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Monitoring voltage before and after solar charger

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TomS_:
Hi all.

Im working on a project, and Ive become a bit stuck. I am trying to monitor voltages of a solar panel, and also a battery connected after a charger that is connected to that solar panel.

The difficulty is that the solar charger uses a common positive rail, and the negative side is switched.

This presents a bit of an issue for how I have designed my circuit so far - my ADC channels are connected to the respective positive terminals, with a common ground. But this simply results in the same voltage reading for both sides of the charger, which is incorrect. Ordinarily I'd expect the positive side to be switched with a common ground.

So I need some help figuring out the best way to connect my ADC channels to be able to read these two voltages when the ground is changing. My current thought is to use an analogue multiplexer, something like a 74HC4052, to change which ground reference is presented to the ADC. But Ive also seen that there are some chips out there that can do voltage and current sensing in one package. The only difficulty is that all of the ones Ive found so far only have a single ground connection, which puts me back at square one - I'd need something that has separate grounds for the digital side and the bus side?

Does anyone have any suggestions? Ive attached some images with basic circuit details including (roughly) how things appear in the charger based on my own investigations, but I can go into more detail as required.

edit: maybe I got the direction of the MOSFETs around the wrong way, but hopefully you get the basic idea - ground is switched

Thanks

Terry Bites:
diff amps? I am guessing no great accuracy is needed so home brew amps will do?
Create a negative supply with a 7660 or similar

Siwastaja:
What kind of charger? If it's a super cheap crap charger (sometimes marketed as "PWM") which is not an actual DC/DC converter but only connects and disconnects the solar panel directly to battery using MOSFETs, ignoring MPP, then there are no two voltages to sense. Just sense the battery voltage, and it will be equal to the panel voltage during energy production. Unless you want to know the OCV of the solar panel, there is no separate input voltage to sense.

Mechatrommer:
there is.. except during disconnection (the time when you want to do PV measurement), the PV -ve terminal will be way below the battery -ve tab, this means you need either dual rail opamp or difference opamp that can accept out of rail (robust) input. once you got |voltage| wrt battery -ve tab, you can just add to battery +ve voltage to get PV open circuit voltage, but this can be much involved such as sensing/tapping mosfet's gate to know its state etc. care also during the night when PV -ve terminal = battery +Ve terminal, your difference amp will supposedly output -ve output, you'll need dual rail anyway, unless maybe you want to do measurement wrt the common +ve terminal. i have such common +ve PPM cheap china unit, made a teardown, and OP's idea passed through my mind, but i think its not worth the effort, so i just let it run as the way it is. i would say they are "not so stupid" at collecting energy from PV, they are less than $10 cheap and are okaish if you dont want to be so particular about wasted energy, a lot better than connecting directly the PV to the battery. ymmv.

Peabody:
I have a more basic question about why you would measure the solar panel voltage, along the lines of Siwastaja's comment.  I recently worked on using a solar panel with a linear Li-ion charger with load sharing, and what I observed was that, depending on the illumination level, the solar panel always provides as much load current as it can at the battery voltage, and its voltage is therefore equal to the battery voltage.  And it's only when illumination increases beyond the point that the panel is providing ALL of the load current that the panel voltage starts to rise.  Well, I really don't understand how these panels work, but I wonder if measuring the panel's voltage really tells you anything even assuming you could figure out a way to do it.  It seems to me that what you need to know is the battery voltage.  Perhaps you could explain how the panel voltage information would be used in your circuit.

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