Shore Power, Solar and Generator: Building a Three-Source Charging System

Most people build a charging system by accretion. The rig came with a converter. Then solar got added, with its own controller and its own idea of what the battery wants. Then a DC-DC charger went in because the alternator wasn’t keeping up. Then a generator arrived, plugged into shore power, and nobody checked what it was actually charging through.

The result usually works, in the sense that the battery is fuller in the evening than it was in the morning. But it works by accident, and it typically has one of three problems: the sources disagree about charge voltage, one of them is throttled to a fraction of its capacity by something upstream, or the alternator is being asked to do something it was never rated for.

Building this deliberately is not hard. It takes one governing decision and about six hours of work.

The governing principle: the battery sets the spec

Every charge source must be configured to the same profile, and that profile comes from the battery datasheet. Not from the converter’s defaults, not from the charge controller’s factory preset, and not from what worked on your last rig.

Write these four numbers down before touching anything:

• Bulk/absorption voltage — typically 14.2–14.6V for a 12V LiFePO4 bank, 14.4–14.8V for AGM, and varying for flooded lead-acid.
• Float voltage — around 13.6V for LiFePO4, higher for lead chemistries.
• Maximum charge current — usually expressed as a C-rate. A 100Ah LiFePO4 rated at 0.5C accepts 50A.
• Low-temperature charge cutoff — the temperature below which the BMS blocks charging entirely, almost always 0°C.

Every source you install gets set to those numbers. Where a source cannot be configured to them — and a lot of older converters cannot — that source gets replaced, not tolerated. A charger stuck on a lead-acid profile will underfill a lithium bank indefinitely, and the owner will conclude the battery is faulty.

The full costed lithium retrofit walkthrough covers the converter replacement in detail, because it is the single most commonly skipped line item in a lithium upgrade.

The three sources are not interchangeable

They have different jobs, different economics and different failure modes. Designing as though they are three flavours of the same thing is where most of the money gets wasted.

Note the last row. The generator does not charge your battery. The generator runs your converter, and the converter’s amp rating is the ceiling on your generator recharge rate, regardless of how many watts the generator can produce. This is the most consequential misunderstanding in the whole subject and step five deals with it.

Step 1 — Audit what you already have

Before buying anything, establish four facts:

1. What converter is fitted, and what profile does it run? It will be a labelled box, often under a dinette seat or in a basement compartment. Note the model number and rated amps — 45A and 55A are the common OEM sizes.
2. What is the alternator rated at, and is it a smart alternator? Vehicles built from roughly 2015 onward, particularly European-designed vans, very often have variable-voltage alternators that drop output to around 12.4–13.0V once the starter battery is full. A small sensor module clamped to the starter battery’s negative terminal is the usual tell.
3. What charge controller is on the solar, and can it be reconfigured? See the MPPT versus PWM comparison if you are also deciding whether the existing controller is worth keeping.
4. Is there a battery monitor, and is every charge and load conductor passing through its shunt? If anything bypasses the shunt, every state-of-charge reading you take from here on is fiction.

Step 2 — Solar, sized as baseload

Solar is the only source with a marginal cost of zero, so it should carry as much of the daily load as the roof allows. Size the array to cover average daily consumption in your worst realistic month, not your best — that single decision determines how often the other two sources have to run.

Configuration points that matter for a multi-source system:

• Set the controller’s absorption and float voltages to the battery’s numbers, and set the absorption time short for lithium. Lithium does not need a long absorption phase, and leaving a lead-acid default of four hours in place holds the bank at elevated voltage for no benefit.
• If the controller supports it, enable the low-temperature charge cutoff and give it a battery temperature sensor. The BMS will block cold charging anyway, but a controller that knows why is a controller that reports usefully instead of throwing errors.
• Fuse the array conductors at the controller, and size the wire for the run length rather than the current alone — voltage drop over a long roof-to-basement run eats more harvest than most people expect.

Step 3 — Alternator charging, sized to the alternator

This is where systems get damaged. A lithium bank has very low internal resistance and will accept current as fast as anything will supply it. Wire it directly to an alternator and it will pull that alternator to its thermal limit and hold it there — at idle, in traffic, in August, with no airflow. Alternators are rated for intermittent peak output, not sustained maximum, and the repair bill is measured in hundreds.

A DC-DC charger solves three problems at once: it limits current to a fixed ceiling, it delivers a correct lithium charge profile regardless of what the alternator is doing, and it boosts the low output voltage of a smart alternator up to something useful.

Sizing rule: never take more than half of the alternator’s spare capacity.

Spare capacity = alternator rating - vehicle's own continuous load
DC-DC ceiling  = spare capacity x 0.5

Worked example: a 150A alternator on a vehicle drawing 60A for ignition, lights, blower and accessories has 90A spare. Half of that is 45A, so a 40A DC-DC charger is comfortable and a 60A unit is not.

Two further corrections most sizing guides omit:

• The alternator supplies more current than the charger delivers. A 40A charger putting 40A into a battery at 14.4V is moving 576W. At around 93% efficiency that is 619W drawn from the input, and if a smart alternator is holding 12.4V, that is 50A off the alternator — not 40A. Size against the input figure.
• DC-DC chargers derate with heat. A 40A unit in a sealed engine bay at 50°C may deliver 20A. If it must live somewhere hot, buy one size up and run it below its rating rather than buying to the number on the box.

