My idea is this. I know voltage is measured in paralel, so I have 2 batteries (12V each) in series. I connect 1 circuit to the upper battery’s + and - (battery 2). I connect the second monitor circuit to the lower battery’s + and - (battery 1). I need ground on the upper circuit and the lower circuit? The grounds must be separated? Can this be done on a PCB? Meaning 1 internal copper layer is ground 1, and botttom copper is ground 2. Do I have to connect the 2 layers toghether (star them) and how do I do it in KiCad?
I am a bit unfamiliar with the ground concept! I know the ground is just a low resistance conductor (10 on -7 ohms) which attracts electrons. It has a low reference potential (and generators are always grounded for defence). But what the heck can a small PCB layer do? Is there any point of putting 2 grounds on 1 PCB even? Should I use blind vias or normal ones? Whats the difference?
Sorry that it looks like many questions in 1 post. its not many questions, its 1 question with all the details combined as 1 full!
Ground is just a name we give to the reference potential. (You might be thinking of copper pour or copper zone. A large copper area instead of a trace.)
If your two monitor circuits communicate with each other then it might be necessary to connect the two “grounds” via a capacitor to provide a low impedance signal return path for high frequency signals. (place this capacitor near the signal traces.)
I made a similar circuit but for a lithium ion monitor. I used two monitor chips who communicate via an integrated isolated SPI interface with each other.
I chose to place 3 capacitors to connect the reference potentials of the two chips. The reasoning is that the area taken up by the communication traces on the pcb is quite large. I wanted to provide short return paths for all 3 high frequency signals.
Here you can see the capacitors i placed for providing the return path:
And here highlighed them on the screenshot of the pcb side. I also highlighted one of the high frequency signals. Note that the dark yellow areas are the copper pours for V- (=GND) and Vmiddle (the things you call ground in your first post.) Both of them are on the same layer. Separated by a small band where no copper is placed.
<some stuff deleted, the second posting made it superfluous.>
There are two types of ground. There is protective ground and reference ground (or reference potential, often 0V). The first one should always be at the potential of the surrounding (building, water pipes, hopefully you too), in many locations on earth either a green or yellow/green insulated wire in the wall plug. The second one is usually a potential in the circuit close to protective ground, but it doesn’t have to be, it can be totally unrelated to it (even as in no meaningful voltage measurable between them), it is only a reference potential for the circuit (and connections between different circuits via connectors and cables). In modern almost standardized times, if there is only a single voltage source, the more negative voltage is declared ground.
Meanwhile you made a second posting, sorry, I had to work a bit after beginning to answer. Now I’m starting to get an idea what you try to accomplish: Two independent circuits, each watching the voltage of a single 12V battery from a 24V series connection.
Yes, connect this middle wire. It is ground (0V) for the +12V of the upper battery, and at the same time +12V in relation to the ground (0V) of the lower one. Ground is just a label.
BTW, I’m not very happy with the placement of your reverse polarity protection diode. C11 can still die.
Thanks for the help and no ground actually:
Thanks to the both of you! So there is no ground actually, we just call the lower voltage ground and put a symbol on it.
Github and no copper fill zones:
I hate github because there is no download button!
How do I create that no copper zone in KiCad? I like KiCad, but I am using it from 4 weeks. Why use 2 parts on the same layer, instead of 2 layers?
Comparators circuits communication, switch, series batteries, MCU:
My comparator circuits will not be communicating, there will only be a switch between the batteries to connect them in series and disconnect them in series so they can be charged separately at 14.4V, 20A, while after that they are used in series at 28,8V. I hope the switch or charging separately will not harm the PCB, which will be 1 PCB with 2 circuits, 1 for each battery and be permanently mounted on the battery bank. I will have an MCU which will monitor the battery voltages separately from the comparator circuits. 1 MCU will monitor both batteries but there might be some communication between the voltage divider to monitor the first battery and the voltage divider to monitor the second battery because they are on 1 ADC pin and are multiplexed.
