Battery and Charging System Repair, in detail

A house bank that used to run overnight and now dies by midnight is rarely one bad battery. Flooded and AGM banks fail as a set, because a weak cell drags the others down on every charge cycle, and one new battery paralleled with three tired ones is pulled to their level within weeks. Testing is a capacity check rather than a voltage check: rested open circuit voltage, then a measured discharge at a known current against a timer, then specific gravity per cell on a flooded bank. Twelve point six volts on a meter proves very little on its own.

Half of the answer is upstream anyway. A bank chronically undercharged by a converter stuck at float voltage will sulphate and lose capacity inside a year, and replacing it without correcting the charger buys the same failure again on the same timeline. House battery replacement runs $500 to $5,500 across two to ten hours. The low end is a like for like AGM swap, and the top is a chemistry change with the whole charging system reworked around it.

Flooded lead acid is cheapest per amp hour, tolerates a crude charger, needs its water checked, vents hydrogen and therefore needs a vented bay, and gives up roughly half its rated capacity before voltage sags badly. AGM costs more, needs no watering, can live in a sealed compartment, and accepts a higher charge current. LiFePO4 costs the most up front, delivers nearly all of its rated capacity, weighs less than half as much for the same usable energy, charges far faster, and lasts many times the cycles, which is why the arithmetic often favours it across ten years even though the invoice is uncomfortable on the day.

Weight matters more than owners expect on a trailer. Four flooded batteries sitting on a fifth wheel pin box is a meaningful chunk of tongue weight, and moving to lithium changes loaded balance in a useful direction. On a diesel pusher with a battery bay the size of a wardrobe, weight is irrelevant and cost per cycle decides it. Neither answer is universal, and a shop that gives every coach the same answer is selling rather than advising.

Dropping lithium batteries into a lead acid coach and calling it done is the most common mistake in this category. Lead acid charging holds an absorption voltage then drops to a float voltage indefinitely. Lithium wants a different absorption voltage, a much shorter absorption stage, and no float at all. A converter that floats a lithium bank at lead acid voltage holds it at full charge continuously, which is exactly the condition that shortens a lithium cell's working life. Every device capable of charging that bank has to be told what changed.

On a typical coach that is three or four devices: the converter or inverter charger, the alternator path if the chassis charges the house bank, the solar controller, and any separate shore side charger. Some are configurable with a dip switch or a remote panel and some are not and have to be replaced, which is a large part of why the band reaches $5,500. A conversion done properly includes the settings for each device written into the job file, so the next technician does not quietly undo the work.

• Converter or inverter charger set to the cell maker absorption voltage with float disabled
• Solar controller profile changed from sealed lead acid to lithium
• Alternator charging routed through a DC to DC charger rather than a direct connection
• Battery management system communication verified where the bank supports it
• Every setting recorded in the job file and a copy left inside the battery bay

Lithium accepts charge and delivers current far faster than the wiring in most coaches was ever sized for, and that is where a conversion becomes an electrical job. Cable gets sized to the actual expected current and run length rather than to whatever the factory used for a smaller lead acid bank, terminals get crimped with a hydraulic tool and heat shrunk, and a proper bus bar replaces the stack of ring terminals that has accumulated on one post over three owners. A class T or equivalent fuse goes as close to the positive terminal as it will physically fit, because an unfused lithium bank can deliver a genuinely dangerous short circuit current.

Battery disconnects get replaced more often than they get reused. A rotary switch that has carried a hundred amps through pitted contacts for a decade is a heat source and a voltage drop, and it usually is not rated for what a new bank can push through it. It also gets labelled, along with the shunt, the fuse and the bus bar, because the next person to open that bay should not have to guess what anything does.

• Cable sized to expected current and run length, not to the factory lead acid loom
• Terminals crimped with a hydraulic tool and covered in adhesive lined heat shrink
• Class T or equivalent fuse mounted at the positive terminal
• Bus bar fitted in place of stacked ring terminals on one post
• Disconnect, shunt, fuse and bus bar labelled before the bay is closed

The four light panel on the wall reads voltage and calls it a fuel gauge, which is roughly as useful as judging a fuel tank by engine noise. A shunt based monitor sits in the negative path and counts amp hours in and out, so it reports state of charge, present draw, time remaining and charge history. Fitting one is a few hours of work and it is the most valuable single addition to a house electrical system, because it converts every future argument about batteries into a number somebody can read.

It also finds problems on its own. A monitor showing a steady draw with everything switched off has located a parasitic load without anyone clamping a meter. A monitor showing the bank never reaching full absorption has found a charging fault. Owners who fit one commonly discover the coach was never charging fully in the first place, which is unwelcome news and considerably cheaper to learn now than after a second bank has died.

Lithium iron phosphate cells must not be charged below zero degrees Celsius, and a battery management system enforces that by opening the charge path. Owners read about it and worry, and along this stretch of coast it is close to a non issue: a bank inside a heated coach or in an insulated basement essentially never gets that cold. It matters if the coach is stored in the mountains, taken to the Sierra in winter, or has its bank mounted in an exposed underbelly where outside air circulates around it all night.

Where it does apply, the answers are a bank with internal cell heating, a heated enclosure, or a charge path interlocked to a temperature sensor. What is not acceptable is leaving it to chance and hoping. A cell charged cold does not fail visibly, it just loses capacity permanently, and the owner finds out a season later with no way to prove what happened or when.

Here is the case against the sale. A coach that spends its life on a shore pedestal at a park never discharges its bank deeply, never needs the extra usable capacity, and never benefits from a faster charge. Its lead acid batteries will last a long time precisely because they are held full and warm. Converting that coach buys a lighter vehicle and a nicer monitor and very little else, for a number that starts mid band and climbs once every charging device is counted.

A conversion earns its money on a rig that dry camps, runs a residential refrigerator through an inverter, or carries a heavy bank on a trailer tongue where weight changes how it tows. If a coach does not fit one of those descriptions, we will say so, quote the AGM replacement, and put the lithium quote alongside it so the decision belongs to the owner with both numbers visible on one page.