RV Solar and House Battery Systems

Why house banks die years before they should

Flooded and AGM lead-acid cells fail by sulfation, and sulfation is a function of time spent below full charge rather than cycle count. Every hour a bank sits at 70 percent, lead sulfate crystals grow larger and harder until the plate area they cover stops participating in the reaction. Owners read that as a battery that suddenly quit. It did not. It lost capacity gradually over two seasons of partial charging, and the day it failed was simply the day the remaining capacity dropped below what the coffee maker needed.

The usual culprit is the converter. A great many factory converters hold a fixed 13.6 volt float and never enter a proper absorption stage, so a discharged bank climbs to roughly 85 percent and stalls there indefinitely. AGM chemistry is less forgiving about this than flooded, because you cannot equalize a sealed recombinant cell to reverse the damage. Add the absence of a temperature compensation probe, which most units ship without, and a bank stored through a hot Southern California summer is chronically undercharged in the exact conditions that accelerate plate corrosion.

Parasitic load finishes the job. A propane leak detector, a slide controller board, a stereo memory circuit, a tank monitor, and an inverter sitting in idle can pull well over an amp continuously. Over a six-week storage period with no charge source, that is enough to drag a pair of group 27 batteries into deep discharge. We measure standby current with a clamp meter on the negative main before recommending anything, because replacing the bank without finding the draw simply resets the clock on the same failure.

Lithium is a system change, not a drop-in

LiFePO4 cells hold a nearly flat voltage across the middle of their usable range, which is exactly why the voltage-based charging logic in a lead-acid converter reads them wrong. A pack sitting at 13.2 volts might be at 90 percent or at 30 percent. The internal battery management system protects the cells from overvoltage and overcurrent, but it protects them by disconnecting, and a BMS that opens under load in the middle of a rainstorm is not a charging strategy. Charge sources have to be reprogrammed or replaced, not merely tolerated.

The alternator deserves specific attention. Lead-acid banks self-limit their charge acceptance as internal resistance rises, so the alternator sees a tapering load. Lithium does not taper. It will accept everything an alternator can produce for as long as the alternator can produce it, and a stock unit with no external cooling and a duty cycle designed for topping off a starting battery can cook its own windings on a long grade. A properly sized DC-DC charger caps the draw and isolates the two systems, and on a diesel pusher it is not optional.

Cold is the other constraint people underestimate. LiFePO4 cells must not be charged below roughly 32 degrees Fahrenheit without internal heating, and many packs simply refuse the charge rather than telling you why. A coach stored in the high desert or towed through the Sierra in shoulder season needs either self-heating cells or a controlled heat pad tied into the charge circuit. We specify one or the other on any conversion that will see winter use, and we test the cutoff behavior before the unit leaves.

Then there is the copper. Lithium banks routinely support continuous currents that the original 4 AWG runs were never rated for, and undersized conductors show up as voltage sag under inverter load and warm terminals after an hour. We size cable to the actual inverter and charger ratings, terminate with hydraulic crimps rather than set screws, and fuse the bank at the positive post with a Class T device, which is the only common fuse type with an interrupt rating high enough for a lithium short circuit.

Panels, controllers, and roof penetrations

The choice between a PWM and an MPPT controller is a math problem, not a preference. A PWM controller drags panel voltage down to battery voltage, so a nominal 100 watt panel with an 18 volt maximum power point delivers closer to 70 watts into a 13 volt bank. An MPPT controller converts the excess voltage into current and recovers most of that loss, and the advantage widens on cold clear mornings when panel voltage peaks. On any array above roughly 200 watts, the controller upgrade pays for itself faster than adding another panel.

Shade behaves worse than most owners expect. Panels are built from series cell strings, and a shadow across one string pulls the whole string out of the circuit through its bypass diode. On an RV roof, the shade sources are permanent fixtures: air conditioner shrouds, vent stacks, antennas, and the refrigerator vent cover. Two panels wired in series will lose disproportionately if one of them lives in the shadow of a shroud at midafternoon. We lay out arrays around those obstructions and choose series or parallel strings based on where the shadows actually fall.

