House battery not charging (12V)

Symptoms

• Voltage sagging overnight; morning lights dim or furnace board won’t stay up
• Converter fan silent; battery never rises above float (or never leaves resting voltage)
• Solar “working” on the controller display but the bank stays low at the posts
• Battery disconnect left OFF (or chassis vs house bank confusion)
• Lithium BMS cut-off — bank looks dead, then wakes after a jump / charger reset / correct recovery
• Alternator / DC-DC charger not contributing while driving
• SmartShunt / monitor SOC drifting far from reality (false “full” or false “empty”)
• Main fuse or ANL/MIDI opens under charge attempt

Quick checks (owner-safe)

1. Confirm the house battery disconnect / kill switch is ON and the main fuse (or fuseable link) is intact — look, don’t guess.
2. Measure resting voltage at the battery posts with a multimeter (not only the panel meter or phone app). Note the number.
3. Plug into shore (or generator). After a few minutes, measure converter/charger output voltage at the bank posts. Compare to resting.
4. Walk the shore path: pedestal → cord ends → EMS/surge protector status lights → coach main breakers. A dark EMS or open breaker kills charging without killing every 12V load.
5. If solar-only: confirm array breaker/fuse, charge-controller status LEDs/app, and that the controller’s reported battery voltage matches your post reading (± a few tenths under load is normal; multi-volt gaps are not).
6. Lithium: open the BMS app / Bluetooth. Note protection state (UVP, OCP, charge inhibit). Do not force-charge blindly past manufacturer recovery steps.
7. While driving (if applicable): confirm DC-DC / isolator / battery-to-battery charger is enabled and receiving ignition/D+ sense — alternator alone often does not float a modern house bank correctly.

When to stop DIY

• Smell of hot plastic or melting near the converter, busbars, or battery bay
• Smoke, arcing, or melted cable lugs
• Battery case swollen, venting, or acid/electrolyte on the tray (lead-acid) — treat as hazardous
• Main fuse keeps opening under light or no load
• You cannot get a stable voltage reading (meter jumps wildly — loose lug or broken conductor risk)
• Lithium BMS will not reset after the correct OEM recovery procedure
• Any work that requires opening a sealed converter/inverter-charger case under warranty or live AC exposure you’re not equipped for

What UMRT diagnoses

United Mobile RV brings diagnostics-first metering — DVOM at posts, busbars, and converter output; thermal imaging where heat at lugs, converters, or high-resistance joints matters. We separate “not charging” from “draining faster than it charges,” then verify the charge path end-to-end.

• Converter vs multi-stage charger vs Victron MultiPlus/Quattro charge path
• Parasitic drain vs failed charging source (clamp / fuse hierarchy)
• Cable drop, bad grounds, and busbar/fuse hierarchy
• Solar controller programming (absorption / float / lithium profiles)
• BMS communication, Victron VE.Bus, and SmartShunt accuracy — Victron Professional Certified Installer
• Alternator / DC-DC contribution vs shore-only expectation

Trip fee quoted before we roll. Diagnostic $150 applied toward repair if you proceed. Labor ~$150/hr (1 hr min / 30-min increments). Active corridor: MT · WY · ID · WA.

Separate “not charging” from “draining faster than it charges”

A bank that drops overnight while the converter was never online is a charging-path failure. A bank that rises under shore then falls hard overnight with loads off (or only “always-on” detectors) is often parasitic draw or a monitor lie — different job.

• Not charging: resting voltage flat or falling while shore/solar/alternator should be pushing current into the bank.
• Draining faster than charge: charger is online and correct voltage appears at posts, but SOC/voltage still bleeds under known loads — or phantom loads with everything “off.”
• False full / false empty: shunt installed on the wrong cable, missing loads on the wrong side of the shunt, or a panel meter fed from a different point than the bank.

Write down: resting V, charging V at posts, whether shore/solar/drive was present, and which loads were on. That note saves a parts-guess trip.

Shore-powered charging path

Converter / charger output checks

• Identify what you have: OEM converter (WFCO, Progressive, PowerMax, etc.), multi-stage charger, or inverter-charger (Victron, Magnum, Xantrex). Labels and model stickers beat memory.
• With shore on and converter breaker closed, expect charge-stage voltage at the battery posts appropriate to chemistry (flooded / AGM / LiFePO4 profiles differ — don’t chase a lithium setpoint on a flooded bank).
• If AC is live at the converter but DC output is dead or weak: internal DC fuses, failed board, or thermal shutdown are common — confirm with meter, don’t swap batteries first.
• Fan silent is a clue, not a diagnosis. Some units are quiet at float; some fail with no fan and no output.

EMS and breaker traps

• EMS / surge protectors that reject reverse polarity, open ground, or low/high voltage will cut the charge path while the pedestal looks “on.”
• Coach converter breaker open or tripped — reset once; if it trips again under no unusual load, stop and diagnose (shorted converter or wiring fault).
• Dogbone adapters and corroded 30/50A pins create voltage drop that looks like a “weak converter.” Feel for heat at the plug face after 10–15 minutes on charge — heat = problem.
• Ties to the broader dead-coach path: start at pedestal inward, same as no power.

