OK, my N900 charges from PC pretty well, while does not charge from other sources when its battery is too low, though does usually charge when battery is above 50%. I suspect it is USB port problem, since device was reflashed recently and has no other problems. USB port doesn't seem to be very loose, though, at least not looser than cheap phone copies that do charge pretty well, but I can't figure anything else that can cause the problem.

First of all, should I be panicking?

Secondly, I have a soldering station and a chinese iPhone 5 with broken USB to get some practice...

Wow, these youngsters nowadays! A cool soldering station like that and you are asking how to solder? I did almost all my soldering for the past 30 years with an older version of sometthing like this. No fine tip, just a loop of copper wire. No temperature control, just plain manual skill. Although, admittedly, I used this for fixing my USB port. Bought it on a promotion for £2.99

Regarding your charging problem, I find it curious that your phone would charge from the PC but not from "other sources", yet those other sources are OK if the battery is half full. To me it suggests not a contact problem but rather something to do with the current. The PC provides a maximum of 500 mA, a charger much more than that. An empty battery takes more current than a half-full one. In short, I would check your "other source", the problem is most likely there.

Other sources include:
1. Coolermaster USNA-120 - 1A max port usually works but not when battery is at very low level, 2.1A max port doesn't usually work unless battery is nearly full.
2. 5V 1A USB chargers, one came with GoClever GPS, the other is HP.

All other devices are working perfectly with the chargers in question.

Also, I am sometimes swapping batteries fast enough so the date&time doesn't drop to default, may it be somehow connected with the issue? Batteries are different types but identical capacity (~1300mAh), one pretends to be genuine but is most likely fake, the other is Gold.

Hard to diagnose this way. If you would search for Pali's BME replacement, install it (backup before!) and provide a dmesg logs (after connecting in every variant/situation), we could get some idea WTF is going on.

Otherwise, current monitoring from device's side (via bq27200.sh script or [shameless plug]BNF from repos[/shameless plug]) could also tell something, but BME repl provide much more debug info than normal BME, allowing to determine what exactly device "thinks" happen, when you plug various stuff.

/Estel

I didn't install the mentioned programs, however, I've tried some 500mA chargers and it works like a charm with all of them, so guess I have no USB problem and will just use different chargers for my phone.

Curioser and curioser. I suspect a fault in your phone's charging circuitry. It seems to cut off at currents exceeding 500mA. A blown fuse or something of that kind. One last thing I would try before consulting the schematics and attempting to replace SMT components (difficult) is... open it and look for dirt that might cause some short circuit.

I've performed some more research on the problem after accidentally getting my N900 successfully charging from another 1A charger.

Conclusion is my N900 usually refuses to charge from chargers which don't have data pins shortened. Is it normal behavior, or should I consider my phone faulty?

Your phone is doing the Right Thing™. Feel free to read up on the USB 2 specification (warning: link leads to a 28 MB .zip). In short,

• USB devices can be low or high power.
• The maximum current is limited to 100 mA in low and 500 mA in high power.
• Any device must start as low power on plugging in to the host and negotiate switching to high power only after a successful negotiation.
• Shortened data pins override the need for power negotiation as well as the 500 mA limit.

Your phone follows these rules. On plugging in, it tries to detect shortened data pins. If not detected, it has to assume that this is a host USB port and not a charger, so it tries to negotiate the high power. This obviously fails so your phone remains in low power mode, where only the USB controller chip is powered (maybe not even that, since the phone has its own power source).

I have problem with my phone. One of issues is that /home partition is broken and fsck can't help. It seems it need at least rootfs reflash, but how to do it properly without doing Vanilla reflash? (I have some nice things on MyDocs I won't want to lose...) - I read rootfs flashing keep /home intact, so I don't know if it would help much. I have backupmenu on my phone if it helps (GUI part of system partly works but mostly not, but this way I have at least working terminal)...

so if anybody could help giving me short walkthrough for my problem?

I've flashed my N900 a few times... all my data in MyDocs was still there..only apps were deleted

i think I used this command in flasher(without -R):

Run the following command (TIP: Do not include '-R' if you are flashing EMMC as well, see below):
flasher-3.5.exe -F RX-51_2009SE_2.2009.51-1_PR_COMBINED_MR0_ARM.bin -f -R