Anyone else had their JK BMS trip out when charging from a DC-DC on engine start?

Bit of a frustrating one this. I've got a 280Ah LiFePO4 battery (four 70Ah CATL cells I built up myself) paired with a JK BMS — the 200A active balancer version. Been running it in my Transit camper for about six months with no real bother, but lately every time I fire up the engine and the Renogy 40A DC-DC kicks in, the BMS trips. Cuts the whole load, inverter goes dark, the lot. Have to manually reset it each time.

I've been through the JK app and I can't see anything obvious — cell voltages are all sitting between 3.28 and 3.31V at rest, so it's not an overvoltage issue. I did notice the overcurrent protection is set to 150A with a 300ms delay, but surely a DC-DC charger isn't spiking that high on startup? I've got a decent 35mm² cable run, about 1.2 metres from battery to the BMS, so I wouldn't have thought voltage drop was causing phantom readings either.

Has anyone seen this with the JK specifically? I'm wondering if it's an inrush current spike from the DC-DC itself rather than anything to do with the cells. Would a pre-charge resistor or a slow-blow fuse on the alternator side help at all, or am I barking up the wrong tree entirely?

Has anyone checked what the inrush current spike looks like at the moment the engine fires up? I had something similar on my boat — the alternator voltage shoots up briefly before the DC-DC regulator catches it, and the JK sees what looks like an overcurrent event even though steady-state charging is fine.

Worth logging with a clamp meter or Victron BMV if you have one connected.

Also — which DC-DC are you running? Some of the cheaper Renogy units have a slower soft-start ramp than a Victron Orion. I switched to an Orion-Tr Smart on my shepherd's hut build specifically because the controlled ramp-up stopped exactly this kind of nuisance tripping.

Might be worth bumping your overcurrent protection delay in the JK settings slightly before you go changing anything else — less destructive as a first step.

KarenEvans | 847 posts

@DODNerd I had almost identical grief with my JK on a Sprinter build last year. The fix that sorted it for me was adding a small delay relay on the DC-DC enable wire — basically holding the B2B charger offline for around 8-10 seconds after ignition. Gives the alternator time to properly come up to speed and stabilise before any load hits it. I used a cheap 12V timer relay from Amazon, wired into the ignition feed.

Also worth checking your BMS overcurrent protection setting — mine was factory set quite aggressively at 150A. Bumping it up slightly (I went to 180A) reduced nuisance trips without compromising real protection.

@12V_King raises a good point about inrush — an oscilloscope on the shunt would tell you a lot if you've got access to one.