Hi,
Have a raspberry and a grovepi+. I want to check the carbatterie. What i have to connect to the grovepi to get the state of charge?
Thx for helping
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The state of charge of a lead-acid battery isn’t a simple “measure the voltage” problem.
If you want to be picky, the correct way to read a lead-acid storage battery’s state (amount) of charge is with a hygrometer, measuring the specific gravity of the electrolyte in each cell.
Assuming that you want a “dry” measurement, 
, the actual state of charge can be approximated by measuring the ratio of the charging voltage across the battery’s terminals and the current drawn at that voltage.
Note that this ratio, known as the battery’s “charging profile”, is different for each different kind of lead-acid battery. Gel-cells, deep-cycle marine batteries, automotive batteries, storage batteries, every one of them have a particular - and different - charging profile.
Using the wrong profile on the wrong battery, (e.g. a car battery charger on a gel-cell), is the best and fastest way to destroy a battery.
There are chips and circuits out there for charging batteries, and they can be controlled by devices like the Raspberry Pi. They can even be controlled to provide the required profile depending on the kind of battery you want to charge.
That is, unfortunately, beyond the scope of these fora, though I would be interested in knowing how you eventually solve this problem.
Thanks for an excellent question and good luck!