs in the same bank: one ageing faster than its neighbours is the useful signal.
Most owners look at the state of health once, see 100 %, and forget it. The value of these numbers is in their history. A pack at 97 % after three years is a good pack; a pack at 97 % after six months, while its neighbours still read 100 %, is worth a phone call to your seller.
What the BMS measures
The state of health is the BMS’s estimate of how much capacity the pack still holds compared with when it was new. The cycle count adds up the energy that has gone through it: two half discharges make roughly one cycle. Both are worked out by the BMS from what it has seen, so they are estimates, not lab measurements.
Why SOH moves in steps
To re-estimate capacity, a BMS needs to see the pack go from full to fairly low. A battery that lives between 40 and 90 % gives it few chances. That is why SOH often sits still for months and then drops a point or two in one go. One step is normal; frequent steps over a short period are not.
What a normal year looks like
A home battery cycled once a day logs around 300 cycles a year, fewer in winter. LiFePO4 packs are typically rated for several thousand cycles, so the first years usually show little change in SOH. A slow drift of a point or two a year is unremarkable.
The signal that matters
With several packs in parallel, compare them. EG4 Monitor reads each pack and keeps its history, so you can see whether one pack’s SOH falls faster, or its cycle count climbs faster, than the others. A pack that works harder than its neighbours often has a connection, an address or a setting that differs, and the earlier you notice, the easier it is to fix.
Keep a record
The free Cloud keeps up to three years of history and exports it as CSV. If you ever need a warranty claim, a dated record of SOH and cycles is far more convincing than a screenshot taken on the day.
Common questions
My SOH dropped two points overnight. Is that a problem?
Usually not. The BMS re-measured after a full cycle. Watch the next months: a stable value afterwards is normal.
How many cycles will my EG4 battery last?
EG4 publishes the rating for each model. Your own count tells you how fast you use them.
Does EG4 Monitor change anything on the packs?
No. It only reads what the BMS reports.
