Hello everybody, this is my first time building a circuit and I'm trying to figure out exactly which transistors I should be using as well as diodes and everything else relevant. The Arduino will control when a battery gets charged or not via the digital pins (5v output). Here's the schematic I have so far.



I've been experimenting with a few transistors that I had, but either they get hot when i put a DC motor or they don't fire at all at 4v. The A0/A0 ports are to detect the battery voltage. The buck up/down circuits are ones i've purchased from ebay so they should be ok. The solar panel is about 400mAh, 7v and the batteries are 3.7v LiPo, 9900mAh

 

TIP120 Transistor should be fine.BY550/1000 diode would be fine.

 

Thank you - gonna try those as soon as the store opens

 

Why are you bucking up to 10V and down to 4 v?

Charging batteries properly is a science. You might want to read up at the Battery University.

 

Thank you - gonna try those as soon as the store opens

Must be nice to have a local shop where you can buy components.

 

Well, it was suggested to me to have it boosted to 10v and then down as the solar panel outputs from 3 to 8v, but changing that is the easiest parts as both the boost up and the buck down circuits have regulators and I can try removing one.

Yeah, having a local store is nice, though half the time they dont have exactly the component you need

 

How much voltage are you getting out of your panel?

 

usually about 4-5v. Haven't done extensive testing there. According to specs it should be: 6v, 3.5W, 580-600mA though direct sunlight tests have shown voltage above 7-8v

 

I really don't think you need either converter, just a charge controller. Those converters are just wasting power. You should check, but I think your battery can take the full current from your panel. You just need to stop it from overcharging the battery, to stop when it's done.

 

It's said that the battery life (max charge cycles) decrease if I feed it more energy (about 2000 at 4v, 500 cycles at 4.2v). The whole point of this is that I can control when a battery get charged via the arduino

 

Yes but do not confuse the battery voltage with the higher voltage potentially available from the charger. The higher voltage of the charger only matters if 1) it causes too high a charging current, or 2) is allowed to overcharge the battery to too high a voltage.

 

the analog pin of the arduino can take only up to 5v so the buck down module is mandatory

 

Or you could use a voltage divider.

 

the analog pin of the arduino can take only up to 5v so the buck down module is mandatory

And maybe also put a 5V zener on it to protect it from over-voltage input.

 

the analog pin of the arduino can take only up to 5v so the buck down module is mandatory

You will need to use a voltage divider if you want to measure the voltage of the panel. Design the divider so you have enough resolution so it can be accurately measured but also so the divided voltage does not exceed the capabilities of your mcu, In other words design it a bit beyond the maximum voltage your panel will put out. With a buck regulator you will get a constant voltage.

 

the analog pin of the arduino can take only up to 5v so the buck down module is mandatory

Two resistors in series is much easier.

 

Thanks, I'll try a version with the 50% voltage divider, though, is that much power lost in the conversion that I should try to avoid that buck down module?

 

As I already mentioned, a step down regulator is not going to work if you want to measure voltage level. A step down regulator is in fact a regulator so it is going to attempt to maintain the voltage to which it is designed till it passes it's minimum operating voltage. Even then the change might not be a true representation of the change at the input because of the circuitry in the regulator.

All you would know is the voltage level dropped below it's operating voltage. If that is the way you want to go then there are better ways to do it like using a comparator. Then all you need to do is use a binary input to test the output of the comparator..

 

Something like this?

 

That schematic could be drawn a whole lot better but yes. Do you want to monitor the two batteries individually?

There is something odd to me about those transistors. It looks to me like you are trying to shut off the step up regulator.

Something like this?

View attachment 97334

Here is a nifty calculator to help out.

http://www.raltron.com/cust/tools/voltage_divider.asp

Oh and you do want a step down for your supply to your MCU.