Advice on project circuit

Community Projects
1

I have this project to monitor a battery used for keeping internet going and provide USB charging during power outages in the Philippines. Well I have the battery, charger, and USB charging hub all there for a year but wanted to monitor and be able to cut the load if the battery got too low during an extended outage.

I came up with this design I made a diagram image of the circuit and hoping that if I have any issues that could cause a problem maybe someone here that knows more than me can spot something I have overlooked or did wrong.

The main load relay is a solid state relay with diodes in it, I haven’t tested it yet but will put a diode in place if needed.

The 12v PC fan I tested it for any voltage spikes and get none, when it shuts down I see almost complete loss of voltage on the voltmeter, on my cheapo oscilliscope I show it drop about half voltage instantly and then fade smoothly away.

https://forthazard.com/images/BattBoxCircuitDiagram.jpg

2

I noticed the solid state relay does not like the voltage drop of the transistor but I am good if I switch it on the low side.

That brings me to another question, is there any down side to changing all of my relay drivers to low side switching?

Although the small coil relays I am using work so far in testing with the voltage drop, would it be more advantageous

to low side switch them to make sure I am always driving them hard enough?

I don’t know a lot, but my reasoning for using the transistors is to limit the amount of current I am drawing from the MCU.

3

Thank you.

I was getting .63v drop from collector to emitter when the base was receiving 3.3v directly from the 3.3v rail without resistor.

Are there any considerations I need to take into account switching low side vs high side?

4

Is the relay reliably act/deactivating on 3.3 volts?

5

The solid state relay says 3 to 32 VDC and works when directly driven from the 3.3v rail, it worked when I used the transistor on the low side because it still got the 3.3v.

The small coil relays are all 3VDC coils and so far in my testing none of them have failed to activate with the transistor on the high side, but I would feel more confident switching the low side if that does not create any issues with the overall circuit.

I have read in articles/forums related to automotive, that your choice of high-side vs low-side switching would be determined by where a short circuit is more likely to happen.

That does not seem to me to be an issue with my project, but this is my first real project, I don’t know what I don’t know, seems that electronics is definitely one of those things

that can appear simple and straightforward until you learn something, if I learn one thing, I find that I now have to learn about five other things I was unaware of, and so on… LOL

For example I did not know about the voltage spike until I saw it in a simulator, then I created the circuit and put it on the oscilliscope and there it was, I was very happy that I did not

damage my ESP32.

I have not physically built this entire circuit, but I have built each part of it separately on breadboards and tested with good results.

I am hoping to learn from other’s experience here before I build this, is there something I am missing?

I do not have reverse polarity protection on the inputs because it played havoc on my voltage divider, and the voltage from the shunts are so small I wouldn’t know how to try to protect that.

I will take extra care to wire it in correctly :wink: