Connect home made lifepo4 battery banks to the Victron cerbo GX

Been down this exact rabbit hole with my shepherd's hut build, so figured it was worth starting a proper thread on it.

I'm running two DIY LiFePO4 banks (280Ah cells from Docan, paired with JK BMS units — the B2A8S20P if anyone's curious). Getting these talking to the Cerbo GX properly took me far longer than it should have.

The key thing I kept getting wrong: the JK BMS uses RS485 communication, not true Ethernet, even though the connector looks like an RJ45. You can't just plug it straight into the Cerbo's LAN port and expect anything to happen. You need a RS485-to-USB adapter into one of the Cerbo's USB ports, then configure the VE Direct/serial port settings in Venus OS accordingly.

A few things worth confirming with others here:

• Are you running Venus OS large or standard? Large gives you more driver flexibility for third-party BMS units
• Which firmware version is your JK BMS on? V11+ seems to have better compatibility in my experience
• Have you set the BMS to the correct Modbus address (default is usually 1, but conflicts can occur with multiple units)

For anyone with multiple banks, I'd also strongly recommend the Victron GX Touch 50 so you can monitor each bank individually rather than squinting at the VRM portal on a phone.

Interested to know what setups others have got working — particularly anyone integrating JK BMS with the Cerbo in a van or static off-grid setup. There seems to be surprisingly little documentation that's specific to UK cell suppliers and builds.

@BatteryBarry JK BMS is your golden ticket here — the RS485 to USB adaptor straight into the Cerbo GX gives you full DVCC control and you'll see cell-level data right in VRM, which is frankly more satisfying than it has any right to be! 🔋

Key bits to get right:

• Set the BMS as the DVCC current limiter so Victron defers to it
• Use VE.Bus BMS profile in the Cerbo settings if prompted
• Double-check your JK firmware is recent — older versions had dodgy comms

The Cerbo will see each bank as a separate battery if you run separate RS485 adaptors, which is tidy for monitoring two banks independently rather than lumping them together like a confused electrician.

@BatteryBarry welcome to the forum — shepherd's hut builds are brilliant projects, great to have you posting about it properly.

One thing worth adding to what @BoatPaddy mentioned: if you're using the JK BMS via RS485, make sure you've got the correct Venus OS firmware version on your Cerbo — older versions have incomplete JK integration and you'll get frustrating dropouts or missing SoC data.

I run a similar DIY setup (Docan 280Ah cells, JK BMS) in my static caravan and had weeks of head-scratching until I updated to Venus OS v3.x large image, which includes the dbus-serialbattery driver natively. That driver is the real key — it handles the JK protocol cleanly and exposes proper battery data to the DVCC system.

Check the dbus-serialbattery GitHub page for the supported firmware/BMS matrix before committing to any particular cable configuration.

@BoatPaddy is right on the JK route but worth noting the Cerbo needs the VE.Can or RS485 path configured properly in Venus OS — don't just plug it in and assume it'll talk.

Had the same setup in my motorhome. JK BMS shows up under Devices but you need to set the correct battery capacity and chemistry in the Cerbo settings manually, it won't pull everything from the BMS automatically.

One thing nobody mentions — if you're running two separate banks, Cerbo will only properly manage one as the "system battery" unless you parallel the BMS comms or use a proper aggregator setup. Bit of a faff but doable.

What capacity are your JK units? The older ones had dodgy RS485 implementation.

Good thread, glad someone's finally written this up properly. Quick question for @BoatPaddy or @DownsCruiser — when you've got two separate banks running through the JK BMS units, does the Cerbo see them as individual batteries or does it aggregate them into one?

I'm planning something similar for my static caravan setup (two 200Ah banks, probably Fogstar cells) and I'm wondering whether the Cerbo dashboard shows them separately with individual SOC readings, or whether you'd need to do something clever in the settings to keep them distinguishable.

Also, did either of you need to do any firmware updates on the JK units before the Cerbo would play nicely with them? I've read some older posts elsewhere suggesting certain firmware versions were a bit temperamental with the RS485 path.

@OldSparky the Cerbo will happily talk to the JK via a cheap USB-RS485 dongle — just don't buy the absolute bargain-bin ones off eBay, learned that one the hard way when mine decided to impersonate a random serial device at 3am and woke every alarm in the system 😅

Key thing