lördag 13 juli 2013

Off-grid charging

Finally we are here! We arrived late last night after a 1 hour and 45 minutes drive from Söderöra. Everything went smooth as a whistle. The boat was packed with supplies but there was only a small increase in energy used during the trip.


Since the trip is one hour longer than what my total battery capacity would count for I had to use the diesel. Another thing to keep in mind is that there is no electric grid here. So how do we charge?

I gave the charging issue some thought this winter and I came up with the following solutions:

A petrol/gasol generator would probably be the quickest way to charge. The downside is noise, local pollution and complexity. I don’t like the idea of burning fuel in order to create electricity.


By using solar panels electricity can be harvest from the sunlight. No moving parts and no noice. The downside is the efficiency of the panels and the dependency of cloud free weather. To get a usable charge current I would have to use about 3-4 square meters of cells, given that there is no clouds.

Using the wind to harvest electricity is common in these areas. Modern wind generators are efficient with a moderate wind force demand. What you need is a generator, small tower and an extension cord.
And the winner is…. The wind generator! Given that we normally have a couple of days with winds around 8-10ms I should be able to get some energy into my batteries. Since I run a 48 volt system I need a 48 volt wind generator. Fortunately Southwest Windpower makes a small, 120cm diameter, generator with an integrated controller. The controller makes sure that the batteries receive the correct voltage while charging.

How long will it take to charge the pack? Well that depends on how strong the wind is. The generator can deliver max 8 Amps. Let's use half of that rating in the equation. My battery pack has a capacity of 60 Amp hours and I used half of that capacity to get to the island. That means that I will have to generate 30 Amp hours in order to have a full pack again. 30 divided by 4 equals 7,5. In 7,5 hours the battery would be fully charged if the generator delivers 4 Amps continuously for 7,5 hours. In real life the numbers will differ but as long as there is wind the generator will run 24 hours a day. 
Right now the battery pack is a 50,6% charge and we have a 5-6m/s wind that should be picking up during the day. I will get back with the charging result tonight, stay tuned!

The wind generator has been working since 06:00 this morning. The wind reached its peak, 14 m/s, around 14:00. As you all know we started at 50,6% charge and during the course of the day the state of charge has increased to 84,0% (22:30).

During peak hours the generator generated as much as 395 watts (7,4 Amps * 53,5 volts). 

During the day the generator has been adding almost 1,5 Kwh to the battery pack. A battery pack full of wind power, does that mean that I'm sailing?
Have a nice vacation, I know I will.
See you soon.

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