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snake eating it's tail?
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Please be positive and constructive with your questions and comments.

snake eating it's tail?

by azs on Tue Mar 03, 2009 10:27 pm

During the day I want to charge a battery with wind, during the night I want to run an 3.3v arduino off the same battery. I have a 3.6v nicad battery pack, a motor, a photoresistor and I plan to get a relay, once I understand what to do. I was told not to use a battery and charge it at the same time. This might mean that I will need two relays, so that the photocell can trigger one to disconnect the battery from the motor and the other one to connect the battery to the microcontroller?

But more importantly, if the battery can't be used and charged at the same time, how do I send voltage to the photo resistor in the first place?

I am hoping that someone will tell me that I can use and charge the battery at the same time and that I can continue to charge the battery during the night, while the arduino is running.

Thanks
azs
 
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Re: snake eating it's tail?

by zener on Wed Mar 04, 2009 3:37 pm

Whoever said that, ask them if they ever listen to the radio while driving their car, and when they say yes, then say "Oh My God!! you could destroy your battery that way!!!"

You need 2 things:

1) A charge regulator to control the voltage and current into the battery

2) A voltage regulator for your load (Arduino). Here is where it is a little dicey since you want to run a 3.3 volt device from a 3.3 volt battery.

Some things to keep in mind: A 3.3v nicad is rarely at 3.3v. When fully charged it could be as high as 4.5 v. And it is not considered "dead" until it gets to 3.0 v.

So it is a bit of "devil in the details". How low can the Arduino really go? Maybe it can actually run down to 3.0V? That would be nice. I am not familiar with that one. Anyway, when you have this situation where you want to run something from a voltage that is nearly the same as the voltage you need, but you also want to regulate that voltage, then you need what is called a LOW DROPOUT REGULATOR. Also known as "LDO". You want one that can handle the current you need (I will assume very little current), and you want the lowest dropout voltage you can find (like .1v or less). Micrel has some nice ones as do the other suppliers, National, Linear etc.

As for the charge control circuit, that is another matter that I am not an expert on. But you can possible get a commercial solution for that part.

Good luck

zener
 
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Re: snake eating it's tail?

by Ran Talbott on Wed Mar 04, 2009 3:51 pm

azs wrote:I was told not to use a battery and charge it at the same time.


This is not always necessary.

If you're doing "fast charging", you need to monitor the battery voltage very precisely to make sure you don't overcharge it and damage it (or even, e.g., with Lithium-based batteries, start a fire). You can't do that if there's a constantly-changing load on the battery.

If you're charging a nicad, NiMH, or lead-acid battery at a relatively slow rate (on the order of 10% or so of the battery's rated capacity), there's no need to do that monitoring: overcharging at that rate does little or no harm to the battery. The excess enery is just dissipated, safely, as heat.

It's routine in solar-charged systems to charge the battery with a load attached. If the energy source is capable of charging the battery at more than that 10%-or-so "trickle" rate, you should have some circuitry (like a relay or power FET) that cuts off charging when the battery voltage is approximately "full".

If you're thinking about using a Li-ion battery, make sure you get advice from an expert: I know that they've designed circuits for systems like PDAs and cellphones that allow the load to be run while safely charging the battery, but I don't know how, myself.

Ran
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Re: snake eating it's tail?

by azs on Wed Mar 04, 2009 4:18 pm

Thanks for the advice. My battery is actually 3.6 and my microcontroller uses 3.3v. I was planning to order L78Lxx series voltage regulators, but they have a drop out voltage of 1.7, so clearly I need to find a different one. Previously I had been using a LD1117V33.


I am using a nicad and it will be charging extremely slowly, much less than 10%. The motor is 5v. I had read somewhere that the input voltage should be higher than the battery voltage, and I am assuming that the 5v motor will charge the battery at 5v. Is this correct?
azs
 
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Re: snake eating it's tail?

by azs on Wed Mar 04, 2009 7:16 pm

Also, I want to monitor the wind speed, by connecting one of the wires from the motor into the analog input of the arduino, as well as into the battery. Will this cause half the voltage to reach the battery? If so, could I built a voltage dividing circuit that sends most of the voltage to the battery and only a tiny bit to the arduino?
azs
 
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Re: snake eating it's tail?

by The_Don125 on Wed Mar 04, 2009 8:01 pm

If you are using the motor to charge the battery, then its extremely hard to say what voltage the motor will charge the battery at. This is because the motor will output a different voltage depending on how fast it is turning, so at low rpm, it might only output 2V, but if you really get it going, it might hit 9V or more!
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Please be positive and constructive with your questions and comments.