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New River, Arizona
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I know there are other ways to do this, but I want to use an arduino!!  The project: a battery charger to keep 4 vehicle batteries topped off during periods of inactivity (months).  To make it harder, there is no power in my barn where the vehicles are stored and it would cost me about US$1800 to have a trench dug and power run.  So, I was thinking about a solar cell or two hooked to an arduino that routes power to the batteries based on their state of charge.  I have a good handle on the methods of charging the batteries and have done several complex projects with arduinos, but now I've got the additional problems of the arduinos not having power for long periods and having to wake up properly then do something that could be interrupted by clouds.  All while running unattended in a barn that actually has rats from time to time.

I don't understand things like keeping the arduino off line until the solar cells provide enough power to run it.  How will I know if there is enough power being supplied to actually float the batteries at 13.5V?  I can measure the voltage easily enough but what happens as it drops late in the day?  That's the kind of stuff I'm wondering about.  I'm not in a hurry to start this project because I have a couple of projects going right now, however I'll be ready to start in a couple of months.  I live in Arizona, USA so sun is not a concern.

Thoughts?
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Trying to keep my house under control http://www.desert-home.com/

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DCon takes care of the rats.

The power consumed by the arduino is going to be minimal compared to the capacity of automotive batteries. Just run off of one of the batteries.

They make solar powered battery minders. Why not just get one for each vehicle.
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New River, Arizona
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I have a lot of rats; they tunnel in and set up shop.  After they empty the dcon trays they use them for building material....sigh.

The main reason I want to control it with an arduino is just fun and bragging rights.  However there is also the advantage that I can use a 100W solar cell and control the current such that the batteries get a regular charge up to the point where a finishing float will carry them.  This will allow a battery that has been pulled down a charge, then maintenance without boiling out the electrolyte. 

Think about it, all those charge points they talk about where you charge at a percentage of the battery's capacity, then lower it to finish and ending with a float to maintain are just made for a computer to monitor.  Periods where the charge is constant current and others where the charge is constant voltage.  Since the times are measured in hours and days, processor speed isn't a problem.  Sure, you may be controlling more than 20A at times, but the robot folks do that all the time.

It could take months to debug something like this with a long time frame though.
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Trying to keep my house under control http://www.desert-home.com/

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