Gypsy Cruising

Solar Panel Upgrade

Short description: increase solar panel capacity from 300 watts to 500 watts

Our cruising freedom is limited by the supply of basic consumables, essentially food, water, fuel and electrical power.  These can be replenished or managed whilst at sea to varying degrees:

• food supplies can be supplemented by catching fish, and some cruisers are very effective at doing this
• reverse osmosis water makers can be installed, ensuring a virtually unlimited supply of water (at a cost)
• diesel consumption can be minimised by not running the engine simply to generate power and by using sail power between anchorages
• electrical power can be generated by solar panels or wind turbines.

I've long been an enthusiastic proponent of solar panels.  They're cheap, quiet, unobtrusive and in Australia's sunny environment they're very effective at pumping out power from dawn to dusk.  I originally installed a pair of 150 watt panels on Gypsy Princess' bimini shortly after I bought her.  I went for rigid panels, partly because I'd heard several stories of soft panels breaking down and partly because fixed panels are cheaper and easy to work with.  It was a relatively simple matter of bolting some rectangular cross section aluminium tubes onto the panels and then mounting those "legs" onto the forward and aft cross beams of the bimini.  The two panels fit side by side running fore and aft in the middle of the boat. I wired them in parallel and ran a single twin sheathed 6mm cable down the bimini frame and into the hull, from where it led to an MPPT solar controller next to the batteries.

I always advocate using MPPT controllers.  Panels normally generate power at voltages in excess of the 14v or so needed to charge the batteries.  Other types of controllers (PWM) waste power by stepping the voltage down whilst leaving the output current unchanged.  MPPT controllers will step the voltage down but will use that excess voltage to increase the current, so conserving the total available power.

I've found that the combined 300 watts of panels have been sufficient, in summer months, to keep the batteries fully charged through multi-week cruises.  That's been enough to run the fridge and freezer, navigation equipment, lights and TV (limit of one DVD movie per night).  However, I decided an upgrade was necessary for the cruise north.  For one thing, the cruise will be during winter and early spring and the sun won't be as high in the sky, even in the tropics, as it is in Sydney summers.  Since the panels lie essentially horizontally on the bimini, the lower sun's angle will cut the panels' efficiency.  Secondly, I've been adding some significant power drawing equipment.  The auto helm can draw up to 9 or 10 amps while running, particularly in a lumpy sea, and the addition of a 2000W inverter will be an inducement to suck up more power from time to time.  Finally, I wanted to minimise the general risk that power could become a problem.  Better to have too much power and not have to worry about it.

So, an additional pair of 100 watt panels seemed to be about right.  They fit on the bimini either side of the existing panels and they're somewhat cheaper and lighter than getting another pair of 150s.  The decision to go smaller created some limitations in the wiring decisions later on, but more on that below.

In any case, the physical installation proceeded easily enough (if slowly, but that's just me).  Each panel bolted directly to the side of its inboard 150W panel neighbour, and I then added a further aluminium tube to the outboard edge of the new panel which was mounted on the bimini frames as for the original panels.  I left the wiring of the 150 watt panels unchanged, and replicated their arrangement for the 100 watt panels, so ending up with two independent wiring harnesses.  I bought a second MPPT controller and connected that to the wiring from the new 100 watt panels.  Both controllers were then wired to the batteries in parallel.

This was where some issues arose, and still remain unresolved.  To test the new system I ran the batteries down to about a 70% charge.  This is lower than I ever permit my batteries to go but it was acceptable for this isolated case.  At that level, even in mid winter, the two controllers both pumped out great levels of power, as efficiently as I think is reasonable to expect.  However, as the charge level approached 90% the new controller attached to the pair of 100w panels began to back off and eventually stopped charging altogether.  I can only assume that it's confused into thinking by the other controller that the batteries are fully charged and it therefore switches into a float charge state.  This is rather disappointing, since I didn't buy the new panels and controller to sit there doing nothing while the batteries are still only 85% to 90% charged.

I began to explore the possibility of rewiring the system so that each 100w panel would be wired in series with one 150w panel, with the two resulting assemblies then being wired in parallel back to a single controller.  That single controller wouldn't be confused by having another controller getting in the way.  I intended that this would increase the voltage while keeping the current within the controller's limits, and delivering maximum available power.  However I discovered that solar panels are built to permit a maximum current to flow through them and that the current limit of my new 100w panels was 5.6 amp as against 8 amp for my original 150s.  So, by wiring a 100 and 150 watt panel in parallel, I'd be reducing the maximum current from my existing 150s to 5.6 amp, effectively turning them into 104 watt panels.  This could be acceptable if the height of the sun was such that the panels were delivering less than about 65-70% of their maximum efficiency, but I don't think this is going to keep me happy.

So, for the time being, I'm living with the original design, and I'll have to moniter performance.  At the very least, regardless of the battery charge state, both controllers will pump out their maximum power if any appreciable load is reducing the batteries' voltage.  This remains a much improved situation from the original setup and it might turn out to be quite acceptable in practice.  I might yet try fiddling with the controllers' charge settings to see if I can trick them into working a bit harder, but I'm not really too hopeful this will work.

So, looking back, my issue could have been resolved if I'd either bought two new 150 watt panels (which wouldn't have fit anyway), or by buying two new controllers that had the capacity to interface with each other and coordinate their charging.