This is pretty boring. I can’t recommend reading it.
We try to be self-sufficient at anchor with Bright Water. We have lots of storage, a pretty good refrigerator and freezer, and we’re happy to eat foods that store well.
But we need electrical power to run the refrigeration and lights and computers and electronics, and we need fresh water to drink, cook, and wash with.
Almost everything electrical on Bright Water runs on 12 volts DC (direct current). It’s the same voltage as your car. The low voltage requires high current to run high power devices like the windlass, but it’s what North America has chosen as the standard. European boats typically run 24 volt systems, have smaller (cheaper) wire, and less resistance loss in the wiring. They also have trouble repairing and replacing their electronics when they fail.
We draw all our electrical power from the battery bank. We use two 270 A-Hr AGM batteries from Mastervolt. These are absolutely top-of-the-line batteries using the standard lead-acid chemistry found under the hood of almost every car made. However, the acid in these batteries is trapped in Absorbed Glass Mats (hence the AGM thing). The batteries don’t leak acid if tipped or cracked.
We’ve covered this before (https://svbrightwater.wordpress.com/2012/09/15/power-for-the-gadgets/), but the best, quickest way to charge our batteries is to run the motor. Our huge alternator will completely charge the batteries almost every time we move the boat. Unfortunately the engine is noisy and hot, so we try and live off solar power at anchor. Solar is quiet and clean and practically useless compared to burning a few dinosaurs.
The key to using solar power is to be smart about power management. The first priority is to keep the batteries charged. Charged batteries are happy batteries and are much, much less likely to fail. Here is a great explanation of battery charging: West Marine Website.
In this graph from the West Marine site above, you can clearly see the three stages of proper lead-acid battery charging. You jam whatever you’ve got into the batteries until their internal (charged) voltage pushes back and you reach, in our case, 14.4 V. Then you hold the charge voltage at 14.4 until the battery is full (current drops to a very small amount). Then you drop the voltage further to maintain, or float, the full charge. Battery makers differ on the details of each voltage value, but the principal is always the same. If you exceed the Acceptance or Float Voltages, you will boil out a flooded battery and create bubbles inside AGM or Gel batteries. The flooded battery can be periodically refilled with water if the plates aren’t exposed, but AGM and Gel batteries are damaged and eventually ruined by overcharging.
So the key to living off solar power is to let the batteries charge until they reach the acceptance phase of the charge. At that point the solar panels are putting out excess power that the battery can’t absorb. Anything we run after that runs at least partially for free – using power that would otherwise be wasted – so this is when we run our watermaker. The watermaker “strains” the salt out of seawater. It runs at high pressure (600 psi at the membrane) and the pump uses a lot of electricity.
It’s difficult to see, but our solar panels (top controller) are putting out 305W (awesome for a 400W panel set, and not typical or sustainable). They’re making 22.9A @ 13.2V. On the battery monitor below, the batteries are at acceptance stage and are absorbing 5.4A @ 13.4V. The missing 17.6A are running the watermaker and this computer and whatever else is on. More power will flow to the batteries when we turn off the watermaker, but they’re in pretty good shape.
We try and keep the batteries between 60 and 85% full with solar power, then top them off when we run the motor when we move. We use an electronic battery monitor to determine our state-of-charge. Monitoring the battery voltage only works if there’s been no load for 12 hours. The definitive way to measure charge level is to check the Specific Gravity (density) of the acid in each cell in the battery, but that only works with flooded, non-sealed batteries like fork lift batteries.
Sitting here in La Paz without moving, we need to run the motor for an hour every week or so to top the batteries off.
Incidentally, the multi-voltage charging regime is why LED lights were such a disappointment when they were first introduced. Old LED’s would run for the promised 50,000 hours at 12 volts. But at the higher charging voltages, they only ran for a few hundred hours. Modern LED bulbs and fixtures have internal voltage regulators and will run on anything from 10 to 24 or 48 volts, last for a very long time, and put out tons of light.
Wikipedia has a good discussion of Lead-Acid design and chemistry http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery, but the basic reaction is:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l), just so you know.
Actual Data for January in La Paz, tilting the panels to try and catch the sun:
Day/Whr – 1/1380, 2/1850, 3/1740, 4/1040, 5/1080, 6/1710, 7/1460. This was for a fairly sunny week, and we increased water stored and battery charge during the week. We have 4ea. 100 Watt panels, approximately 26 square feet total area.