Every hole in a roof is a future leak unless it is built correctly. The membrane matters: EPDM, TPO, and laminated fiberglass all take different sealants, and self-leveling lap sealant applied over a dirty or incompatible surface releases within a season. We bed feet in butyl tape, mechanically fasten into structure rather than into skin alone, and top with the sealant the membrane manufacturer specifies. Cable entry goes through a proper gland with a drip loop, not through a slot in the refrigerator vent, which is a common shortcut and a common source of water in the sidewall.

Inverters, transfer switching, and real load profiles

Modified sine output is cheap and it is hard on anything with a motor or a switching supply. Residential refrigerator compressors run hot on it, variable speed fans hum, and some medical equipment refuses to start. Pure sine costs more and eliminates the entire category of complaint. Sizing follows the load profile rather than the nameplate wish list: a 2000 watt unit runs a residential refrigerator, a microwave, and a coffee maker if you do not run all three at once, and a 3000 watt unit only helps if the bank can deliver the current behind it.

Air conditioning off the inverter is possible and it is a different conversation. A 13,500 BTU rooftop unit draws a large locked rotor current at startup, and a soft start module cuts that surge enough for a 3000 watt inverter to carry it. Without one, the inverter faults on overload while the compressor stalls, which is how start capacitors and compressor windings get damaged. We install the soft start and verify the actual inrush with a meter rather than assuming the published figure applies to a fifteen year old unit.

The wiring downstream is where safety problems hide. An inverter feeding a subpanel needs the neutral and ground bonded in exactly one place, and a bonded inverter tied into a shore-bonded system creates a parallel neutral path that trips GFCI devices and defeats the protection you thought you had. Automatic transfer switches need to be rated for the load and mounted where their contactors can be inspected. We label every circuit and leave a one page diagram in the unit, because the next person to touch it will not have our notes.

How we scope and price the work

Power work starts on the bench with measurement. Diagnostics run at $285 per hour with a one hour minimum, and that hour is credited against an authorized repair, so a customer who approves the work is not paying twice. Electrical labor is posted at $260 per hour. We give a written scope after the diagnostic rather than before it, because quoting a solar and lithium package from a phone description reliably produces a number that changes once someone actually reads the charge profile on the existing converter.

All of this happens in the shop at Yorba Linda. We do not do mobile electrical work, and that is deliberate: verifying a bank under load, running an inverter through a full transfer cycle, and confirming a controller harvest curve across a day takes bay time and instrumentation, not a van and an hour in a customer driveway. Coaches up to 45 feet fit inside, which matters when the array work and the interior wiring have to happen on the same unit in the same week.

Parts pricing is the other half of the conversation and we keep it plain. Cells, controllers, and inverters vary enormously in build quality behind similar specifications, and the cheapest listing for a given rating is usually cheap for a reason that shows up in year three. We quote a component we are willing to stand behind, we tell you what the alternatives cost, and if you want to supply your own hardware we will install it and note on the invoice which portion of the system we selected.

What is included

Bank load test and parasitic draw measurement

Standby current read at the negative main and traced to the offending circuit before any bank is quoted.

Charge source audit

Converter profile, alternator path, and solar controller checked against the chemistry actually installed.

Array layout and mounting

Panels placed around shroud and vent shading, bedded in butyl, fastened to structure, and sealed to the membrane type.

Conductor sizing and overcurrent protection

Cable rated to the real continuous load, hydraulic crimped terminations, and Class T fusing on lithium banks.

Inverter and transfer switch integration

Single-point neutral bonding, subpanel separation, and a verified transfer cycle under load.

Documentation left with the unit

Labeled circuits and a one page system diagram so the next technician is not guessing.

Signs you need this work

• Batteries that read full in the morning and will not run the furnace overnight
• A converter that never gets the bank past roughly 85 percent no matter how long you stay plugged in
• Solar controller showing good panel voltage but almost no amps into the bank
• Inverter faulting or clicking off when the microwave and the refrigerator overlap
• Warm battery terminals or cable insulation that has discolored near a lug
• A new lithium pack that shuts off entirely on cold mornings