I use my Lumia 925 in the car as a GPS. I power it using a car charger but have noticed that even though the phone is plugged in and the charging icon is showing, using Here Drive + causes the battery to discharge. In a 2 hour journey I can use as much as 40% of the battery.
Even if the phone is not running any apps, it still charges very slowly, on a 90 minute drive I only managed to charge by about 10%. I currently use a Nokia DC-6 car charger but prior to this I have used a couple of generic chargers. I have experienced the same issue with all three chargers.
Is there a problem with all of the chargers (is there some in-car fast charger I could use instead) ? Is there an issue with my particular phone or are the power demands of the phone simply too high for an in-car charger to keep up ?
The DC-6 has an output of only 550mA. Your AC charger is at least 1A (1000mA), if not 1.3A (1300mA), so you can see why your handset is losing battery charge under heavy use with a charger supplying just about 50% of the required charge.
You would need a charger with at least 1A output, something like the DC-17 from Nokia for example. The DC-20 gives you 2 outputs with 1A each if you need to charge something else at the same time.