Why Your Onan Generator Only Charges Lithium Batteries at 5 Amps: Troubleshooting Guide for RV Power Systems

The Quick Answer

Your Onan 4k generator is likely operating in "float" mode rather than bulk charging mode when your lithium battery is at 40%. The PD 9360 converter you installed has a multi-stage charging profile designed for lead-acid batteries, not lithium. At 40% state of charge, your lithium battery should be pulling maximum amps, but the converter is seeing the higher voltage of your lithium battery and switching to a lower amp maintenance charge. Meanwhile, your Growatt system is designed to deliver consistent high amperage regardless of battery type, which explains why you're seeing 25 amps from it versus only 5-6 amps from the generator setup.

What's Causing This

The root cause is a mismatch between your PD 9360 converter's charging algorithm and your lithium battery's needs. Progressive Dynamics converters use a three-stage charging system (bulk, absorption, float) that works well with lead-acid batteries but can be problematic with lithium. When the converter senses what it thinks is a "full" battery based on voltage readings, it drops into float mode at around 13.2-13.6 volts and only 2-8 amps.

Your lithium battery likely has a resting voltage around 12.8-13.0 volts even at 40% capacity, which tricks the converter into thinking the battery is nearly full. Lithium batteries can accept much higher charge rates throughout their charging cycle compared to lead-acid batteries. The PD 9360 is capable of 60 amps output, but it's being conservative because it's reading your lithium battery's voltage profile incorrectly.

Your Growatt portable power station (or other Growatt charging system), on the other hand, likely has a lithium-compatible charging profile or simply operates as a constant current source that delivers maximum available amperage regardless of battery voltage, which is why you're seeing the expected 25+ amp charging rate.

Tools You'll Need

• Multimeter for voltage testing and verifying converter output
• Progressive Dynamics Charge Wizard (PD91-60LI-A) - lithium-specific pendant
• Wire strippers and electrical tape
• Small Phillips head screwdriver
• Optional: Clamp-on ammeter to verify actual output current

Step-by-Step Fix

1. First, verify the issue by checking your converter's output voltage with a multimeter while the generator is running and charging. You should see around 13.2-13.6 volts if it's in float mode, rather than the 14.2-14.6 volts needed for bulk charging lithium batteries.
2. Purchase and install a Progressive Dynamics Charge Wizard PD91-60LI-A, which is specifically designed for lithium batteries. This pendant plugs into your PD 9360 and changes the charging algorithm to be lithium-compatible. It costs around $50-70 and takes about 10 minutes to install.
3. Locate the small round port on your PD 9360 converter (usually on the front face) and plug in the lithium Charge Wizard. The converter will automatically detect it and switch to lithium charging mode, which uses proper lithium charging voltage of 14.2-14.6 volts and maintains higher amperage during the bulk charging phase.
4. Test the system by running your generator with the battery at a lower state of charge. You should now see charging rates in the 20-40 amp range (depending on your battery's acceptance rate) instead of the 5-6 amps you were experiencing.
5. If you still see low charging rates after installing the Charge Wizard, check that your generator is producing proper AC voltage (around 120V) and that all connections between the generator, converter, and battery bank are tight and corrosion-free.

When to Call a Pro

Call a professional RV technician if you're uncomfortable working with 120V AC electrical systems or if the Charge Wizard installation doesn't resolve the issue. Also seek professional help if you discover voltage irregularities from your Onan generator (should be 120V ±5%), as this could indicate internal generator problems that require specialized diagnostic equipment.

If your solar panels are consistently underperforming (you mentioned only 170-190 watts from 600 watts of panels), consider having a solar specialist check your MPPT charge controller settings, wiring connections, and panel orientation. While not directly related to your generator charging issue, maximizing your solar input will reduce your dependence on generator charging and extend your boondocking capabilities.