First of all, the power of the transformer is enough, the 24V transformer is removed part of the secondary, there is about 17V on the line, (you can directly find a black and white TV transformer) with a diode half wave rectification of more than 6A, no need to limit the current resistance, directly charging the 12V lead-acid battery, whether it is full only their own estimates, the market simple charger is like this. If it is not a lead-acid battery, but a lithium battery, it requires a charging circuit.