One of my 2 SIM cards drains battery of my smartphone. Why and How?

This I haven't tested yet, but indeed I should test this possibility too. Thx.
...but OTOH I doubt this test can help to fix the problem.

The said SIM card is the only one that has also a data plan booked by me (the other one is just voice and SMS only), so I suspect that as soon as this card gets activated in the phone, it tries to connect to servers in the Internet, but since there is another setting in Android to enable/disable data services (and since it gets enabled by me only on demand), then its connection attempts of course fail, but that dumb program seems to try it endlessly.
This is just my attempt to explain it, I'm not sure b/c it all happens under the surface, ie. in the background, there is no indication in the GUI nor in the log(s) [also maybe due to the fact that it fails and it only logs successes, if any...].
 
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I've a cheap smartphone (with Android 9 OS) that can hold 2 SIMs. It's for my needs very practical as one can disable/enable and also chose which SIM to use for voice, SMS, data etc. The SIM cards are from different providers here over in f'upd Germany: o2/Telefonica and Telekom; both SIMs are prepaid ones.

If I activate just the o2 SIM then the battery holds about 4 days. If I activate the Telekom SIM then within 3 or so hours the whole battery gets drained and the smartphone shuts off b/c of no power anymore. That latter case happens when either activating just the Telekom SIM or activating both SIMs.

Anybody else has/had such a similar nightmarish experience? What's going on here? How to explain what possibly is going on with this Telekom SIM?

Is this maybe a sign for the bigbrother-surveillance-state coming onto my phone via this Telekom SIM? As said, happens only with this Telekom SIM! And the SIM is functionally OK, ie. is not defect.

Or is this maybe the latest fraud/scam in the world of the mobile providers, practiced by the provider: to force the user not to make much use of the SIM?... :) Just guessing... :)
one sim may be set to
data: always on
 
one sim may be set to
data: always on
There is a setting called "Data usage" under Settings. There the two SIMs appear, and under them is then a setting named "Cellular data" for enabling/disabling. Ie. only after manually activating here does data connection to Internet work. In my case this is disabled by default and gets activated by me manually only on demand.
Other than that I'm not aware of any other setting that says "always on".
And: I also have not installed any other program beyond the built-in standard defaults on this phone.
 
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The setting I mention is burried in the settings menu. It isnt about data transfer it is about keeping the data connection open even when no data is being transferred.

However its no use to you. I found it and it is a device level setting, not sim.

Another possibility is one sim is set to not use roaming, and is trying to work off a tower further away.
 
Found a suspicious setting:
Settings / Security / Apps with usage access / Google Play Store
was enabled all the time (ie. "On")! Now I disabled it and testing...

After 1h: LOOKS GOOD SO FAR!... :-)
 
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The setting I mention is burried in the settings menu. It isnt about data transfer it is about keeping the data connection open even when no data is being transferred.

However its no use to you. I found it and it is a device level setting, not sim.

Another possibility is one sim is set to not use roaming, and is trying to work off a tower further away.
I've Data roaming disabled as it brings the warning "Attention! You may incur significant charges" when activating it.
Btw, this error happens at home...
 
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