Winnebago Solis – 12V Lithium Battery Bank & Solar Upgrade

Call a Winnebago dealer about powering a Solis off-grid for a week, and they’ll tell you the stock 60Ah AGM battery setup is plenty—you just need to manage your usage. Ask that same question in any RV forum, and you’ll get a dozen voices saying flat-out that those batteries are undersized and you’ll be in the dark by day two. One of these answers is dead wrong, and I learned which one the hard way during a ten-day boondocking stretch in the high desert where the closest shore power was seventy miles behind me. The factory AGM pack drains fast enough that running a fridge, heated water, and basic lighting at night becomes a cold calculation: what stays on, what turns off, and how deep do I let this battery go before I’m stuck. A lithium upgrade with proper solar isn’t some luxury add-on for the comfort-seeking crowd—it’s the bedrock of actually living in a Solis without constantly rationing power or praying the sun comes out tomorrow. I’m walking you through the exact rebuild I did, which components moved the needle, which ones didn’t, where I cut corners (and paid for it), and how to wire it all yourself without hiring someone else to flip the switches.

Trading Your Wimpy Factory Batteries for a Lithium Bank That Actually Lasts

The Winnebago Solis arrives with a 60Ah AGM battery that hemorrhages capacity the moment you disconnect from hookups and start running a fridge, water heater, and cabin lights from your own power. Stepping up to lithium is the single most transformative upgrade available for truly independent off-grid living.

Parts and Tools

• 12V 100Ah LiFePO4 lithium battery with integrated or external battery management system (BMS) rated for RV duty — check current price
• Lithium-compatible DC-to-DC charger or multi-stage charger capable of accepting 13.6V+ input from alternator and solar controller
• Heavy-gauge marine battery cable (2/0 or 4/0 AWG) with crimped lugs rated for continuous 200+ amp service
• Inline fuse holder and 150–200A ANL or MEGA fuse matched to cable gauge and charger output
• Battery disconnect switch (manual or solenoid) rated for 12V DC lithium systems, minimum 200A continuous
• Lithium-safe solar charge controller (MPPT or PWM) with low-voltage cutoff and BMS communication capability
• Crimper, wire strippers, multimeter, and torque wrench for terminal connections (typically 10–15 ft-lbs on battery posts)

Step-by-Step Instructions

Step 1: Isolate all 12V power and document the factory setup

Turn off the main battery disconnect switch in the Solis. Photograph the factory AGM battery terminals, cable routing, and any existing fuses or breakers before touching anything. Note the gauge and color of each wire and where it terminates—this is your wiring map. Disconnect the negative terminal first, then the positive. Remove the factory battery hold-down bracket and any thermal insulation. Take a photo of the empty battery bay to confirm clearance for the new lithium unit.

Step 2: Verify the lithium battery’s BMS is armed and test voltage

Unbox the new LiFePO4 battery and check that the integrated or external BMS is in the correct state (usually armed or ready mode per manufacturer instructions). Use a multimeter to confirm the battery reads 13.2–13.6V on the terminals. If it reads below 12V or shows no voltage, contact the seller—a dead-on-arrival battery must be replaced before installation. Confirm the BMS has no error lights or beeps indicating internal faults.

Step 3: Install the battery disconnect switch and fuse holder in the positive line

Mount the disconnect switch and inline fuse holder in a clean, accessible location near the battery bay—typically on the frame or inside the cabinet wall. Use heavy-gauge cable (2/0 or 4/0 AWG) to run from the positive battery terminal to the fuse holder, then from the fuse holder to the disconnect switch. Do not install the fuse yet. Ensure all connections are crimped and tight; loose terminals will cause voltage drop and heat. Leave enough slack in the cable to allow battery removal without strain.

Step 4: Connect the new lithium battery and install the fuse

Carefully place the lithium battery into the factory battery bay, ensuring it sits level and does not rock. Connect the positive cable from the disconnect switch to the positive terminal of the new battery, tightening to 10–15 ft-lbs. Connect the negative cable directly to the negative terminal (no switch in the negative line). Install the 150–200A fuse into the holder. Do not turn on the disconnect switch yet. Double-check all connections are tight and no wires are pinched or touching metal edges.

