In short
- In parallel, modules share the current according to their resistance and their state; small differences are normal.
- A module that ends every evening lower, or runs warmer, limits the whole cluster sooner or later.
- The trend over weeks matters more than any single reading.
WeCo 4K4 and 5K3 modules are made to be stacked. In a low-voltage installation they are connected in parallel, and the cluster talks to the inverter as one battery. That is convenient, and it hides something useful: how each module is really doing.
Why modules drift apart
Modules in parallel are tied to the same voltage, but they do not carry the same current. A module with slightly longer cables, a looser connection or cells of a different age takes a smaller share of the charge and gives a smaller share in the evening. Over days, its state of charge settles a few points away from the others.
A few points of difference is normal, especially between modules bought at different times. What matters is whether the gap is stable or growing.
What a weak module costs
The cluster is only as good as the module that reaches a limit first. If one module hits its lowest cell voltage before the others, the battery management system has to reduce or stop the discharge for the whole cluster. You lose the energy still stored in the other modules that evening. At the top, the same thing happens in reverse: one module full early slows down the charge of all.
What to look for
| In the data | What it may mean |
|---|---|
| One module always a few points lower in the evening | Higher resistance on its path: cable, terminal, busbar |
| One module warmer than the others under load | More current through it, or a connection heating up |
| A growing gap between its lowest and highest cell | Cells drifting inside that module |
| A state of health falling faster than the rest | That module is ageing first |
None of these is an emergency on its own. All of them are worth showing to your installer, who can check the connections long before a module limits the battery.
Seeing the cluster as it is
WeCo Monitor reads the battery management system on a free RS485 port and shows each module on its own line: charge, lowest and highest cell, temperature and health. The link with the inverter is not touched. Over a few weeks, the dashboard makes the pattern obvious: modules that move together, or one that keeps falling behind. Home Assistant can send you a message when the gap between modules passes a threshold you choose.
Common questions
Is a difference of 2 or 3 points between modules a problem?
Usually not, especially between modules of different ages. Watch whether it stays stable over the weeks.
Can I add a module to an existing cluster?
WeCo designs its modules for that, within the limits of its manual. Your installer knows the rules for mixing ages and versions.
Does the box balance the modules?
No. It only reads. Balancing and limits remain the job of the battery management system.
