Mobile phone chargers usually have output voltages from 3.5 to 5.5V, and all recent phones with a microusb plug are charged at 5V.
For a trickle charger you need 13.8V output (presuming a 12V battery).
This means you need to add a voltage step-up circuit, and a shut off circuit. Sometimes it is possible to modify the charger to double the voltage (and half the current) by simply replacing one or 2 components ... but lets be realistic, it is not worth neither the time nor money. A month ago I payed 5$ for a brand new trickle charger (free shipping from China) and it works just fine.
For the price I could not get the empty plastic housing in my local electronics shop
Since the advent of selfadhesive LED strips, I found myself quickly run out of old chargers because all of my friends wanted a lenght of LED for aquariums, PC keyboard illumination etc ....