Solar battery charger w/ load, first time building a circuit

Thread Starter

Атанас Баротов

Joined Oct 7, 2015
18
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.

circuit.png

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
 

Thread Starter

Атанас Баротов

Joined Oct 7, 2015
18
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 :)
 

Thread Starter

Атанас Баротов

Joined Oct 7, 2015
18
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
 
Last edited:

wayneh

Joined Sep 9, 2010
16,162
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.
 

Thread Starter

Атанас Баротов

Joined Oct 7, 2015
18
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
 

wayneh

Joined Sep 9, 2010
16,162
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.
 

spinnaker

Joined Oct 29, 2009
7,837
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.
 

spinnaker

Joined Oct 29, 2009
7,837
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..
 

spinnaker

Joined Oct 29, 2009
7,837
Top