Solar-only and hybrid charging

Controller status vs real battery voltage

• Trust the posts. Controllers can show array watts while a blown battery fuse, open disconnect, or wrong battery sense lead leaves the bank untouched.
• Confirm PV breaker/fuse, MC4/connector integrity after travel days, and that night-time resting voltage isn’t being misread as a daytime charge failure.
• Hybrid systems (shore + solar + DC-DC): note which source was online when the bank failed to climb. One failed source with others healthy is a narrow fix; all sources failing points at the bank side (fuse, BMS, disconnect, cabling).
• Lithium charge profiles must match BMS limits. Wrong absorption/float or “lead-acid” defaults on a LiFePO4 bank cause chronic undercharge or charge-inhibit fights.

Lead-acid vs LiFePO4 differences

No brand-bashing — chemistry honesty only.

• Flooded / AGM: resting ~12.6–12.8V healthy; chronic undercharge sulfates; overcharge cooks water/AGM. Equalization is OEM-specific — don’t invent a schedule.
• LiFePO4: resting voltage sits high across much of the SOC curve; a “12.9V” reading is not a state-of-charge guarantee. BMS can open charge or discharge FETs and look like a dead bank or a dead charger.
• Recovery after BMS UVP often needs a controlled charge source the BMS will accept — not random jumper chaos.
• Mixing old lead-acid habits (hydrometer-only thinking, ignoring temperature compensation / profiles) on lithium installs wastes time.

Alternator and DC-DC while rolling

• Many modern coaches need a DC-DC charger (e.g. Victron Orion) — chassis alternator smart regulators may not push a house bank correctly.
• Check ignition sense / D+ / engine-run signal. No sense = no charge while driving, even with a healthy alternator.
• Cable gauge and fuse size between chassis and house must match the DC-DC rating. Undersized cable = voltage drop that looks like “alternator won’t charge.”

When a bank is actually done vs misdiagnosed

• Load test / capacity test beats a single resting voltage. A bank that holds 12.6V unloaded and collapses under a known load is tired; a bank that never sees correct charge voltage is unfinished diagnosis.
• One bad battery in a series/parallel string can drag the set — isolate and compare when safe.
• Corroded lugs, painted ground points, and stacked ring terminals create intermittent “dead bank” stories that clear when the wrench turns — until the next washboard road.
• Replace the bank after the charge path proves healthy — not before.

Converter internal fuses and thermal shutdown

• Many OEM converters hide DC output fuses under a cover — open only if you’re comfortable with AC locked out and OEM access guidance.
• Thermal foldback: unit hot, output sagging, fan failed or blocked — cool the bay, clear lint, re-test before condemning the board.
• WFCO-style “mystery float forever” after a low-voltage event: some units need a reset sequence or a healthy bank to exit a protective mode — confirm with OEM notes, don’t assume the batteries are fine because the display blinked.
• Progressive / multi-stage units: verify stage with a meter over time (bulk → absorb → float). Stuck in float with a depleted bank is a programming or sense-lead problem until proven otherwise.

Grounds, busbars, and cable drop

• Measure voltage at the charger output and at the battery posts under charge. More than a few tenths of a volt difference under modest charge current means cable, lug, or ground drop.
• Frame grounds painted over, rusty, or stacked under unrelated rings are frequent. Clean metal-to-metal; torque to OEM/spec — don’t gorilla-strip threads.
• Busbar hierarchy: main fuse → shunt → bus → loads/chargers. A charger landed on the load side of a blown fuse looks “alive” on the charger and “dead” at the bank.
• Aftermarket dual-bank isolators and poorly fused add-ons create parallel paths that confuse shunt SOC and DIY meter checks.

Parasitic drain isolation (high-level)

• With coach “off,” note resting voltage trend over an hour. Steep drop with known loads off → hunt always-hot circuits (detectors, stereos, aftermarket trackers, USB hubs).
• Pull fuses one at a time only if you understand what you’ll lose (slides, brakes, alarms). Label what you pulled.
• Clamp meters on the negative (or positive per your training) help; guessing by feeling cables does not.
• A dying bank can look like a massive parasitic — capacity test after the charge path is proven.

Book a diagnostic

If posts never see charge voltage, fuses open, or lithium BMS won’t recover cleanly, Prefer Text. We meter the path, quote the trip before we roll, and apply the diagnostic if you proceed.

Prefer Text (616) 606-5277 · Book a Service · trip fee quoted before roll · diagnostic $150 applied if proceed

Pricing reference: trip $75/30 mi then $1.50/mi · labor ~$150/hr (1 hr min / 30-min) · diagnostic $150 applied. Credentials (text): Victron Professional Certified Installer · weBoost Authorized Installer · Peplink Certified Associate · Dometic Professional Certified · Starlink install capability. Corridor: MT · WY · ID · WA.

Related guides

• No power / dead coach
• Inverter / shore-power faults
• Electrical troubleshooting
• Solar & battery troubleshooting
• Victron fault code guide
• Converter / inverter / charger guide
• Services · Pricing · Book a Service
• Troubleshoot hub: /troubleshoot/