Low battery level

[28i]

2012 F31 , 8 speed auto.
I can sort of understand why we are told at our local BMW garage that we have a low charge on the battery. We do short runs so the battery isn't able to charge fully. That is why I bought the battery charger that BM use and charge maybe once a fortnight.
The sunroof stopped working. Have had this once before under warranty and was told it was the "controller" that had failed. So I went back to the BMW garage. They put the car on diagnostics and that gave a "low Battery Level" plus other codes. They reset the codes and the sunroof now worked. They explained that as the battery was low the system automatically shut down parts of the vehicle.
I've read about this on this and other forums.
I think I've also read that the alternator only comes to full power at about 1500 revs. On my journey back from the garage I only reached 1500 RPM at 50 mph, so in theory driving around at 30 mph the battery will never get to be fully charged.
My questions are,

1, How do we owners know when the battery is low, to stop things shutting down.
2, Do we all need a code reader and be able to reset them. Which one is best?

[Maracus]

The only time I've seen a 'Battery Low' in my 2013 F31 328i was when I'd left a DVD player plugged in the rear 12V socket for about 4 days, whilst the car was parked up!

Never seen it since.

[sun.and.rain]

My retired father has a 2008 318d, immaculate with around 60k miles. Owned for 4yrs.

"Battery low" warning has started coming up, especially on cold mornings.
Battery voltage is fine (replaced last yr with HD item), alternator is charging (around 14V with engine running).
Car is used mainly for short trips but monthly round trip of 160miles.

I've charged the battery with a charger overnight but the problem is still occurring.

Can anyone advise based on experience? The car appears to be running perfectly but I don't want my elderly dad to find himself stranded somewhere with a flat battery.

Thx.

[helix402]

Does the car have an agm battery? (Probably). Was the new battery registered to the car, is the new one the same ampage as the old one?

[sun.and.rain]

Thx for reply. Battery was replaced with a new one as recommended by local garage (reliable local garage we've used for years, not bmw or bmw indy). I don't know the battery rating right at the moment but can check at the weekend. Knowing my dad the new battery will be rated at BMW recommended spec or maybe slightly higher and it wom't be a cheapo budget one.

Does the new battery have to be "matched" to the car's electronics in some way? Have BMW even made changing the battery complicated?

[wild rover]

check with BMW to ensure whether the ecu requires to be reset. Also consider cleaning the earth connections to the body, also body to engine. Fitting an additional earth lead from battery to starter motor can also make difference, cost of made up cable £10 must be worth a try. Best of luck.