A word of warning.
The output voltage from most mains chargers is higher when at no or little load than when it is providing its full rated output. It's not unusual for something rated at 12V to produce over 14.5 volts when lightly loaded. Your ICOM uses much more power on transmit than when idle so keep the battery connected. The additional load from charging the battery will keep the output voltage of the charger within bounds.
(The ICOM charger should have been designed to cope with the fluctuation in load, someone else's may not have been)
That said I do use a third party car charger with mine, but always with the battery in place and I unplug it once it is fully charged.
Mike