I'm making a basic solar power phone charger using solar panels and a voltage regulator. Even with a limit on the current and voltage, is there any risk of overcharging the phone if left to charge too long?

 

Cell phones almost always have the charging circuits built in to them, unless they're quite old. You could probably tell by discharging the phone, and then monitoring the current vs voltage of the external adapter. Initially, the charge rate will be fairly high, and when the battery gets a reasonable charge, the current flow to the phone will fall.

At that point, if the external voltage has dropped along with the current, you'll know if the regulator is external.

If the external voltage has increased with the drop in current, you will know that the charger circuit is built in to the phone.

 

Thanks, that's a great suggestion.