Have seen many attemps to make a battery charger for mobil etc.
They are all expensiv and difficult to make :-(
My idea was to make a cheap charger with thing that many of us have at home
Take your auto USB charger and solder a red wireon the bottom plug a blue on the side put on lugs on the ends.
Step 1: Step 2
Find a batteri you have for gardentool or tools must be at 12-24 volt
Charges it as usual
Push the + and - lugs on the battery (REMEMBER POLARITY)