Hi,

I have problems to recharge the N900 battery with the Proporta battery charger for car and with a Kensington portable battery pack (k38021 with output 5VDC - 1.50A Max).
I must insert the USB cable several times before it starts charging the battery.
This does not happen with the battery charger of the Nokia N900 and connecting it to the computer,

Has anyone encountered the same problem?

Thanks

It sounds like you are talking about a simple power supply, sometimes known as a phone charger, not a battery charger, right?

I have had trouble trying to charge my BL-5J in an external "universal" LiIon charger.

Other Nokia phones can be very fussy about their power input, and refuse to charge from car chargers that work fine with other devices.
So no surprise if the N900 does the same. Maybe the input voltage is a little high? Or too much impedance?

please use search and read the thread about usb chargers for N900 and why some work and others don't. Often due to missing data pin connection.