12V Triage: Everything Is Off and Nothing Is Broken

Every few weeks a van forum produces the same post in a different accent. Everything is off. The lights, the fridge, the fan, the sockets, all of it, at once. Nothing smells burnt. Nothing is visibly damaged. The battery was fine yesterday.

The instinct is to name a culprit, and the most expensive component in the chain usually gets the blame. One owner on r/vandwellers spent a 2am outage certain his DC-DC charger had taken down his entire system. It had not. He later found the cause was a nearby lightning strike, and his account of it is the clearest one-line summary of this whole failure mode:

“I found out that nearby lightning strikes can cause a voltage spike in your 12v system, which will cause your smart lipo’s BMS to shut off your battery.”

— /u/SqueezeMyLemmons, r/vandwellers

Nothing was broken. A protective device had done exactly what it exists to do, and the system looked identical to a catastrophic failure from the outside.

This is a community-evidence guide, per How We Test. VanTested has not bench-tested the parts below, and the diagnosis chain is a synthesis of owner reports rather than a lab procedure. Every product figure is a manufacturer claim, labelled as one.

The ladder, in order

Work these top to bottom and stop at the first one that explains what you are seeing. The order is not arbitrary: it runs from most likely and free to least likely and awkward, and the top rung is the one people skip because it does not feel like a cause.

1. The battery’s BMS has disconnected the pack.

Every modern lithium pack has a battery management system whose job is to open the circuit when it sees something outside its limits: cell voltage too low or too high, current too high, temperature out of range, or a spike. When it does, the pack stops presenting voltage to the outside world and the whole van goes dark in one step.

The test is two measurements. Put a meter on the battery terminals, then on the distribution bus. Normal voltage at the terminals and nothing at the bus means an open circuit between them. If the main fuse is intact and the disconnect switch is on, the BMS is what is holding it open.

Most packs re-engage when they detect a charge voltage on their terminals, so connecting a charge source is usually the test and the fix at once. What re-arms your specific pack is in its manual, and manufacturers differ enough that this is worth reading before you conclude the battery is dead. A BMS that cuts off repeatedly with no obvious trigger is a different conversation and a warranty one.

2. A fuse has blown.

Check the main fuse between the battery and the bus first, because that one takes everything down at once. Branch fuses only take their own circuit with them, which is a useful diagnostic in itself: if the fridge is out and the lights are fine, you are on rung two looking at one circuit, not rung one looking at the whole system.

Do not diagnose a blade fuse by eye. The element can fail with barely visible separation. Pull it and check continuity with a meter, or swap in a known-good fuse of the same rating.

3. A breaker has tripped.

Same function, different recovery. Breakers are quiet about it — a tripped breaker looks almost identical to a set one at a glance, and in a dim cabinet under a bed it is easy to walk past. Check every one physically rather than visually.

4. A switch is in the wrong position.

The battery disconnect, the isolator, the master switch under the seat that got nudged loading gear. This rung is embarrassing and it is also common enough that it belongs above the expensive rungs. Check it before you get the meter out again.

5. A DC-DC charger, MPPT or inverter is dead or faulted.

Now you are into components. Note that a dead charger explains a battery that has not been charging, not a system that shut off instantly — that distinction is exactly the one the lightning owner got wrong at first. If the pack is holding charge and the bus is live, a faulted charger is a recharge problem rather than a blackout, and it belongs in the sizing and recharge conversation instead.

6. A connection is loose or corroded.

Last on the list because it is hardest to find and easiest to misread. The signature is intermittency: it works, then it does not, then it works when you lean on the furniture. Negative and ground connections produce the most confusing version of this, because a marginal negative makes several unrelated circuits misbehave in ways that look like several unrelated faults.

Symptom to cause, at a glance

What you see	Most likely rung	The test
Everything off at once, no warning	1, 2 or 4	Voltage at battery terminals vs at the bus
Everything off, battery reads normal at its terminals	1 (BMS cutoff)	Apply a charge source and see if the pack re-engages
One circuit dead, rest fine	2 (branch fuse)	Pull the fuse, check continuity, do not judge by eye
Fuse blows immediately every time you replace it	Real fault, usually a short or reversed polarity	Check the connector polarity with a meter before fuse number three
Works, then does not, then works again	6 (loose connection)	Wiggle test with a meter on the suspect circuit
Bus is live, battery never fills up	5 (charge source)	Check charger output while the engine runs or the sun is up
Everything dead after sitting a long time	Low-voltage cutoff, which is rung 1 with a mundane cause	Charge it, then work out what drained it

The trap that costs three fuses

One roof fan wiring account is worth more than a paragraph of general advice because it is so specific and so repeatable. An owner reported blowing three fuses across three annual cleanings of the same fan, and the cause was that in that loom black is positive and white is negative — the opposite of the convention almost everyone reassembles by.

