The four numbers
Everything else in the build is downstream of these, and they have to be consistent with each other before anything is ordered. Two of them come from the inverter, not from the battery, which is the part most first builds get the wrong way round.
| Number | Where it comes from | What it has to agree with |
|---|---|---|
| Series count | The nominal system voltage the inverter expects. Sixteen cells for a 51.2V system. | The BMS series range, the enclosure, and the inverter battery voltage window. |
| Charge voltage | The cells, not the charger. It is a per-cell figure multiplied by the series count. | The inverter charge setting and the BMS overvoltage protection, which must sit above it with a margin. |
| Continuous current | The inverter rating divided by the pack voltage. Not the cell capacity. | The BMS rating, busbars, cable size and the fuse, with headroom above the calculated figure. |
| Protocol | The inverter manual, including its firmware version. | The BMS family and firmware, the port, and the cable pinout. |
Sizing continuous current from the battery capacity rather than the inverter is the single most common correction made after a first order. A 280Ah pack does not need a 280A board; a 5kW inverter on a 51.2V pack draws roughly 100A, and the board should sit comfortably above that.
Then size the energy from measurements
Capacity is a separate question from the four numbers and answers a different one: how long, not how hard. Take a measured daily kWh figure from the meter rather than adding up appliance labels, decide how much of that day you want covered, and add back the state-of-charge window you intend to leave unused.
If the pack is also standing in as backup, decide now what fraction is reserved and never spent on the everyday function, because that reservation comes out of the usable figure.
Order the cells before the box
The enclosure is chosen to fit the cells, so the cell decision comes first. Capacity does not predict dimensions, and stack length with compression allowance is what actually determines whether a set of cells goes into a given box.
The kit pages cover the box and the hardware listed for the selected package. Cells, inverter, external protection, installation and commissioning are outside that unless a written quotation says otherwise.
The measurements to take are in the enclosure fit note. Formats are compared on the battery kits page, and the caster and rack versions differ mainly in how the loaded mass is supported.
Build in this order
Each step below is verifiable on its own, and each one rules out a class of fault that would otherwise be indistinguishable from the next step failing. Skipping ahead is what turns a two-evening build into a week of debugging.
- Top-balance the cells at the bench, before they go anywhere near the enclosure
- Assemble and compress the stack, torque the busbars to specification, and re-check after settling
- Fit the sense harness in cell order, routed away from the power path, and verify per-cell voltages against a meter
- Power the BMS and confirm every cell reads on the app before any load is connected
- Set series count and capacity first, then protection thresholds, recording the original values as you go
- Fit the fuse or DC breaker and terminal covers before the pack is capable of delivering current
- Bring up a small load and confirm current reads correctly and in the right direction
- Only then connect the inverter link, and only then set the inverter charge and discharge limits
The reason the balance step is first is that it cannot be done well afterwards. Cells assembled at different states of charge will read as badly unbalanced under load, and a balancer correcting that from scratch takes weeks of idle time it may not have.
Choose the balance current from the cells you actually bought
If the cells are matched, from one batch, and the pack sits idle overnight, the lower balance class converges on the same result as the higher one and produces less heat inside a sealed box. Mixed-age or salvaged cells, or a pack with almost no idle time, are where the higher class earns its cost.
The arithmetic for working out how long a given correction takes is in the balance current note.
The inverter link is the last step, and the most version-sensitive
A supported protocol is a claim about two firmware versions rather than two brands, so record both before ordering and again once the link works. Bring the pack up on local monitoring first; debugging a handshake against a pack with an unrelated wiring fault is how commissioning days disappear.
Start from the inverter link planning page, and select the cable by protocol and pinout rather than by connector shape from the cable catalog.
What to have ready before asking for help
- Cell brand, model, capacity, measured dimensions and quantity
- Series count and target nominal voltage
- Inverter brand, model, power rating and firmware version
- Continuous and peak current the load will actually draw
- Caster or rack installation, and the space available for it
- Whether the BMS and display are already owned or need to be included
- Destination country, for the shipping rule that applies to kits
Settle the four numbers, then pick the parts
With the series count, charge voltage, continuous current and protocol fixed, the shortlist is short.
Questions this raises most often
How big should a home pack be?
Work from a measured daily kWh figure and the highest simultaneous load, then decide how many hours of that you want to cover. A single appliance rating tells you nothing useful about either.
Do the cells have to be top-balanced before assembly?
Yes, if you want the pack to behave. Cells assembled at different states of charge read as badly unbalanced under load, and the balancer will spend weeks correcting what an afternoon of preparation would have avoided.
Can cells of different capacities be mixed?
The pack behaves as its weakest cell, so a mismatched cell caps the whole thing and drifts faster than the rest. Match capacity, internal resistance and age, or accept the lowest common denominator.
The inverter and BMS both list CAN. Why is there no link?
A shared bus name is not a shared message dictionary. Confirm the protocol revision, the pinout at both ends, the bitrate and which battery type is selected on the inverter.