Wiring: use pure copper, not copper-clad aluminium, and fuse both ends of the run — one fuse within a few inches of the starter battery, one within a few inches of the house bank. The long cable between them is energised from both directions and needs protecting from both.

Step 4 — Shore power, and the case for consolidation

You have two architectures available.

Converter plus separate inverter. The converter charges from shore; the inverter makes 120V from the battery. Cheap, simple, and two boxes that never talk to each other. It is the right answer when the existing converter is modern and lithium-capable and the inverter needs are small.

Inverter/charger as a single unit. One device does both, with a transfer switch inside it that automatically hands over between shore power and battery. It costs considerably more, but it buys three things worth having: a single configurable charge profile, pass-through charging while loads run, and — the underrated one — power assist, where the unit supplements a limited shore or generator supply from the battery so a 15A pedestal can run a 2,000W load without tripping.

If you are rewiring anyway, consolidate. If you are not, do not tear out a working converter for the sake of tidiness.

Either way: set the charge voltages to the battery’s numbers, and if the unit has a lithium preset, verify what that preset actually contains rather than trusting the label.

Step 5 — The generator, sized to the converter

Here is the arithmetic nobody runs before buying.

A generator plugs into the shore inlet. Everything it charges, it charges through the converter or inverter/charger. So the recharge rate is:

Recharge rate = converter amps (not generator watts)
Generator load = converter amps x 14.4V / efficiency

A 55A converter charging at 14.4V is drawing about 790W from the generator, plus perhaps 15% for conversion losses — call it 900W. Add a 400W load elsewhere in the rig and you are at 1,300W.

A 2,000W inverter generator is therefore ample for that system, and a 3,500W unit is dead weight you carry, fuel and maintain for no benefit. People routinely buy the larger unit because they are sizing against the generator’s output rather than against the converter’s intake. If you want faster generator recharge, upgrade the converter — not the generator.

The corollary matters too: recharging a 400Ah lithium bank from 20% through a 55A converter takes roughly six hours of runtime. If that is unacceptable, the answer is more solar or a bigger converter, not more generator.

Sizing, fuel cost per hour, and the carbon monoxide question are covered properly in the inverter generator guide. Read the CO section before you decide where the generator lives. Exhaust from a generator running near a vent, window or open compartment kills people every year, and it is the one part of this build where there is no acceptable margin for improvisation.

Step 6 — Making the three coexist

The good news: they need no coordination logic. Charge sources in parallel on a common bus simply share according to voltage — whichever source is holding the highest voltage supplies the current, and the others taper. There is no arbitration to build, no priority controller to buy, and no risk of them fighting, provided their voltage setpoints match.

That proviso is the entire job. Mismatched setpoints are what produce the classic symptom of a battery that never quite reaches full: the solar controller is set to 14.4V, the converter to 13.6V, and the converter is effectively holding the bus down and starving the solar of the voltage headroom it needs to finish the absorption phase.

Physical build points:

• One common negative bus bar and one common positive bus bar. Everything lands there. No daisy-chaining charge sources off each other’s terminals.
• The shunt goes between the battery negative and the common negative bus, with nothing else on the battery side of it. Every amp in and out passes through it or your state of charge is guesswork.
• Every source gets its own appropriately rated fuse at the bus end, sized to the conductor.
• Label everything. Six months from now, at night, in the rain, you will want to know which of five identical black cables is the alternator feed.

Step 7 — Commission it properly

Do not declare victory when the lights come on. Verify:

1. Each source in isolation. Disable the other two, run one, and confirm with a clamp meter that the current at the battery matches what the source reports. A source reporting 40A while the clamp reads 22A means voltage drop in the cable.
2. Terminal voltage at full charge. Measure at the battery posts, not at the charger. A 0.6V difference between charger output and battery terminals is common on undersized cable and causes the charger to taper early.
3. All three together. Confirm no source is being pushed into an error state and that total current stays under the battery’s maximum charge rate.
4. Cold behaviour. If the bank will see freezing temperatures, confirm the BMS blocks charge and that each source handles the block gracefully rather than faulting.

Photograph the finished installation, keep the receipts and the wiring diagram, and store them somewhere that is not the vehicle. Substantial owner-installed electrical work is one of the things that turns a routine claim into a disputed one, and standard vehicle policies handle custom conversions badly at the best of times. Documentation is free insurance for your insurance.

Symptoms, and what each one points at

The advantage of a deliberately built system is that faults become diagnosable rather than mysterious. Common symptoms and their usual cause:

The last two are the ones people spend money on incorrectly — buying a bigger generator when the converter is the constraint, or replacing a battery monitor that is reporting accurately on incomplete data.

What good looks like

A correctly built three-source system has one profile, one bus, one shunt and one set of numbers. Solar covers the day. The alternator tops up in transit. Shore power handles bulk recovery when it is available, and the generator exists for the week in November when neither of the other two is delivering.

If you find yourself running the generator in summer, the array is undersized. If you find yourself arriving at camp with a flat bank after four hours of driving, the DC-DC charger is undersized or absent. Each symptom points at exactly one component, which is the practical benefit of building it deliberately instead of by accretion.