RV Second Battery Not Charging from the Tow Vehicle or Alternator: Isolator Checks

Canadian RV owner’s repair guide · Educational information • Updated October 23, 2026 • About 4 minute read

A house battery that stays flat while you drive, or a starter battery that drains with the house battery, often involves a battery isolator or a charge feed. This guide explains how isolators work and which owner-level checks can find the problem before a technician opens the wiring.

It covers FET and diode battery isolators (splitters) on motorhomes and vans. It does not cover DC-to-DC chargers, which have different behaviour, or any installation work.

Disconnect the battery negative cables before making any change to the electrical system. ^[1] An isolator can get hot, so it must be mounted on a non-flammable surface and given room to cool. ^[1] Do not bypass a fuse, short across terminals, or work on charging cables with metal tools. One guide notes "do not short-circuit batteries. This can cause fire or explosion." ^[2] If a cable or isolator smells hot, stop and get an RV electrician.

1. Look for the LED. On one FET isolator, a blue LED lights when voltage is present at the input. ^[1] No light means no input voltage. 2. With the engine running, measure voltage at the alternator input, at the isolator output to the house battery and at the house battery itself. Large differences point to loose or undersized cables. ^[1] 3. Inspect the connections for corrosion or heat marks. Poor connections add resistance. ^[2] 4. Check each battery's positive fuse by sight. 5. Ask whether your alternator needs an energize signal. Some do, and a splitter blocks the battery voltage that would start them. ^[1] 6. Check the battery type. The isolator manual does not discuss lithium batteries, so confirm charging compatibility with the battery maker. ^[1]

Cleaning and tightening accessible terminals and replacing a blown fuse are owner tasks. Changing cables, adding an energize wire or replacing the isolator is for an RV electrician. A DC-to-DC charger is often considered for lithium house batteries or modern alternators, but ask the battery and vehicle makers first.

After a drive of at least half an hour, check that the house battery has risen in voltage, and that the starter battery stays charged when the house loads run with the engine off.

Batteries charge poorly when cold, and lithium batteries have charging limits in freezing weather. Salt and moisture corrode terminals under the vehicle, so check them each spring.

Before you book, send the component label, the RV year and model, the exact symptom and any displayed code, when it happens, photos taken from a safe position and the checks you have already completed. Mention recent repairs, freezing, impact or water exposure. Ask for a written quote in Canadian dollars that separates diagnosis, parts, labour, mobile travel charges and applicable taxes. The final cost depends on the confirmed fault, access and parts availability, so be wary of a firm price quoted before anyone has looked at the problem.

For this repair: Tell the electrician your battery chemistry and alternator type, because they affect which charging solution suits.

These are original manufacturer or component-maker resources to help you find the instructions for your exact equipment. A manual for a named model is an example, not authority for every RV. Match the manufacturer, model and serial number before applying any procedure, and follow the matching manual where its details differ.

1. Victron Energy: ARGOFET battery isolator manual (example)How a FET isolator works, voltage drop, energize input and installation warnings.
2. Victron Energy: Wiring Unlimited (DC cabling, voltage drop, fusing)Cable size, connection quality and fusing.

Reference links were selected on October 23, 2026. Manufacturers move and rename documents; if a link has moved, search the maker’s support site for your model number.