C11 and a second filter capacitor:
What do you suggest about C11? Should I put a reverse diode (1n4148) on it too? Its suppose to be a filter capacitor for the comparators which are on 1 IC chip (4 comparators on 1 chip). Should I add a 100uF capacitor in paralel, because there will be a 90W motor when the 2 batteries in series are discharging.
As the two filled zones are different nets you can simply either place two zones with their outline apart by some amount. Or you make one zone such that it fills the whole board and give the second zone a higher priority and place it at the exact place where you want it. (Giving one of them a higher priority might be a good idea anyways. This ensures that the clearance settings of the zones will be used and you do not need to be as careful placing their outlines.)
In this case i doubt you would need a connection capacitor. (You don’t have hight frequency signals that would need a return path.)
As you plan on using one MCU it might happen that some state of your system can fry it if you are not careful. It might pay to use external ADCs with an isolated interface (example an isolated I2C bus) and have them as part of your comparator circuit.
It sounds like you are over-engineering a rather simple project. Why disconnect the batteries to recharge them? Why the comparators if you have an ADC?
My ADC is an Amica r.2 wifi module with 9 digital I/O pins and 1 ADC pin. Adding another microcontroller at the moment is not a good idea.
The batteries will be charged separately with 2 chargers (1 for each battery) at 20A per charger. But they will be used togheter to power a load at 24V.
As I said earlier, you are making this project far more complicated than it needs to be. Do you realize you have over 100ma of load for the LEDs alone? The LM117 is not the best choice for stepping 12V down to 3V3. The compatators are completely unnecessary but there are better ways to generate a reference voltage. Why do you have multiple circuits for generating the same reference voltages? Your “multiplexer” is flawed. Is C4 intended to be a 1F capacitor? Does it provide power when the batteries are removed? C5 has way too low a value. Your note mentions debouncing but that is not it’s purpose, but even it it were 22pF would not provide any debounce for a mechanical switch. Why do you have numerous 3v3 and Gnd pins of your MCU module unconnected? Why do you have so many reverse polarity protection diodes? One would suffice but I would consider using a MOSFET instead of a diode.
1. Better solution:
If you can give me a circuit for a simple solution I would be gratefull. Its hard wih 9 digital I/O pins and 1 ADC pin.
2. Step down voltage regulator:
The other option is LM2576 for a step down voltage converter.
3. Reference voltages:
How do I generate a reference voltage otherwise? I am meeting serious problems with resistor dividers as the batteries will be with a switch between them to disconnect the series connection. Is it necessary to disconnect the 2 batteries with the switch, when 2 chargers are connected (1 charger to each battery).
4. Multiple circuits that do the same:
The idea was to have 1 circuit for each battery, as both of them have to be monitored and they will be connected in series with a switch between them to disconect the upper battery’s “-” from the lower battery’s “+”, when charging (I dont know if its needed to disconect the series connection while 2 chargers are connected, 1 to each battery).
5. 1F capacitor:
Yes its 1F, I was told this is how much it has to be and I shouldnt change it. it should provide power when the batteries are removed or discharged.
6. 22pF capacitor on the reset button:
C5 (22pF on the reset button) is also something I was told not to change as its good.
7. Unconnected pins:
The numerous pins that are unconnected I am also told not to touch, because they dont need to be connected since they are used as outputing 3.3V or inputing to ground.
This whole post is only because of OP’s unfamiliarity with GND, Earth, grounding etc.
In pcb design managing the gnd is often a very important part of the design. In mixed (analog / digital) circuits the “analog ground” often has to be separated from the “digital ground”, even if they are electrically connected (often at a single specific point on the PCB). The very “noisy” currents through the digital ground plane must be separated from the sensitive analog ground plane to make things work properly.
The (copper) resistance and also the inductance of pcb traces or copper wires is very real and can not be neglected. Take your battery circuit. Attach a meter of wire and put some current through it. You can measure a very real voltage difference over the wire with a simple DMM.