The narrow lesson is: check the polarity on your own fan before you reconnect it. The broad lesson is the useful one. Colour is a convention, not a specification, and it is not consistent across the mixed set of automotive, marine and consumer parts that end up in a van. Label the connections you make while you can still see both ends of the wire, and verify with a meter when you reassemble something you took apart a year ago.

What actually shortens the next outage

None of the parts below prevent a fault. What they do is turn a two-hour flashlight investigation into a five-minute one, which is the realistic goal.

Terminate every branch in one labelled place. A distribution block with a negative bus and written-on labels is the single biggest difference between a system you can triage and one you cannot. Blue Sea’s 5026 ST block claims 100A total with up to 30A per ATO/ATC circuit, tin-plated copper buses, a clear cover with spare fuse storage and twelve write-on circuit labels. That last feature is not a gimmick — a labelled block is what lets you go straight to rung two instead of pulling twelve fuses in the dark.

Carry spare fuses in every rating you use. A 120-piece ATC/ATO assortment costs less than one night of being stuck, and the point is having the right rating available so nobody is tempted to fit a bigger one. Fitting a larger fuse to stop a symptom removes the protection from the cable, and the cable is the thing that catches fire.

Put a monitor on it that can shout. A shunt-based battery monitor with programmable high and low voltage alarms turns “everything is off” into “you were told about this two hours ago”. Renogy lists its 500A shunt monitor as covering 10V to 120V with high and low voltage programmable alarms and a 20ft shielded cable. Claimed figures; we have not verified them.

Own a meter and know two measurements. Voltage across the battery terminals and continuity through a fuse are the only two skills this entire page requires. An auto-ranging meter such as the Klein MM400, listed at 600V with AC/DC voltage, current, capacitance, frequency, duty cycle, diode and continuity, is more instrument than a van needs and cheap enough that buying less is a false economy.

Be able to make a proper joint. Half of rung six exists because somebody twisted wires together at 11pm. A ratcheting crimper made for heat-shrink connectors and a box of adhesive-lined marine-grade butt connectors means the repair you make on the roadside is not the fault you chase next season.

Have a way to kill it. A battery disconnect switch you can reach without unloading the van is both a safety device and a diagnostic one — isolating the bank is how you test a suspected short without holding a spanner near a live terminal. Blue Sea claims 300A continuous and 48V DC maximum for the m-Series mini, with IP66 sealing and ignition protection.

The surge protection question, answered honestly

The lightning story invites an obvious follow-up: what do I buy so that never happens again? The honest answer is nothing, and it is worth saying plainly rather than selling around.

Consumer 12V DC surge suppression is a thin market, most of what exists is aimed at shore-power AC rather than the DC side, and none of it is a credible defence against an induced spike from a nearby strike. The protective device in that story already worked. The BMS saw something out of range and opened the circuit, which is the whole reason the pack was undamaged when he plugged a charger in.

If you want to spend money on resilience, spend it on the things above: correct fusing at every connection to the battery, labelling, a monitor with an alarm, and a disconnect. Those improve every failure mode. A surge product would improve one, badly, and only if it worked.

Where this fits in the bigger build

Triage is downstream of architecture, and some outages are really design decisions surfacing late. If your system goes down because a single point of failure took everything with it, that is worth thinking about at the level of power station versus DIY bank rather than at the level of fuses. If you are running an inverter permanently and wondering what it costs you in idle draw and complexity, 12V vs 48V and the always-on inverter covers the trade.

And if the underlying issue is that the bank never quite keeps up and the low-voltage cutoff is doing the shutting off, that is a sizing problem wearing a failure costume. The amp hour arithmetic is the fix.

What would settle this properly

Everything on this page is a diagnosis order derived from owner reports plus manufacturer claims about the parts. What would make it better is measurement: logged bus voltage through an actual BMS cutoff and recovery, so the shape of the event has a trace rather than a description, and a bench comparison of how different packs re-arm. That is an instrumented job on a van we control, and it is on the list.

Until then, the ladder is the useful part. The order is what saves the evening, and the top rung — nothing is broken, something protected itself — is the one worth remembering at 2am.