Solar Powered Li-Ion battery?

I have this Carrera RC car, and it runs on a Li-Ion battery. Li-Ion battery: 7,4 V – 900 mAh
It is the Red Freezer Watergun Car, and the battery is on the bottom of the car. I wondered if anyone knows how i could cut the recharger wire (8,4 V – 500 mA) and attach it to a solar panel on the top of the car to constantly charge the battery. 

Cut the recharger wire?

You can certainly splice in a solar panel to the car but the panel will need to be able to offer the 500mA needed and you will need the right charging circuity to monitor the battery as it charges. Li-ion batteries have pretty specific charging needs. But the problem is adding a solar panel to the car isn't going to give you longer driving times. Any power brought in by the panel as you drive isn't going to charge the battery or be much use in running the motor(s). It will take upwards of a full day or 2 of good sunny conditions to take the battery from low to fully charged. So long as the car isn't being used.