The motorhome that would not start in April

You turn the key in April and get a click. The house bank is fine — it has been fine all winter, you watched it, the chart is flat and healthy and completely irrelevant, because the battery that failed is the other one. Nothing was watching that.

A motorhome has two batteries and they fail differently. The house bank gets emptied by something that switched on: a residential fridge falling back to DC, a furnace running more than you expected, an inverter left on with nothing plugged into it. The chassis battery gets emptied by things that never switch off at all — the ECU, the alarm, the steps, the radio memory — over months, with nobody looking.

Both are the same question: something is drawing and you did not know. The readings are different for each, and the pair of them together says something neither says alone.

The house bank: a draw that appeared

a. Current, set just above what you run on purpose

Learn your rig’s idle first — many sit at one to two amps overnight — and then set the threshold above the largest thing you run deliberately. An example on that rig: warn below minus 8 amps, critical below minus 20. Negative is discharge, and a SmartShunt reads it every 60 seconds.

It will still catch things you meant to do. A water pump, a furnace blower at seven to ten amps, a microwave on the inverter pulling over a hundred for two minutes — all of those cross any threshold low enough to be useful. That is the trade, and it is why the number goes above your normal peak rather than above your idle.

b. The overnight slope, which is a chart and not an alert

The better signal is the one the app cannot express. A bank that used to lose eight points of charge overnight and now loses twenty has acquired a load, and that is obvious on a week of history and invisible to any single threshold — there is no “dropped X in Y hours” rule to set.

So make it a weekly glance rather than an alert, and keep the state-of-charge floor as the backstop that catches whatever the current threshold missed. One caveat that is really a season: the first cold week of autumn genuinely does add a load, because the furnace really is running more.

c. What turns “a draw appeared” into “and here is why”

Neither of the readings below detects a parasitic draw. Both explain one, which is more useful.

A propane sensor reading empty at the same moment the draw jumped ten to fifteen amps is an auto-select fridge that lost gas and switched itself to DC. It will empty the bank, and refilling the bottle fixes it. A door sensor reporting the entry door opened just before the draw appeared means somebody left something on; no door event and a draw that appeared anyway means a device turned itself on, which is a different and more interesting problem.

Voltage sag is the weak method here and worth naming so nobody relies on it. On lead-acid a bank that was at 12.7 an hour ago and reads 12.3 now is under a heavy draw — but it cannot tell a big load from a low bank, and on lithium the voltage barely moves until the very end, so it says nothing at all.

The chassis battery, which nothing is watching

The cheapest answer is the crude one: a second Lancol clipped to the chassis post, reading voltage every 120 seconds with no key and no connection to set up. Thresholds are per sensor, so it gets its own — warn below 12.4 volts, which is roughly 75 per cent on a resting starter battery, and critical below 12.1, which is around 40 per cent and where sulfation starts making a cold crank doubtful.

Read it in the morning and compare like with like, because cold moves it: resting voltage falls roughly a tenth of a volt for every twenty degrees. A January reading is lower than the battery really is, and comparing a January morning against an October one will convince you of a failure that has not happened.

And the obvious device is not available. The small blue BM2 monitor that half the internet has clipped to a starter battery is detected but not decoded — its voltage sits behind an encrypted connection and a key exchange, which is a different path from the passive listening the hub does today. Do not buy one for this on our account.

The combinations

Two batteries watched at once is the whole reason to do this, and it is the cookbook pattern with an unusually clean payoff.

• Chassis declining over days, house steady — a chassis-side draw or a tired battery. Pull the negative for storage, or fit a maintainer.
• Both declining together, in step — they are tied. A battery isolator stuck closed, a “both” switch left on, or the converter that was floating the pair has died.
• Chassis fine every morning for a week, then 12.0 volts in one step, with a door event — something was left on. Without a door event, a cell has shorted and the battery is finished.
• Chassis dropped overnight and the outdoor sensor fell twenty degrees — temperature, not the battery. Compare mornings at the same temperature before deciding anything.
• A steady ten amps out overnight with the pedestal reading 120 volts — the converter is not carrying the load. That is the not-charging article, arriving from the other direction.
• Chassis steady while the house sits under 12.9 with shore power present — a chassis maintainer doing its job and quietly masking a dead house converter.

What none of this tells you

Which circuit. One shunt sees the sum of everything, and anything wired around it onto the battery post is invisible to it entirely. Several DC monitors, one per circuit, is the only way this catalog answers “which one”, and they are placeholder layouts rather than products we can point you at.

Cranking capacity, which is the number that actually decides whether the engine starts. A battery can rest at 12.5 volts and still fail to turn a cold engine; only a load test says otherwise and nothing here does one.

What to do about it

For a rig in use: the current threshold above your biggest deliberate load, the charge floor as the backstop, and a weekly look at the overnight slope. Three things, one of which is a habit.

For a rig in storage: put a cheap voltage monitor on the chassis post. It is the battery nobody instruments and the one with the most predictable failure, and the fix once you can see it is a maintainer or a disconnected negative — both of which cost less than the battery and far less than the April morning.

Where to get one

A shunt for the house bank if you do not already have one, and a second cheap voltage monitor for the chassis post — that one being a rare case where the crude device is genuinely the right answer, because all it has to do is notice a slow decline. The compatibility database has the alternatives.