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I need help on my design which includes a wind turbine, charge controller, battery and load.

The question is, what will happen to the load if the wind turbine is charging the battery and while it is not charging the battery? My load is a Raspberry Pi 3. What I mean is, when the battery is full, I know the charger controller will do its thing which cuts off current to the battery. Will the Raspberry Pi turn off? I need it to always run.

Also, if it does not turn off, wouldn't the Raspberry Pi discharge the battery while it is being charged by the turbine? Would I need a current greater than the load?

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I know it's cliche, but you could use a solar panel, there's a lot of variables on the water when it comes to wind. Specifically how are you going to manage your windmill extracting energy from the motion of your boat when you're trying to get somewhere (relative wind from the boat's driving through the air), and what are you going to do when there's a storm?

If you're concerned about system longevity, you should consider a Lithium Iron Phosphate (LiFePO4) battery which has a cycle life of 2,000 (capacity degraded to 80% of original) but it comes with the added complication of needing a Battery Management System to keep the cells balanced. Otherwise a deep cycle lead acid battery will work great but may only last a year if it's cycled every day (cycle life is around 300, again this it to 80% original capacity). Regardless of the battery choice, you'll want a low voltage indicator on your battery to protect the battery from overdischarge and give the Pi time to gracefully shutdown to prevent data corruption.

For some intuition on how a self-contained system might work, this is a site hosted from a raspberry pi running on solar/battery power. It explains some of the configuration: https://solar.lowtechmagazine.com/about.html

You're going to have to know how much power generation time is available per day (to determine the turbine size) and how long you need the Pi to run without replenishment (to determine the battery size). This site will give you an idea of Raspberry Pi power usage to help you size your battery and wind turbine/solar panel. http://www.pidramble.com/wiki/benchmarks/power-consumption

If you're looking for the ultimate green solution, you should really just wire your inverter (from wind or solar) directly into your boat's electrical system and power the pi out of a normal outlet. I know I know, that defeats the purpose, but it'll actually be more "green" because you won't lose energy in the battery charge/discharge cycle since it'll be used immediately (best case scenario you'll probably get around 80-90% energy back out of a battery) and you won't miss out on energy that is being generated because your battery is full and the Pi is only sipping a few watts.

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  • \$\begingroup\$ Hi, thank you for your reply. But i forgot to add, we're just students and we dont want to mess up a boat's electrical system. We're proposing this for our project. The only thing left for us is the conceptual framework for the wind turbine. Thank you very much! \$\endgroup\$ – rlyn09 Jul 18 '19 at 7:08
  • \$\begingroup\$ Well in that case, I think the solar argument still stands. Your project focus may be the AIS implementation from a pi, so if there's already enough work to get that up and running, it might be nice to go with a tried and true solar/battery system. \$\endgroup\$ – Kent Altobelli Jul 18 '19 at 10:55
  • \$\begingroup\$ As far as configuration goes, I'd recommend a 4 cell Lithium Iron Phosphate (LiFePO4) due to its high cycle life (better than PbA), safety and common voltage level (around 12V). Find a charge controller that can extract maximum energy from the turbine to charge the battery. Get a 12V to USB converter and also connect that to the battery terminals. Energy will flow from the turbine to charge the battery and/or run the Pi. When there's no wind the battery will continue to supply power until it's empty (detect this early since you'll want to give the Pi an opportunity to shutdown gracefully). \$\endgroup\$ – Kent Altobelli Jul 18 '19 at 12:08
  • \$\begingroup\$ Thank you. that really helps. For the 12V to USB converter, can we just use a Buck Converter instead? or is it the same? the 4 Battery will total 12V right, so we need to find a charge controller that can use a 12V input? is a 20W wind turbine enough? \$\endgroup\$ – rlyn09 Jul 18 '19 at 14:49
  • \$\begingroup\$ I don't know what your budget is, but try to find a "12 volt" LiFePO4 battery with built in protection and BMS (for example you may be able to find one that's designed for motorcycles or something). A buck converter will work great from an efficiency standpoint, but make sure it has a low voltage shutdown do it doesn't overdischarge your battery. The LiFePO4 chemistry is tolerant of overdischarge but it will degrade the battery faster, plus there's no reason to flirt with danger. \$\endgroup\$ – Kent Altobelli Jul 18 '19 at 14:54
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We can't answer your question (except in general) without knowing the specifics of your system.

In general with these kinds of systems the battery can supply the load both while charging and when there's no wind (which is the point of the battery).

If the charging current exceeds the load current then the battery will continue to charge until full. If not, then the battery will discharge at a rate equal to the load current minus the charging current.

Details depend on the load, the kind of battery, the specific charge controller, the battery capacity, the generator type and the system implementation.

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  • \$\begingroup\$ So what you mean is, the charging current needs to always be greater than the load current so that it will not discharge the battery hence turn off the raspberry pi? The load will be a raspberry pi which has an input requirement of 5V 2A. For the batteries, we dont know yet. But for the wind turbine, we saw a 200W wind turbine. Will that be okay? \$\endgroup\$ – rlyn09 Jul 17 '19 at 15:12
  • \$\begingroup\$ Also, we will use this on a fishing boat. Which means we can have at least a good amount of wind. We only worry that it will turn off the raspberry pi being used for AIS \$\endgroup\$ – rlyn09 Jul 17 '19 at 15:13
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    \$\begingroup\$ @rlyn09 No, the charging current doesn't have to be always greater than the load current, the battery will keep the raspberry pi running while there is no charge current, at least until the battery is discharged. (Which is the main reason for having the battery). So you need the total energy supplied by the generator to be more than the total energy removed by the load over a time shorter than the battery discharge time to guarantee that the raspberry pi never shuts off and that the battery never fully discharges. \$\endgroup\$ – John D Jul 17 '19 at 15:22
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    \$\begingroup\$ @rlyn09 why is using the boat electrical system “not efficient”? Relying on wind - sailors have the word “becalmed” any idea what it means? \$\endgroup\$ – Solar Mike Jul 17 '19 at 16:12
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    \$\begingroup\$ It would be more efficient to use an inverter to put the turbine power directly into the boat's electrical system (as long as the boat will always be able to consume the turbine's output to prevent reverse current INTO the generator). This way you avoid potential power loss to your isolated battery system, avoid the conversion loses charging and discharging the battery and use the full turbine output. Another thing to consider is that when you're driving the boat, the turbine will try to extract energy from the relative wind generated by your motor/propellor... Lots of inefficiency there. \$\endgroup\$ – Kent Altobelli Jul 17 '19 at 20:42
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Your load must be smart enough to go to sleep so when the Under Voltage is detected , the battery is locked out from drain. This is usually called UVLO and you may choose to create an early warning threshold should your design choose to increase your broadcast interval.

You must get smart enough to design your load demand to always be less than supply including wind power, efficiency losses, battery energy capacity, converter losses, aging and calm spells in a 100 year weather cycle.

So you need an accurate power, energy demand profile then an adequate battery energy capacity for the current range in use, then an adequate MPPT impedance matching converter for maximizing wind-power capture and to battery charge manager independent of load current.

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