# Help with Building a Solar Battery Charging Circuit

#### Chris Verdegan

Joined Nov 7, 2017
2
Hey guys!
I am currently embarking on a project of a little bot that moves around to find sun and uses it to charge its batteries through a solar panel. The solar panel voltage is voltage dividered read through my arduino counterpart, the Teensy's ADC pin. I am using a 5V solar panel and four 1.2 volt NiMH batteries. I want this solar panel to simultaneously charge the batteries while it is running off the power supplied by the batteries. Is this possible with some sort of formation of schottky diodes or is impossible because it will have current trying to run in two directions? Is there any work around for something like this?

Thank you!!

#### Colin55

Joined Aug 27, 2015
519
4 x 1.2v = 4.8v You need over 7v to charge the batteries. The panel will not work.

#### Picbuster

Joined Dec 2, 2013
1,020
Look at the lt1513-2 it's step-up ort step down battery charger.
Step-up when source < wanted cell floating voltage and step_down when source is higher.

A few external components and you have a charger.
( I am very pleased with the chip).

Picbuster

#### Bernard

Joined Aug 7, 2008
5,624
What is the short circuit current rating of panel ? Is 5V open circuit or under rated load ? Motor load ? Or you might want to try a " Solar Bot ", where a capacitor is charged to almost open circuit V, about6.6 V & then runs the Bot a few inches, & repeats.
All I could find is BEAM 1381 Solar Engine.

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#### wayneh

Joined Sep 9, 2010
16,399
I want this solar panel to simultaneously charge the batteries while it is running off the power supplied by the batteries. Is this possible with some sort of formation of schottky diodes...?
You need just one diode (a schottky is a good choice because of the low voltage drop but a silicon diode would work almost as well) to prevent reverse current from the battery into the panel. If the panel has enough EMF to overcome the battery voltage (plus the diode drop), current will flow from the panel to the battery plus its load. It usually doesn't matter how that current distributes between the battery and the load and you don't have to worry about it.

An exception to that is if the panel is powerful enough to over-charge the battery at too high a charging rate. In that case you would need additionally circuitry (called a charge controller) to maintain safe conditions for the battery no matter what the panel or the load is doing - for example at peak panel power with the load turned off.

As noted, your 5V panel will not be able to charge 4 cells in series. Most small systems I've seen have a panel voltage roughly double the nominal battery voltage.

#### alfonsoM

Joined Nov 8, 2017
41
You use the solar panel to charge the battery, and you power your circuit from the battery, you don't need a switch.
The circuit in this website is very efficient because it uses all the energy the panel can give even when it's down to 1V: http://mfprojects.co.uk/charger.php

#### alfonsoM

Joined Nov 8, 2017
41
The main idea of the circuit in the website is that it charges when the cell voltage is down to 1V. It also uses a few uA to power itself in compare to the one transistor circuit that needs 10mA to drive itself, on cloudy days 10mA is all you get.