A rack of EG4 batteries in parallel behaves, from the inverter's point of view, like one large battery. The inverter receives a combined state of charge and a combined current. That is convenient, and it also hides what happens pack by pack, which is where most problems start.
Why parallel packs never share perfectly
In a parallel bank, current divides according to the resistance of each path. Every cable, lug, busbar and breaker adds a little resistance. A pack connected through a slightly longer cable, or one whose terminal is a little less tight, will carry less current than its neighbours; the closest pack to the inverter connection often carries more.
- Cable length and gauge: equal-length cables to a common busbar give the most even sharing.
- Connection point: taking the inverter cables from opposite ends of the bank (positive at one end, negative at the other) evens out the paths.
- Terminal torque: a loose terminal adds resistance and heat. It is the first thing to check when one pack behaves differently.
- Age and capacity: mixing an older pack with new ones means the newer packs end up doing more of the work.
What an uneven bank looks like in the data
| Symptom | Likely cause |
|---|---|
| One pack's current consistently higher in charge and discharge | Lowest-resistance path, often the pack nearest the inverter connection |
| One pack's charge level lags the others by more than about 10 % | Higher-resistance connection, or lower real capacity |
| One pack runs warmer | Carries more current, or has a connection heating up |
| Packs end the night at very different levels | Uneven sharing combined with different capacities |
Why it matters
A pack that carries more current cycles more and warms more, so it ages faster. Over a few years the gap widens: the hard-working pack loses capacity, then trips its protection first at the end of a discharge or the top of a charge, and the whole bank delivers less than it should. Catching an uneven bank in its first months usually comes down to tightening a lug or re-routing a cable.
Checking it pack by pack
EG4 Monitor reads each pack of the bank on the communication bus and shows them side by side: charge, current, power, temperature and cell spread per pack. On its screen and on your Wi-Fi you see the live picture; with the free Cloud you keep up to three years of history, enough to see whether two packs drift apart over a season.
- Look at current per pack during a steady discharge in the evening. Within a bank of identical packs, the values should be close.
- Compare the charge levels at the end of a full night.
- If one pack stands out, switch the system off safely and check its terminals and cable before anything else.
In Home Assistant each pack also appears as its own device, so you can create a simple notification when the gap between packs passes a threshold you choose. The box only reads; it never changes a setting on your batteries or your inverter.