How to Set Up Solar Charging for a Lithium Bank

Solar is sized from the worst month you cruise in, not the average one. An array that covers an annual mean will leave a boat short every winter, which is when the deficit is least convenient and hardest to fix.

How do I work out how much solar I need?

Divide your daily energy use in watt-hours by the peak sun-hours available in your cruising ground in the worst month, then divide again by system efficiency to allow for controller losses, temperature, orientation and wiring. The result is array watts. Using winter sun-hours rather than an annual figure is the single decision that most changes the answer — in southern Australia the winter array can be several times the summer one. The solar sizing calculator includes regional sun-hours.

How do I find the sun-hours for my cruising ground?

Take peak sun-hours from Bureau of Meteorology solar exposure data for the region and month you care about, rather than from a national average. Australia spans a very wide range: a boat wintering in the tropics and a boat wintering in Tasmania are solving different problems with the same equipment. If you cruise seasonally between regions, size for the worst place you will be without shore power. Southern Australia winter boat lithium and tropical Australia marine lithium cover the two extremes.

How do I choose between rigid, semi-flexible and walk-on panels?

Choose by where the panel will be mounted and what will stand on it, then accept the efficiency and lifespan that follow. Rigid framed panels on a gantry or arch are the most efficient and longest-lived because they are stiff and air can circulate behind them. Semi-flexible panels bonded to a coachroof are lower profile but run hotter and generally have shorter service lives. Walk-on panels trade output for deck space. Best marine solar panels in Australia compares the types.

How do I size an MPPT controller for my array?

Size the MPPT controller on two limits at once: the array's open-circuit voltage at the coldest temperature you will see, which must stay inside the controller's maximum input voltage, and the controller's output current, which must cover the array's output at your battery voltage. Panels produce higher voltage when cold, so the voltage limit is checked at cold, not at typical. MPPT solar controllers and best MPPT controller for a boat cover the selection.

How do I configure an MPPT controller for LiFePO4?

Select the LiFePO4 or user-defined profile and enter the battery manufacturer's absorption and float voltages exactly, then disable any equalisation or reconditioning stage. Lead-acid default profiles apply voltages and timings that do not suit LiFePO4 and can leave the BMS doing the limiting. If the controller supports a battery temperature sensor, fit it. Record the settings alongside the rest of the system documentation so the next person does not have to guess.

How do I wire panels in series or parallel on a boat?

Wire in series to raise voltage and reduce current in the cable run, and in parallel to keep partial shading on one panel from dragging the rest down. Boats shade unpredictably from masts, booms, rigging and sails, which usually favours parallel strings or separate controllers over one long series string. Whichever you choose, check the resulting open-circuit voltage against the controller's cold-weather limit. Solar panels for boats covers the layouts.

How do I deal with shading from the mast and rigging?

Reduce the electrical consequence of shade rather than trying to eliminate the shade, because a sailing rig will always shade something. Splitting the array across multiple MPPT controllers, or into parallel strings, limits the loss to the shaded panel rather than the whole array. Mounting panels on a stern arch or gantry clear of the boom is the other half of the answer. Expect real-world output below the nameplate figure as a matter of course. Solar panel sizing allows for it.

How do I run more than one solar controller into one lithium bank?

Configure each controller with identical charge voltages and connect each to the bank through its own fuse and cable, sized for that controller's output. Multiple controllers are common and work well, because each tracks its own array's maximum power point independently. The failure mode to avoid is mismatched settings, where one controller holds the bank at a voltage another has already finished at. Multiple charge sources covers coordinating them with the alternator and shore charger.

How do I check my solar is actually producing what it should?

Compare controller output in watt-hours per day against the array's rating multiplied by that day's sun-hours, using the controller's own logging or a shunt. A large gap points at shading, soiling, a hot panel, a wiring loss or a controller in the wrong profile. Measure on a clear day so the comparison means something. A battery monitor with a shunt lets you see generation and consumption on the same scale.

How do I size cable between the panels and the controller?

Size the cable from the array's short-circuit current, the run length and a voltage-drop target, exactly as you would any other DC run. Losses in the panel-to-controller run are energy that never reaches the bank, so a run that would be acceptable on a lightly loaded circuit can be expensive here. Marine-grade tinned copper, correctly terminated and protected from chafe, is the baseline. Run the figures in the cable size calculator.

How do I fuse a solar array on a boat?

Fuse each string at the combiner and fuse the controller's battery output close to the battery, sizing each to protect the cable it feeds. The battery-side fuse matters most, because that conductor is connected to a source that can deliver very high fault current. Fit a means of isolating the array as well as the battery, so the controller can be worked on safely. Marine battery fuses and breakers covers the selection.

How do I combine solar with an alternator and shore power?

Set all three to the same charge voltages and let the bank take what it needs from whichever source is available. Solar typically does the daily work at anchor, the alternator provides bulk charge when motoring, and shore power finishes and holds the bank in a marina. The only coordination required is agreement on the target voltages and confidence that combined current cannot exceed the battery's maximum charge rate. Shore power lithium charging and inverter chargers cover the AC side.

How do I decide between more solar and a bigger battery?

Add solar when the deficit is in generation and battery when the deficit is in overnight or bad-weather reserve, and tell the two apart with a shunt rather than by impression. A bank that reaches full most days but runs low overnight needs capacity; a bank that never reaches full needs generation. Adding capacity to a generation-limited system makes the problem worse, because there is more to fill. Solar to battery ratio for a liveaboard works through the balance.

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