Tips for Identifying Early Float in Victron Chargers: Diagnose Voltage Drops and Battery Imbalance in Your Campervan

When your Victron MPPT or DC‑DC charger slips into float mode while the batteries are still not 100% full, something is wrong. An early float transition means the charger thinks the batteries are full when they aren’t, leading to under‑charged lithium cells and shortened lifespan. In the worst case, you could end up stranded with no power.

This post walks through a real‑world scenario diagnosed remotely with amazing assistance from the customer, explains what causes premature float, and shows you how to diagnose and fix it using a multimeter. Whether you’re an installer or a van builder, you’ll find step‑by‑step instructions, diagrams and practical tips.

🛠️ The Scenario

A customer’s electrical system looked like this:

• 3× 230 Ah lithium batteries in parallel
  
  

• Victron SmartShunt connected on the negative side
  
  

• Victron SmartSolar MPPT charger (x 3) and Orion‑Tr DC‑DC charger feeding a positive busbar
  
  

• A Victron MultiPlus inverter/charger for AC charging
  
  

• Solar panels feeding the MPPT, alternator feeding the DC‑DC, and a fuse block and isolator connecting everything to the loads
  
  

Despite having plenty of solar and alternator power, the MPPT and DC‑DC chargers switched to float mode long before the batteries were full. The MultiPlus charger, however, charged normally. The VictronConnect app showed 14.4 V at the MPPT and DC‑DC outputs, but a multimeter on the battery terminals read only about 13.9 V. The SmartShunt agreed with the chargers, not with the batteries.

Why This Matters

According to Victron’s own manual, if the battery cables experience a voltage drop of more than 2.5 %, the charger will produce the correct voltage but the batteries will see a lower voltage. That drop can be caused by loose connection, loose lugs or faulty fuses, faulty isolators and more. A charger will interpret the higher voltage at its own terminals as a full battery and prematurely switch to float mode.

🔎 How We Diagnosed the Problem

Before testing, ensure your chargers are in bulk mode (not float). You may need to put a load on the system or temporarily reduce the target voltage to force the chargers to ramp up.

Test Equipment and Settings

• Digital multimeter with 20 V DC range and resistance mode
  
  

• Red probe always goes to the positive point under test
  
  

• Black probe goes to ground or the negative point under test
  
  

1. Check Battery Voltage at Rest (No‑Load Test)

1. Power off all charging sources.
   
   

2. Measure each lithium battery individually: red probe on its positive terminal, black probe on its negative terminal.
   
   

3. All batteries should be within 0.05 V of each other. A battery that’s more than 0.2 V lower likely has a weak BMS or poor interconnect.
   
   

2. Check Battery Voltage Under Load (Bulk Charging)

1. Turn on the solar MPPT and DC‑DC so they charge at full current.
   
   

2. Measure each battery again. They should still be within 0.05–0.1 V.
   
   

3. A difference greater than 0.2 V indicates a bad interconnect or an internal battery imbalance.
   
   

3. Measure Voltage Drop from MPPT to Positive Busbar (On‑Load)

1. With the MPPT charging in bulk, place the red probe on the MPPT’s positive output terminal and the black probe on the positive busbar.
   
   

2. Expect a drop of < 0.1 V. Anything over 0.2 V points to a loose crimp, loose lug or undersized cable between the MPPT and busbar.
   
   

3. Repeat the same test for the DC‑DC charger.
   
   

4. Compare Charger Output Voltage to Battery Voltage

1. Place the red probe on the charger’s positive output and the black probe on the battery negative terminal.
   
   

2. Then measure directly across the battery terminals.
   
   

3. If the charger “sees” a voltage that’s 0.1 V or more higher than the battery terminals, it will think the battery is full and go into float early.
   
   

5. Measure Battery Interconnect Resistance (System Off)

1. Switch off all sources.
   
   

2. Set the meter to resistance (Ω).
   
   

3. Measure between the positive terminals of each pair of parallel batteries.
   
   

4. Resistance should be < 0.1 Ω; anything higher indicates a loose or corroded interconnect.
   
   

6. Check Ground Path Voltage Drop (On‑Load)

1. With the chargers running, place the red probe on the battery negative post and the black probe on the negative input of the MPPT or DC‑DC.
   
