Monday, January 24, 2011

The Other Use For Solar Panels

Boot Key Harbor, Marathon, FL
34 42.54 N 081 05.58 W

I seemed to be confessing a lot of mismanagement lately. Oh well, c`est la vie.

Bob Williams of Salt Technologies is a local businessman here in Marathon. He has been presenting a number of free seminars on marine topics which are well attended and much appreciated. Two weeks ago, I went to Bob's seminar on solar and wind technology. There I learned something that totally changed the way I manage my batteries, and with great success.

First some background. We have a small 50 watt solar panel on Tarwathie. We also have two deep discharge batteries on board. Both are the biggest I could find, group 31 size, and rated 100 to 125 amp-hours each. There is no more room in our double ended boat for more batteries, so we have to live with just two.

I confess that this is the first year, out of 6 on board, that we have not bought new batteries. In every case, the batteries went bad weeks after arriving in Florida for the winter.

I know that 2-3 years seems to be the minimal life, and that many cruisers go 5-6 years before changing batteries. Somehow though, it didn't work at all for me. I might have investigated deeper except that West Marine provided me with replacement batteries free of charge three times. That insulated us from a lot of the financial consequences (e.g. $400-$500 per year). Money aside the question remained; what was I doing wrong?

Here's what Bob said at the seminar. “Lead acid batteries get sulphated and lose much of their capacity. Sulphation is caused by not charging the batteries fully. You may charge your batteries to 14 volts, but to fully charge it, you need to run your generator 7-8 hours more at only 1-2 amps charging current. Most people don't do that. The nice thing about solar panels is that they can provide that low charging current all day and thus prevent sulphation.” WOW! I didn't know that what caused sulphation, nor had I ever thought of using the solar panels that way.

At the time I went to the seminar, I was on the verge of buying new batteries for the 6th time. They had degraded so much that I could charge them up to 14.2 volts, and 4 hours and 10 amp-hours later they would be down to 11.5 volts. I had to run my Honda generator 3 times per day. I suspected sulphation, but neither my alternator, nor my shore power controller had the so-called equalizing cycle that helps desulphate the batteries. I looked around on shore but I never found anyone who could desulphate them for me. I was ready to throw in the towel.

Bob's talk turned me around. I had been managing my solar charge backward. My practice was to let the solar panel generate whatever was possible during the day, then to run my generator before sunset to finish the job, and (hopefully) give us enough juice to get through the night with the refrigerator/freezer running. Using the computer, such as to watch a movie online, was such a big drain that we would either have to run the generator again at night (not nice) or shut off the fridge for the night (also not nice).

Since the panel was so small, the difference between a sunny day and a cloudy day was that I might have to run the generator 80 minutes instead of 90. The contribution of the panel was so small, that I lost interest in maintaining it and operating it. The wiring was old and I suspected might have parasitic resistance. I had a charge controller. It prevents overcharge by cutting the solar current off if the voltage reaches 13.8. But the controller it never did anything because the battery never got close to 13.8 during the sunny part of the day.  Basically, I thought the small solar panel was useless and I lusted after 240 watts of solar capacity.

After hearing Bob's talk, I changed everything around:

  1. I put in brand new wiring to make sure that the solar panel worked at peak efficiency.
  2. I wired in a DPDT (double-pole-double-throw) toggle switch. Now I have off-on-controlled modes of operation.  In the on mode, the charge controller is bypassed.
  3. I took more interest in the solar panel and adjusted the tilt to point at the sun 3-4 times during the day.
  4. I ran the Honda generator in the morning until the battery voltage reached 14-14.2 volts. Then I allowed the panel to generate 2.5 amps through the day with the charge controller switched off.

The results were immediate and dramatic. On the first day, the solar panel brought the battery voltage up to 15 volts for two hours. On the second day I did the same. Since then, battery performance has been like new. Instead of 4-6 hours (10 amp-hours) between generator runs, I can now go 24-36 hours (40-60 amp-hours). WOW! That's a huge improvement.

So here's my new policy.

  1. Run the generator in the morning.
  2. 6 days per week, leave the charge controller in the circuit.
  3. Equalize: one sunny day per week, remove the charge controller from the circuit an allow the batteries to charge to 15 or more volts for at least one hour. That's my equalizing cycle.  I also monitor the battery temperature to make sure I don't boil the water.  So far, the temperature never exceeded 80F at 15 volts overcharge.

So far, things are much better. Thank you Bob Williams.  Who says you can't teach an old dog new tricks?


  1. I feel that a pair of 220 amp/hr 6V deep cycle golf cart batteries would work well when it comes time to replace. A pair connected in series would give the 220 amp/hr @ 12V that you have now. There are some advantages to the 6V choice and the current Sam's Club version is about $90 each. I've used a pair aboard my sailboat and also many pairs at my "off the grid" home for years.
    I plan to take some of what you posted and use it with my batteries.


  2. Thanks, Dick. I've copied this and sent it to friends who have been struggling with their batteries. Hope this helps them too!

  3. Awesome ! i really found very informative article here and bookmarked this blog. Thank energy


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