Step 5: Reconnect the DC-to-DC charger and solar controller to lithium settings

Locate the Solis’s factory DC-to-DC charger (usually mounted near the engine bay or under a cabinet). Verify it is set to lithium charging profile (typically 14.2–14.6V absorption, 13.6V float) or replace it with a lithium-compatible unit. Reconnect its positive and negative leads to the battery through the disconnect switch. Similarly, reprogram or replace the solar charge controller to lithium mode, ensuring it communicates with the BMS if equipped. Do not power on either charger until the battery is fully connected.

Step 6: Turn on the disconnect switch and monitor for faults

Flip the main battery disconnect switch to the ON position. Listen for any clicking, buzzing, or alarm sounds from the BMS. Check the BMS status lights—they should show green or normal operation within 5–10 seconds. Use a multimeter to confirm 13.2–13.6V at the battery terminals and at the main fuse. If the BMS shows red lights, beeps, or the voltage is below 12V, turn off the switch immediately and troubleshoot the connection or contact the battery manufacturer.

Step 7: Test 12V loads and verify charging from alternator and solar

Start the Solis engine and confirm the DC-to-DC charger is supplying current to the battery (voltage should rise to 14.2–14.6V within 30 seconds). Run the cabin lights, water pump, and fridge for 10 minutes and confirm they operate normally. If the rig has solar panels, place them in direct sunlight and confirm the solar controller is charging (check the controller’s display or LED). Measure voltage at the battery after 15 minutes of charging—it should be stable and rising toward the absorption setpoint. If any load causes the voltage to collapse or the BMS to fault, stop immediately and check for loose connections or an undersized fuse.

Step 8: Perform a discharge cycle and verify low-voltage cutoff

With the engine off and solar panels shaded, run the cabin loads (fridge, lights, water heater) for 30 minutes and monitor the battery voltage with a multimeter. It should drop gradually from 13.2V toward 12.8V. The BMS should not cut power until the battery reaches its low-voltage threshold (typically 10.5–11V for LiFePO4). If the BMS cuts power prematurely or the voltage drops too fast, check that the DC-to-DC charger is not still drawing current and verify the BMS low-voltage setpoint matches the battery spec. Once satisfied, reconnect shore power or solar to recharge and confirm the battery accepts current without faults.

What works

• Fits straight into the OEM battery enclosure with no cutting, welding, or frame modifications—install boils down to removing the old unit, sliding the new one in, and reconnecting two cables to unlock 100Ah of actual usable power instead of the 45Ah you were getting before.
• An onboard BMS (battery management system) prevents runaway discharge and protects you from dead shorts, which means you stop treating your power budget like you’re defusing a bomb and can actually live normally without second-guessing every amp you draw.
• Charges quickly from a 40A DC-DC charger and accepts solar input without degradation risk—unlike lead-acid, lithium cells thrive on rapid charge cycles, so your Solis tops up in roughly half the time an AGM would require.

What doesn’t

• The price tag is heavy—plan on spending three to four times what a comparable AGM costs—although that math flips quickly when you factor in lifespan (5,000+ full cycles versus maybe 500 from lead-acid chemistry).
• Lithium performance drops off sharply in freezing temperatures compared to AGM chemistry, so if your plan involves winter boondocking in unforgiving climates, you’ll need to either adjust your charging approach or install a heating pad underneath the battery.

I was honestly nervous the first time I drained the lithium to 20% while parked in the mountains with no sun forecast for two days—but it held rock-solid, the BMS never cut me off, and it charged right back up the moment the weather cleared. That confidence shifted everything about how I camp. If you’re serious about boondocking on a Solis, grab a 12V 100Ah LiFePO4 Lithium Battery.

Part-Level Diagnostics

This guide covers the install on this coach. These go deeper on the sizing and wiring decisions behind it:

• Sizing a Victron SmartSolar MPPT: what the 100/30 numbers limit
• DC-DC charger vs. alternator: why your alternator won’t charge LiFePO4
• Battery cable and ANL fuse sizing for a lithium bank
• BMV-712: the shunt wiring mistake that ruins state of charge