A Pytes battery keeps a running record of itself inside its BMS. Most owners only ever see one figure, the charge level on the inverter screen, and that figure alone says very little about how the battery is ageing. The other numbers are there, on the console port, and they are worth a few minutes of your attention once in a while.
State of charge (SOC): useful, but counted rather than measured
The BMS does not measure the charge directly. It counts the amp-hours going in and out and keeps a running total, then corrects that total when the battery reaches a clear reference point, usually a full charge. Between two full charges the estimate drifts a little, which is why a battery can show 30 % one evening and seem to give more, or less, than expected.
- If your system rarely reaches 100 %, the displayed charge can be several points off after a few weeks.
- A full charge from time to time, held for a while at the top, lets the BMS reset its count and lets the cells balance.
- A jump from 96 % to 100 % at the end of a charge is the BMS correcting itself, not a fault.
State of health (SOH): compare it over months, not days
SOH is the BMS's estimate of how much capacity is left compared with a new battery. It moves slowly and in steps. A drop of one point in a month means nothing on its own; a steady slide over a year, or one battery losing ground faster than its neighbours in the same stack, is what matters.
| What you see | What it usually means |
|---|---|
| SOH 100 % for months on a young battery | Normal: the estimate only starts moving after some use |
| All batteries of a stack within a few points | Healthy, evenly used stack |
| One battery 8–10 points below the others | Worth a look: cell imbalance, temperature or a connection issue |
| A sudden fall after a firmware or configuration change | Often a recalculation; watch the trend for a few weeks before worrying |
Cycles: a measure of use, not a countdown
The cycle counter adds up energy throughput: discharging the equivalent of one full capacity counts as one cycle, whether it happened in one night or over four days of shallow use. A home battery doing one cycle a day reaches about 365 a year. That is a normal figure, far from the several thousand cycles lithium iron phosphate cells are built for. Cycles are most useful to compare two batteries bought at the same time, or to check a second-hand one.
Cell spread: the earliest warning
Inside each Pytes module, the cells are in series. The difference between the highest and the lowest cell voltage, the spread, is the first number to move when something starts to go wrong. In the middle of the charge range, lithium iron phosphate cells sit on a flat voltage plateau, so a spread of a few millivolts is normal. Near full charge the voltage curve steepens and the spread naturally widens; the BMS balancing then pulls the cells back together.
- A spread that stays small in mid-range and widens only at the very top: normal behaviour.
- A spread that grows week after week, even in mid-range: one cell is drifting; a long full charge often helps.
- The same cell always being the lowest at the end of discharge: note its position and keep an eye on it.
Temperature: the quiet factor
Lithium batteries age faster when kept hot and accept less charge when cold. The BMS reports several temperatures; a battery that runs consistently warmer than the others in a stack is often the one in the least ventilated spot. Moving it a few centimetres can make more difference than any setting.
Reading all this without a laptop
Pytes Monitor reads these values from the console port and shows them on its screen, on your home Wi-Fi and, if you turn the free Cloud on, on your phone anywhere. The Cloud keeps up to three years of history, which is exactly what you need for SOH and spread: trends, not snapshots. You can also set an e-mail alert on low charge, high temperature or a BMS alarm.
If one of your batteries shows a pattern you are unsure about, export the period as CSV and send it to us: we are happy to take a look.