“just connecting all gnd pins” together is almost guaranteed to deliver a non working design if high speed logic is used. Circuits like these need “ground planes” to make them work reliably.
Analog circuits such as audio amplifiers are also very sensitive to wronly routed ground connections.
Then there is the difference between “ground” and “earth” and “neutral”
“Ground” (gnd) is often an arbitrary conductor chosen as a “0” reference.
“Earth” is quite literally a metal shaft driven into the earth with a conductor into your home / circuit.
“neutal” is the wire in your mains socket which you are most likely to survive if you touch it.
There are at least 9 different symbols for “ground/Earth/earth” in the “power” library of KiCad. And they are there for a reason. Not because KiCad is an Open Source project and everybody likes their own symol better than the neighbours.
@Arak_Rachael ignore my post it has nothing to do with your question. It should mainly show you that some of the things written by @paulvdh should be taken with a grain of salt.
This was the advice back when you had pure analog circuits. Today you mostly have a small filter followed by an ADC which then communicates via a high frequency (or fast rise time) digital signals with your MCU. All the fancy signal processing is then done in the MCU instead of having complicated analog circuitry. If this signal now crosses over your gap between your two grounds you have created a nice slot antenna.
So the advice nowadays seems to be not to bother with splitting ground. (If there is a really bad circuit like a quarz one would still guard it. But that is a different story.) Yes some applications can benefit from it but it is not as simple as “always split the analog and digital ground”.
@rene: You are of course also right, but these “old” techniques will probably still be used in 20 years time. One of my first noise problems was with a single chip uC (AVR) and it’s internal 10 bit ADC. The uC was simultaneously scanning a multiplexed keyboard. Such projects are real good learning moments.
Main point here is that OP seems to think that everything is ok if the right components are connected to “gnd” in a spice simulation.
Even in something as simple and low frequency as a batery management system a circuit can be seriously compromised by a badly routed wire and high currents cause a sense circuit to trip at the wrong levels.
Ok, this is very interesting and useful, but I cant really start reading all the books on 9 grounds at the moment.
What I need to know is, can the batteries be charged by 2 separate chargers (1 for each battery), whlie there is still a series connection or do I need to filp the switch between the batteries for charging and discharging? In both cases, how should I measure the voltage for every battery since any of them might get damaged. I will connect the high current paths with wires not on the PCB. On the PCB there will be 1 MCU with 1F capacitor and 4 to 7 LEDs, and the wifi module thats built in the MCU Amica board.
This is far from the only question you should be asking, I have pointed out several others but you seem to be following some misguided advise so I’m not sure how to help you any further.
As far as your question is concerned, you seem to have already found your answer. In general, yes, you can connect two chargers to two batteries in series without isolating the batteries. As @Rene_Poschl points out you need to be careful that the output terminals of the chargers you use are isolated from the mains or earth, but it is unlikely that any modern charger wouldn’t be. Make sure it carries the appropriate approvals and you should be fine.
Dont take it personal, I am not trying to offend you.
Yes I can read!
The advices became a lot and the information also and the reading and sometimes it becomes too much.
As for the advice I am willing to hear it non the less (your advice that is), just because until now I have been told that I need this, does not means that I want to make mistakes.
Sometimes the devil part gets the best of me, and my text seems worse than my bite.
And when you get get overwhelmed by (possibly contradictory) information that can be pretty confusing.
'Been there, experienced that.
So what I normally do:
Read and follow the original documentation. (That seems pretty clear to me in this case).
Do some (carefull) experiments and deductions.
Measure all the currents and voltages you think that are relevant.
If you are in doubt add some fuses or resistors to limit the current.
Remember that currents can only flow in circles (or squares, or whatever).
If you put current into a component, that same current also has to come out of the component.
True for resistors, capacitors, IC’s PCB’s, cars.
On sattelite systems with an Plasma or Ion drive they once tried to let electricyty flow in only one direction.
Guess what, it didn’t work.