   

2. A drop greater than 0.05 V suggests poor ground connections or undersized wire.
   
   

7. Positive Busbar to Isolator and Fuse Block

1. Busbar → Isolator: With charging active, measure from the positive busbar to the input side of the isolator switch. Drop should be < 0.05–0.1 V.
   
   

2. Busbar → Fuse Block: Measure from the positive busbar to the input of the fuse or fuse block. Drop should also be < 0.05–0.1 V.
   
   

3. Shunt → Negative Busbar: With charging active, measure from the battery negative (on the shunt) to the negative busbar. A drop above 0.05 V means the shunt or its connections have excessive resistance.
   
   

By following these steps, we discovered a faulty isolator in the system, causing resistance. After the customer changed this, the chargers stayed in bulk mode until the batteries reached absorption voltage.

🧠 What Causes Early Float and How to Fix It

✅ Tips for Installers and DIY Builders

• Size your cables correctly: Use Victron’s Wiring Unlimited guide or our own wiring tables to choose the right gauge. Undersized cables not only reduce charging efficiency but create heat and fire risk.
  
  

• Crimp and torque your lugs properly: Loose or poorly crimped lugs are a common cause of voltage drop. Use a proper crimp tool and always torque nuts to the manufacturer’s spec.
  
  

• Enable remote voltage sensing: A Smart Battery Sense or VE.Smart network measures battery terminal voltage and sends it to the charger, so it can correct for small cable losses.
  
  

• Check your settings: Make sure the MPPT and DC‑DC are set for lithium batteries and not using temperature compensation meant for lead‑acid. Adaptive charging can be great, but a fixed absorption time sometimes prevents premature float.
  
  

• Use a battery monitor: A SmartShunt or BMV monitor gives accurate state of charge, unlike simple voltage‑based monitors which don’t give much information on Lithium batteries.

🤝 How We Can Help

At Rayne Automotive, we design and supply complete campervan electrical systems. Our custom wiring looms and pre‑wired power boards save hours of installation time. We also stock Victron Multiplus inverter/chargers, Orion‑Tr DC‑DC chargers, SmartShunts, solar panels, Smart Battery Sense modules, and more.

Need help selecting or troubleshooting? Book a 1:1 technical support call with our engineers, or browse our buying guide for Victron products for more background on chargers, inverters and battery monitors.

Related Posts & Products

• Complete Guide to Victron Products for Campervans – Learn about DC‑DC chargers, MPPTs, Multiplus units and more
  
  

• Portable Power Stations vs Traditional Leisure Battery Setups – Compare self‑contained power stations with full systems.
  
  

• Custom Wiring Looms – Pre‑assembled looms built to your van’s measurements.
  
  

• Book a Technical Support Call – Schedule a one‑to‑one call with our engineers.
  
  

❓ FAQs

Why does my Victron MPPT or DC‑DC charger switch to float when the batteries aren’t full?

Because the charger is sensing a higher voltage at its terminals than at the battery, often due to voltage drop in the cabling or poor connections. Victron notes that even a 2.5 % drop is too much.

What’s a normal voltage drop between the charger and battery?

Under load, aim for less than 0.1 V drop on a 12 V system. Larger drops (0.2 V or more) suggest undersized cables, too long cables or bad crimps.

How do I size my cables correctly?

Use the current rating of your chargers and the cable length to choose the right cross‑section. The Wiring Unlimited book provides tables and formulas; our own instructions in the Help Centre also cover this.

Do I need a Smart Battery Sense?

For small systems with short cables, maybe not. For longer runs or high‑current setups, adding remote voltage and temperature sensing ensures the charger compensates for cable losses.

What if my batteries are unbalanced?

Check each battery’s voltage under load. If one is 0.2 V lower, inspect its interconnects or BMS. You may need to re‑balance or replace that battery.

📞 Need Expert Help?

If your camper solar panels and Victron chargers are acting up, don’t guess. Our engineers troubleshoot dozens of systems each month and can help you fix yours. Book a support call or drop us a message today – we’re here to power your adventures.