Bestehende Pylontech Akkus um DIY Akkupack 48V (15S) mit JK BMS erweitern

Right, this is something I've been mulling over for a while and wondered if anyone else has gone down this rabbit hole.

I'm running a fairly hefty Victron setup on my static caravan — a Multiplus II 48/5000, a couple of MPPTs, CerboGX, the whole lot — currently paired with two Pylontech US3000C batteries. Works brilliantly, no complaints there. The Pylontechs talk to the Victron gear over CAN bus and everything just... behaves itself.

Here's my problem though. I want more capacity without paying Pylontech's frankly eye-watering prices for additional units. I've been looking seriously at building a supplementary 15S LiFePO4 pack using Fogstar cells and a JK BMS, which would sit alongside the existing Pylontechs on the same 48V bus.

Now, the BMS protocol mismatch is what's keeping me up at night. The Pylontechs handle their own charge/discharge communication with the Victron system natively. A JK BMS speaking a different protocol into the same CerboGX — will it actually play nicely, or will the system get confused about which battery to believe when they report different SOC figures?

Has anyone actually done this in practice? Mixed managed batteries with a DIY pack on the same bus? I've read vague things about setting the DIY battery to "dumb" mode and just letting the Pylontechs control everything, but that feels like a bodge.

Interested to hear from anyone who's genuinely attempted this rather than just theorising. Especially on UK installations where the grid/off-grid situation adds its own complications.

@FormerTeacher interesting situation — mixing chemistries or even just different pack configs on a shared bus is where things get spicy.

I went through something adjacent with my shepherd's hut build. The moment you put two battery systems with different BMS logic onto the same Victron bus, you're essentially letting them fight over who gets to call last. Pylontech's CANbus comms will be shouting charge parameters at the Multiplus whilst your JK sits there doing its own thing on a separate protection layer.

The critical question is whether your JK pack's cell-level voltage window genuinely overlaps with what Pylontech is negotiating. If the Multiplus honours Pylontech's CVL and your 15S pack's full-charge sits slightly higher, one of them will always feel unloved.

Worth checking Victron's DVCC documentation — specifically what happens when multiple BMS sources conflict. It's not pretty.

@FormerTeacher the thread title's in German but your problem's universal — Pylontech speaks CANBUS fluently to Victron, your JK BMS speaks... interpretive dance.

The real gotcha isn't the 15S voltage (49.2V nominal sits close enough to Pylontech's 48V profile), it's that the Victron DVCC system will take charge parameters from one BMS source and apply them globally — so whichever pack shouts loudest wins, and the other suffers.

Practical options:

• Isolate the DIY pack behind a DC-DC converter and let it run as a secondary buffer
• Use VenusOS with a custom driver to aggregate both BMS feeds (there's a GitHub repo for JK integration)
• Accept manual management and monitor obsessively via Cerbo

I've got a Fogstar Drift alongside older cells and learned the hard way that "same bus" doesn't mean "same rules."

Good shout from @OldSailor on the CANBUS point — that's the real sticking wicket here.

Your JK BMS won't be chatting to the Victron ecosystem the same way Pylontech does. The Pylontech will effectively "win" the conversation and dictate charge params to your Multiplus II, which may not suit your DIY pack at all.

Few options worth looking at:

• Separate the packs onto different charge sources if possible
• Look into whether your JK firmware supports any Victron-compatible protocol (some newer units do via RS485)
• Victron's DVCC settings become critical — worth a deep dive in VRM

What cell chemistry is your 15S pack? LFP I'd assume, which at least means your voltage windows are comparable to the Pylontechs. That helps a bit, but doesn't solve the comms issue.

@OldSailor @BirchLover both right on the CANBUS issue, but there's another wrinkle nobody's mentioned — cell count mismatch.

Pylontech runs 15S internally (nominally 48V), same as your DIY pack, which is fine. But the charge profiles differ. Pylontech uses a fairly conservative top-of-charge voltage. If your JK BMS is configured for, say, 3.65V/cell (54.75V total), and the Pylontech CANBUS is telling the Multiplus II to cap at 52V-ish, your DIY pack never fully charges.

On my boat I run a standalone Victron MPPT alongside a separate Victron Battery Protect — keeps chemically-mismatched banks genuinely isolated. Parallel-bushing packs with different BMS personalities is asking for trouble regardless of chemistry matching.

What's your JK BMS model exactly? The protocol bridging options vary quite a bit between firmware versions.

@CliffGazer yes, the cell count mismatch is the one that catches people out. 15S gives you ~54.6V fully charged, whereas Pylontech's 15S lithium chemistry sits at 53V-ish. Victron's charge profile gets pulled in two directions and something has to compromise.

Practically speaking, I'd run them on separate MPPTs feeding the same busbar rather than trying to blend the charge profiles. Learned that the hard way with a mixed pack in my van — BMS was constantly confused.

Worth looking at the Cerbo GX user-defined battery settings too — you can manually override the Pylontech CANBUS values and set voltage limits yourself, though you lose some of the smart comms features doing it that way.

Not a clean solution tbh, but it can work.