# Charging batteries with solar panel

Before I ask my question, I would like to say that im not exactly from the EE field, so please be easy on me.

I would like to use a Solar Panel and Rechargable Batteries as input to raspberry pi zero w that needs 5v.

I came across the CN3722 charging controller with battery protection. it can handle 1 or more multi-cell lithium battery

So this is the circuit I thought about (high level):

These are my main questions:

1. What will happen when the batteries are charging? The load still getting current? im not sure if there is cut out between the CN3722 to the Pi (as the two pictures I uploaded). If the load is still getting current, how can I be sure it will no go over\under from what it needs and ruin the Pi?

2. The Pi needs 5 V. does the CN3722 output voltage is basically the batteries output 7.4 V? The resistor should take 2.4 V?

3. What does it mean that the CN3722 can handle 1 or more multi-cell lithium battery? Are those batteries connected in serial \ parallel \ combined. How does it know how they are connected?

I’m a bit confused and ill be happy for guidance. Thanks a lot!

EDIT

This situation of connecting the load to the battery, in parallel, can damage the load? i guess my question is divided to 3 questions. what will happen (current wise) to the load when:

1. the batteries are full
2. the batteries are half full
3. the batteries completely empty


how can I know\make sure, the Pi gets the current it needs and not more or less?

from my understanding, when the batteries are half full, the current (that the solar outputs on the exact moment) will divide somehow between the batteries and the load.

1. The chip tries to charge the batteries, but it can't know if there is only battery or load too, so it is unlikely that the charger chip can determine when the batteries are full.

2. The charging chip does not output anything, it just charges batteries. The output is the battery output. You can't use a resistor to drop voltage, you need a buck converter.

3. You can freely set the total expected battery voltage with resistors. So you can connect two batteries in series if you want 7.4V output. Or two batteries in parallel, which just looks like one larger 3.7V battery.

• (1) Charger has to provide current to load and batteries (2) same thing (3) same thing. Pi gets all the current it needs from the battery, it's just that it needs 5V, and that is not the problem of CN3722 chip, that is a completely other problem that must be solved with a buck/boost converter to convert whatever battery voltage you use to 5V. Aug 15 '20 at 15:06
• sorry, I didnt seeyour answer, I put mine in edit to the original question Aug 15 '20 at 15:15
• so when the batteries are fully charged + the load still get current through the controller, doesnt it damages the batteries? because one of the controller's purpose is to detach the battery when fully charged, but from the controller perspective, it still needs to 'charge' Aug 16 '20 at 11:03
• That is the problem with your block diagram. When there is external supply connected, the load should be disconnected from the battery, so the charge controller chip can only charge the battery, and the load should get power from the external supply. Aug 16 '20 at 15:06

First off, this is a not a well designed product so the datasheet is incomplete. If you can't find the answers it is probably because of the datasheet and product information and no fault of your own.

In the diagram above, you'd want a step down regulator if using 2C (7.4V) or step up regulator if using 1C (3.4V) where the resistor is. A resistor will not work well (if at all) because the PI is not a constant current source.

I would use a 2C configuration with a step down regulator to 5v for the PI.

The top diagram makes more sense than the bottom one, you would want to connect the load to the battery.

• You'll also want to make sure that you are connecting your regulator "upstream" of RCS to prevent the charge controller from lumping current to the RPI in with battery charge current.
– vir
Aug 14 '20 at 22:00
• Is there a need to connect any capacitor between the step down regulator? or just connect it directly to the Pi.. Aug 15 '20 at 14:46
• The datasheet will explain that, spoiler alert: you need a capacitor on the output and input Aug 15 '20 at 14:53
• thanks! ill look for it Aug 15 '20 at 15:17

The PV source is not a voltage, but rather a current source with an apparent impedance of Voc/Isc=Zpv.

The MPT operating point needs a voltage from 82%Voc down 72% in late afternoon then drops with clouds.

The CN3722 expects a low impedance voltage source input, so when the battery demands CC charge the PV may collapse the voltage to very low % of MPT. Therefore it will not be matched impedance.

There are many other details missing for specs.